
                     University of Amsterdam

                        Dept.  of Social
                       Science Informatics

                              (SWI)
                      Roeterstraat 15, 1018
                          WB  Amsterdam
                         The Netherlands
                     Tel.  (+31) 20 5256121

                          SWI-Prolog 4.0

                         Reference Manual
              Updated for version 4.0.3, April 2001

                          Jan Wielemaker

jan@swi.psy.uva.nl  http://www.swi.psy.uva.nl/projects/SWI-Prolog/

 SWI-Prolog  is  a Prolog  implementation based  on a  subset of
 the  WAM (Warren Abstract  Machine).   SWI-Prolog was developed
 as  an  open  Prolog  environment,  providing  a  powerful  and
 bi-directional  interface  to  C in  an  era  this was  unknown
 to   other  Prolog  implementations.      This  environment  is
 required  to  deal  with XPCE,  an  object-oriented  GUI system
 developed  at SWI. XPCE is  used at SWI for  the development of
 knowledge-intensive graphical applications.

 As  SWI-Prolog  became  more  popular,  a large  user-community
 provided  requirements that  guided its  development.   Compat-
 ibility,  portability,   scalability,  stability and  providing
 a   powerful  development   environment  have  been   the  most
 important  requirements.     Edinburgh,  Quintus,  SICStus  and
 the  ISO-standard  guide  the  development  of  the  SWI-Prolog
 primitives.

 This  document gives an overview of the features, system limits
 and built-in predicates.

 Copyright Oc 1990{2001,University of Amsterdam


Chapter 1.  INTRODUCTION


1.1 SWI-Prolog

SWI-Prolog  has   been  designed  and  implemented   to  get  a   Prolog
implementation which can be used for experiments  with logic programming
and  the  relation  to other  programming  paradigms.     The  intention
was  to  build  a Prolog  environment  which  offers  enough  power  and
flexibility to  write substantial applications,  but is  straightforward
enough to  be modified for experiments  with debugging, optimisation  or
the introduction of  non-standard data types.  Performance  optimisation
is  limited due  to the  main objectives:    portability (SWI-Prolog  is
entirely written in C and Prolog) and modifiability.

SWI-Prolog  is based  on  a very  restricted  form  of the  WAM  (Warren
Abstract Machine) described  in [Bowen & Byrd, 1983] which defines  only
7  instructions.   Prolog  can  easily be  compiled into  this  language
and the  abstract machine code  is easily  decompiled back into  Prolog.
As  it is  also  possible to  wire a  standard  4-port debugger  in  the
WAM interpreter  there is  no need  for a  distinction between  compiled
and interpreted  code.   Besides  simplifying the design  of the  Prolog
system  itself this  approach has  advantages  for program  development:
the  compiler is  simple and  fast, the  user  does not  have to  decide
in  advance whether  debugging  is required  and  the system  only  runs
slightly  slower when  in debug  mode.   The  price we  have  to pay  is
some  performance degradation  (taking  out the  debugger from  the  WAM
interpreter improves performance  by about 20%) and somewhat  additional
memory usage to help the decompiler and debugger.

SWI-Prolog  extends  the  minimal  set  of   instructions  described  in
[Bowen & Byrd, 1983]  to improve  performance.    While  extending  this
set care  has been  taken to  maintain the  advantages of  decompilation
and  tracing of  compiled  code.    The extensions  include  specialised
instructions  for unification,  predicate  invocation,  some  frequently
used built-in  predicates, arithmetic, and  control (;/2, |/2),  if-then
(->/2) and negation-by-failure (\+/1).


1.1.1 Other books about Prolog

This manual does not  describe the full syntax and semantics  of Prolog,
nor how  one should  write a  program in Prolog.    These subjects  have
been  described extensively  in  the literature.    See  [Bratko, 1986],
[Sterling & Shapiro, 1986],   and   [Clocksin & Melish, 1987].       For
more  advanced  Prolog   material  see  [OKeefe, 1990].     Syntax   and
standard  operator declarations  confirm  to the  `Edinburgh  standard'.
Most  built  in  predicates  are  compatible  with  those  described  in
[Clocksin & Melish, 1987].      SWI-Prolog  also  offers  a  number   of
primitive  predicates compatible  with  Quintus Prolog  [Qui, 1997]  and
BIM_Prolog [BIM, 1989].

ISO  compliant  predicates  are based  on  ``Prolog:    The  Standard'',
[Deransart et al., 1996], validated using [?].


1.2 Status

This manual describes  version 4.0 of SWI-Prolog.   SWI-Prolog has  been
used now for many  years.  The application range includes  Prolog course
material,  meta-interpreters, simulation  of parallel  Prolog,  learning
systems,  natural  language processing  and  two large  workbenches  for
knowledge  engineering.    Although we  experienced rather  obvious  and
critical bugs  can remain  unnoticed for  a remarkable  long period,  we
assume the basic Prolog  system is fairly stable.  Bugs can  be expected
in infrequently used built-in predicates.

Some bugs are known to  the author.  They are described as  footnotes in
this manual.


1.3 Compliance to the ISO standard

SWI-Prolog 3.3.0 implements  all predicates described in ``Prolog:   The
Standard'' [Deransart et al., 1996].

Exceptions  and warning  are still  weak.    Some SWI-Prolog  predicates
silently  fail on  conditions where  the ISO  specification requires  an
exception  (functor/3 for  example).    Many predicates  print  warnings
rather than raising an  exception.  All predicates where  exceptions may
be  caused due  to a  correct program  operating in  an imperfect  world
(I/O, arithmetic,  resource overflows)  should behave  according to  the
ISO standard.   In other  words:  SWI-Prolog  should be able to  execute
any program  conforming to [Deransart et al., 1996]  that does not  rely
on exceptions generated by errors in the program.


1.4 Should you be using SWI-Prolog?

There are a number of reasons why you better  choose a commercial Prolog
system, or another academic product:

  o SWI-Prolog is not supported
    Although  I usually fix bugs shortly  after a bug report arrives,  I
    cannot  promise anything.   Now that the  sources are provided,  you
    can always dig into them yourself.

  o Memory requirements and performance are your first concerns
    A  number  of  commercial compilers  are  more  keen on  memory  and
    performance  than SWI-Prolog.   I do not  wish to sacrifice some  of
    the  nice features of the system, nor its portability to  compete on
    raw performance.

  o You need features not offered by SWI-Prolog
    In  this case you  may wish to  give me suggestions for  extensions.
    If  you  have great  plans, please  contact me  (you  might have  to
    implement them yourself however).

On the other hand, SWI-Prolog offers some nice facilities:

  o Nice environment
    This includes `Do  What I Mean', automatic completion of atom names,
    history mechanism and  a tracer that operates on single key-strokes.
    Interfaces  to  some  standard  editors are  provided  (and  can  be
    extended), as well as a facility to maintain programs (see make/0).

  o Very fast compiler
    Even  very  large applications  can  be loaded  in seconds  on  most
    machines.  If  this is not enough, there is a Quick Load Format that
    is slightly more compact and loading is almost always I/O bound.

  o Transparent compiled code
    SWI-Prolog  compiled code can be  treated just as interpreted  code:
    you  can list it, trace  it, etc.  This  implies you do not have  to
    decide  beforehand whether a module  should be loaded for  debugging
    or  not.  Also, performance  is much better than the  performance of
    most interpreters.

  o Profiling
    SWI-Prolog offers tools  for performance analysis, which can be very
    useful  to optimise  programs.   Unless you  are very familiar  with
    Prolog  and Prolog  performance  considerations this  might be  more
    helpful than a better compiler without these facilities.

  o Flexibility
    SWI-Prolog  can  easily   be  integrated  with  C,  supporting  non-
    determinism  in Prolog calling  C as well  as C calling Prolog  (see
    section  5.  It can  also be embedded embedded in external  programs
    (see  section 5.7).  System  predicates can be redefined locally  to
    provide compatibility with other Prolog systems.

  o Integration with XPCE
    SWI-Prolog   offers  a   tight  integration  to   the  Object   Ori-
    ented   Package  for   User  Interface   Development,  called   XPCE
    [Anjewierden & Wielemaker, 1989].    XPCE  allows you  to  implement
    graphical  user  interfaces  that are  source-code  compatible  over
    Unix/X11 and Win32 (Windows 95 and NT).


1.5 The XPCE GUI system for Prolog

The  XPCE GUI  system  for dynamically  typed  languages has  been  with
SWI-Prolog for  a long time.   It is  developed by Anjo Anjewierden  and
Jan  Wielemaker from  the department  of SWI,  University of  Amsterdam.
It  aims at  a  high-productive  development environment  for  graphical
applications based on Prolog.

Object  oriented  technology has  proven  to  be a  suitable  model  for
implementing GUIs, which  typically deal with things Prolog is  not very
good  at:   event-driven  control  and global  state.    With  XPCE,  we
designed  a system  that has  similar characteristics  that make  Prolog
such  a powerful  tool:   dynamic typing,  meta-programming and  dynamic
modification of the running system.

XPCE is  an object-system written  in the C-language.   It provides  for
the implementation of methods  in multiple languages.  New  XPCE classes
may be defined from Prolog using a simple, natural syntax.   The body of
the method is executed  by Prolog itself, providing a  natural interface
between the two systems.  Below is a very simple class definition.

:- pce_begin_class(prolog_lister, frame,
                   "List Prolog predicates").

initialise(Self) :->
        "As the C++ constructor"::
        send(Self, send_super, initialise, 'Prolog Lister'),
        send(Self, append, new(D, dialog)),
        send(D, append,
             text_item(predicate, message(Self, list, @arg1))),
        send(new(view), below, D).

list(Self, From:name) :->
        "List predicates from specification"::
        (   catch(term_to_atom(Term, From), _, fail)
        ->  get(Self, member, view, V),
            pce_open(V, write, Fd),
            set_output(Fd),
            listing(Term),
            close(Fd)
        ;   send(Self, report, error, 'Syntax error')
        ).

:- pce_end_class.

test :- send(new(prolog_lister), open).

Its  165  built-in   classes  deal  with  the  meta-environment,   data-
representation  and---of  course---graphics.     The   graphics  classes
concentrate on direct-manipulation of diagrammatic representations.

Availability. XPCE runs  on most  Unixtm platforms, Windows  95, 98  and
Windows NT. It has been connected to SWI-Prolog, SICStustm and Quintustm
Prolog as well  as some Lisp dialects and  C++.  The Quintus version  is
commercially distributed and supported as ProWindows-3tm.

Info. further        information        is         available        from
http://www.swi.psy.uva.nl/projects/xpce/   or   by   E-mail   to   xpce-
request@swi.psy.uva.nl.  There are demo versions for Windows  95, 98, NT
and i386/Linux available from the XPCE download page.


1.6 Release Notes

Collected release-notes.   This section  only contains some  highlights.
Smaller changes to especially  older releases have been removed.   For a
complete log, see the file ChangeLog from the distribution.


1.6.1 Version 1.8 Release Notes

Version  1.8 offers  a stack-shifter  to  provide dynamically  expanding
stacks  on machines  that  do  not offer  operating-system  support  for
implementing dynamic stacks.


1.6.2 Version 1.9 Release Notes

Version  1.9  offers  better portability  including  an  MS-Windows  3.1
version.  Changes to the Prolog system include:

  o Redefinition of system predicates
    Redefinition  of system  predicates  was allowed  silently in  older
    versions.    Version 1.9  only allows it  if the  new definition  is
    headed by a :- redefine_system_predicate/1 directive.

  o `Answer' reuse
    The  toplevel maintains  a table  of bindings  returned by  toplevel
    goals  and  allows for  reuse  of these  bindings by  prefixing  the
    variables with the $ sign.  See section 2.8.

  o Better source code administration
    Allows  for proper updating of multifile predicates and  finding the
    sources of individual clauses.


1.6.3 Version 2.0 Release Notes

New features offered:

  o 32-bit Virtual Machine
    Removes various limits and improves performance.

  o Inline foreign functions
    `Simple'  foreign predicates no  longer build a Prolog  stack-frame,
    but are directly  called from the VM. Notably provides a speedup for
    the test predicates such as var/1, etc.

  o Various compatibility improvements

  o Stream based I/O library
    All  SWI-Prolog's I/O is now  handled by the stream-package  defined
    in  the foreign include file SWI-Stream.h.   Physical I/O of  Prolog
    streams  may be  redefined through the  foreign language  interface,
    facilitating much simpler integration in window environments.


1.6.4 Version 2.5 Release Notes

Version  2.5  is  an  intermediate release  on  the  path  from  2.1  to
3.0.    All  changes are  to  the foreign-language  interface,  both  to
user- and system-predicates implemented  in the C-language.  The  aim is
twofold.   First of all  to make garbage-collection and  stack-expansion
(stack-shifts)  possible  while  foreign  code  is  active  without  the
C-programmer having  to worry  about locking  and unlocking  C-variables
pointing  to Prolog  terms.    The new  approach is  closely  compatible
to the  Quintus and  SICStus Prolog  foreign interface  using the  +term
argument specification (see their respective manuals).   This allows for
writing foreign  interfaces that  are easily portable  over these  three
Prolog platforms.

Apart from  various bug fixes  listed in the  Changelog file, these  are
the main changes since 2.1.0:

  o ISO compatibility
    Many   ISO  compatibility  features   have  been  added:     open/4,
    arithmetic functions, syntax, etc.

  o Win32
    Many  fixes for the Win32 (NT,  '95 and win32s) platforms.   Notably
    many  problems related  to pathnames  and a problem  in the  garbage
    collector.

  o Performance
    Many  changes to  the clause  indexing system:   added  hash-tables,
    lazy computation of the index information, etc.

  o Portable saved-states
    The   predicate  qsave_program/[1,2] allows  for   the  creating  of
    machine independent saved-states that load very quickly.


1.6.5 Version 2.6 Release Notes

Version 2.6  provides a stable implementation  of the features added  in
the 2.5.x  releases, but  at the same  time implements  a number of  new
features that may have impact on the system stability.

  o 32-bit integer and double float arithmetic
    The  biggest change is the  support for full 32-bit signed  integers
    and  raw  machine-format  double precision  floats.    The  internal
    data  representation as well as  the arithmetic instruction set  and
    interface to the arithmetic functions has been changed for this.

  o Embedding for Win32 applications
    The  Win32 version has been reorganised.   The Prolog kernel is  now
    implemented  as Win32  DLL that may  be embedded in  C-applications.
    Two front ends  are provided, one for window-based operation and one
    to run as a Win32 console application.

  o Creating stand-alone executables
    Version  2.6.0 can create  stand-alone executables by attaching  the
    saved-state to the emulator.  See qsave_program/2.


1.6.6 Version 2.7 Release Notes

Version 2.7  reorganises the  entire data-representation  of the  Prolog
data  itself.   The  aim is  to remove  most of  the  assumption on  the
machine's memory  layout to  improve portability in  general and  enable
embedding on  systems where the memory  layout may depend on  invocation
or on how the executable is linked.  The latter  is notably a problem on
the Win32 platforms.  Porting to 64-bit architectures is feasible now.

Furthermore, 2.7 lifts the  limits on arity of predicates and  number of
variables in  a clause considerably and  allow for further expansion  at
minimal cost.


1.6.7 Version 2.8 Release Notes

With version  2.8, we declare  the data-representation changes of  2.7.x
stable.   Version  2.8 exploits  the changes  of 2.7  to support  64-bit
processors like the DEC Alpha.  As of  version 2.8.5, the representation
of  recorded  terms  has  changed,  and  terms   on  the  heap  are  now
represented  in a  compiled format.    SWI-Prolog no  longer limits  the
use of malloc()  or uses assumptions on  the addresses returned by  this
function.


1.6.8 Version 2.9 Release Notes

Version  2.9  is  the next  step  towards  version  3.0,  improving  ISO
compliance  and introducing  ISO  compliant  exception handling.     New
are catch/3, throw/1, abolish/1, write_term/[2,3], write_canonical/[1,2]
and  the C-functions  PL_exception() and  PL_throw().    The  predicates
display/[1,2] and displayq/[1,2]  have been moved to  library(backcomp),
so old code referring to them will autoload them.

The interface  to PL_open_query() has changed.    The debug argument  is
replaced  by a  bitwise or'ed  flags argument.    The  values FALSE  and
TRUE have their familiar meaning, making old code  using these constants
compatible.   Non-zero values  other than TRUE  (1) will be  interpreted
different.


1.6.9 Version 3.0 Release Notes

Complete  redesign   of  the   saved-state  mechanism,   providing   the
possibility of  `program resources'.   See  resource/3, open_resource/3,
and qsave_program/[1,2].


1.6.10 Version 3.1 Release Notes

Improvements   on  exception-handling.       Allows  relating   software
interrupts (signals)  to exceptions,  handling signals in  Prolog and  C
(see on_signal/3  and PL_signal()).   Prolog stack  overflows now  raise
the resource_error  exception and thus  can be handled  in Prolog  using
catch/3.


1.6.11 Version 3.3 Release Notes

Version 3.3  is a  major release,  changing many  things internally  and
externally.   The highlights are a  complete redesign of the  high-level
I/O system, which is  now based on explicit streams rather  then current
input/output.  The  old Edinburgh predicates (see/1, tell/1, etc.)   are
now defined on top of this layer instead of the other  way around.  This
fixes various internal problems and removes Prolog limits  on the number
of streams.

Much  progress  has been  made  to  improve ISO  compliance:    handling
strings  as  lists  of  one-character  atoms  is   now  supported  (next
to  character   codes  as  integers).      Many  more  exceptions   have
been  added and  printing  of exceptions  and messages  is  rationalised
using   Quintus   and   SICStus   Prolog   compatible   print_message/2,
message_hook/3 and print_message_lines/3.   All predicates descriped  in
[Deransart et al., 1996] are now implemented.

As of version  3.3, SWI-Prolog adheres the  ISO logical update view  for
dynamic predicates.  See section 3.13.1 for details.

SWI-Prolog  3.3  includes garbage  collection  on  atoms,  removing  the
last serious memory  leak especially in text-manipulation  applications.
See  section 5.6.2.1.    In addition,  both the  user-level and  foreign
interface supports atoms holding 0-bytes.

Finally, an  alpha version of a  multi-threaded SWI-Prolog for Linux  is
added.    This version  is still  much slower  than the  single-threaded
version due  to frequent access to  `thread-local-data' as well as  some
too detailed  mutex locks.   The basic thread  API is ready for  serious
use and testing however.  See section 3.39.


1.6.11.1 Incompatible changes

A number  of incompatible changes  result from this upgrade.   They  are
all easily fixed however.

  o !/0, call/1
    The  cut now behaves  according to the ISO  standard.  This  implies
    it  works in compound  goals passed  to call/1 and  is local to  the
    condition part of if-then-else as well as the argument of \+/1.

  o atom_chars/2
    This  predicate  is now  ISO  compliant and  thus generates  a  list
    of  one-character atoms.    The behaviour  of the  old predicate  is
    available  in the  ---also ISO compliant---  atom_codes/2 predicate.
    Safest  repair is a  replacement of all  atom_chars  into atom_codes.
    If you do not want to change any souce-code, you might want to use

    user:goal_expansion(atom_chars(A,B), atom_codes(A,B)).

  o number_chars/2
    Same applies for number_chars/2 and number_codes/2.

  o feature/2, set_feature/2
    These  are  replaced  by  the  ISO  compliant  current_prolog_flag/2
    and  set_prolog_flag/2.    The  library  library(backcomp)  provides
    definitions for  feature/2 and set_feature/2, so no  source has to be
    updated.

  o Accessing command-line arguments
    This  used  to   be  provided  by  the  undocumented  '$argv'/1  and
    Quintus  compatible library unix/1.   Now  there is also  documented
    current_prolog_flag(argv, Argv).

  o dup_stream/2
    Has  been deleted.   New  stream-aliases can deal  with most of  the
    problems  for which  dup_stream/2 was  designed and  dup/2 from  the
    clib package can with most others.

  o op/3
    Operators  are now local to modules.  This implies  any modification
    of  the operator-table does  not influence other  modules.  This  is
    consistent  with the proposed ISO behaviour and a necessity  to have
    any usable handling of operators in a multi-threaded environment.

  o set_prolog_flag(character_escapes, Bool)
    This  prolog flag is now an interface to changing attributes  on the
    current source-module,  effectively making this flag module-local as
    well.   This is required for consistent handling of  sources written
    with  ISO (obligatory) character-escape sequences together  with old
    Edinburgh code.

  o current_stream/3 and stream_position
    These predicates have been moved to library(quintus).


1.6.12 Version 3.4 Release Notes

The  3.4 release  is  a consolidation  release.    It  consolidates  the
improvements  and  standard conformance  of  the  3.3 releases.     This
version  is closely  compatible  with the  3.3  version except  for  one
important change:

  o Argument order in select/3
    The  list-processing  predicate select/3  somehow  got into  a  very
    early  version of SWI-Prolog  with the wrong  argument order.   This
    has been fixed in  3.4.0.  The correct order is select(?Elem, ?List,
    ?Rest).

    As  select/3  has  no  error  conditions,  runtime  checking  cannot
    be   done.       To   simplify   debugging,   the   library   module
    library(checkselect)  will  print  references to  select/3  in  your
    source  code  and  install  a  version of  select  that  enters  the
    debugger if select is called and the second argument is not a list.

    This   library   can   be   loaded   explicitely   or   by   calling
    check_old_select/0.


1.6.13 Version 4.0 Release Notes

As of  version 4.0 the  standard distribution  of SWI-Prolog is  bundled
with a  number of its  popular extension packages,  among which the  now
open source XPCE GUI toolkit (see section 1.5).   No significant changes
have been made to the basic SWI-Prolog engine.

Some useful tricks in the integrated environment:

  o Register the GUI tracer
    Using  a call to guitracer/0,  hooks are installed that replace  the
    normal command-line driven tracer with a graphical forntend.

  o Register PceEmacs for editing files
    From    your   initialisation   file.        you   can    load   li-
    brary(emacs/swi_prolog)  that  cause  edit/1  to  use  the  built-in
    PceEmacs editor.


1.7 Acknowledgements

Some  small  parts  of  the  Prolog  code  of  SWI-Prolog  are  modified
versions of  the corresponding  Edinburgh C-Prolog code:   grammar  rule
compilation  and writef/2.    Also some  of the  C-code originates  from
C-Prolog:  finding the path of the currently running  executable and the
code underlying  absolute_file_name/2.   Ideas  on programming style  and
techniques originate from  C-Prolog and Richard O'Keefe's thief  editor.
An  important  source  of inspiration  are  the  programming  techniques
introduced by Anjo Anjewierden in PCE version 1 and 2.

I also  would like to thank  those who had the  fade of using the  early
versions of this system,  suggested extensions or reported bugs.   Among
them are Anjo Anjewierden, Huub Knops, Bob  Wielinga, Wouter Jansweijer,
Luc Peerdeman, Eric Nombden, Frank van Harmelen, Bert Rengel.

Martin Jansche  (jansche@novell1.gs.uni-heidelberg.de) has been so  kind
to reorganise the sources for version 2.1.3 of this manual.

Horst  von Brand  has been  so  kind to  fix many  typos  in the  2.7.14
manual.  Thanks!


Chapter 2.  OVERVIEW


2.1 Getting started quickly


2.1.1 Starting SWI-Prolog


2.1.1.1 Starting SWI-Prolog on Unix

By default, SWI-Prolog is installed as `pl',  though some administrators
call  it  `swipl'  or `swi-prolog'.     The  command-line  arguments  of
SWI-Prolog  itself  and  its  utility  programs   are  documented  using
standard  Unix  man pages.     SWI-Prolog  is normally  operated  as  an
interactive application simply by starting the program:

machine% pl
% /staff/jan/.plrc compiled 0.00 sec, 1,260 bytes
Welcome to SWI-Prolog (Version 4.0.3)
Copyright (c) 1990-2000 University of Amsterdam.
Copy policy: GPL-2 (see www.gnu.org)

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?-

After  starting Prolog,  one  normally loads  a  program into  it  using
consult/1,  which---for  historical  reasons---may   be  abbreviated  by
putting the  name of  the program  file between  square brackets.    The
following  goal loads  the  file  likes.pl containing  clauses  for  the
predicates likes/2:

?- [likes].
% likes compiled, 0.00 sec, 596 bytes.

Yes
?-

After this point, Unix and Windows users are united again.


2.1.1.2 Starting SWI-Prolog on Windows

After SWI-Prolog has been  installed on a Windows system,  the following
important new things are available to the user:

  o A  folder  (called  directory in  the  remainder of  this  document)
    called  pl  containing the  executables,  libraries, etc.    of  the
    system.  No files are installed outside this directory.

  o A  program  plwin.exe,  providing  a  window  for  interaction  with
    Prolog.  The  program plcon.exe is a version of SWI-Prolog that runs
    in a DOS-box.

  o The  file-extension .pl  is associated with  the program  plwin.exe.
    Opening  a .pl file will cause plwin.exe to start,  change directory
    to  the directory in  which the file-to-open  resides and load  this
    file.

The  normal   way  to  start  with   the  likes.pl  file  mentioned   in
section 2.1.1.1  is by simply double-clicking  this file in the  Windows
explorer.


2.1.2 Executing a query

After loading a program,  one can ask Prolog queries about  the program.
The query below  asks Prolog to prove  whether `john' likes someone  and
who is  liked by  `john'.   The system  responds with X = <value> if  it
can prove  the goal for a  certain X. The  user can type the  semi-colon
(;) if  (s)he wants another solution,  or return if (s)he is  satisfied,
after which Prolog will say Yes.  If Prolog answers  No, it indicates it
cannot find any more  answers to the query.  Finally, Prolog  can answer
using an  error message  to indicate  the query or  program contains  an
error.

?- likes(john, X).

X = mary


2.2 The user's initialisation file

After  the necessary  system  initialisation  the system  consults  (see
consult/1) the user's startup file.  The base-name  of this file follows
conventions of  the operating  system.   On MS-Windows,  it is the  file
pl.ini  and on  Unix systems  .plrc.   The  file is  searched using  the
file_search_path/2 clauses for user_profile.  The table below  shows the
default value for this search-path.

     _______________________________________________________________
     |________|Unix_|Windows________________________________________|
     | local  |.    |.                                              |

     | home   |~    |%HOME% or %HOMEDRIVE%\%HOMEPATH%               |
     |_global_|____|SWI-Home_directory_or_%WINDIR%_or_%SYSTEMROOT%_|_

After the  first startup  file is found  it is  loaded and Prolog  stops
looking for further startup files.  The name of the  startup file can be
changed with the  `-f file' option.   If File denotes an absolute  path,
this file is loaded,  otherwise the file is searched for using  the same
conventions as for the default startup file.  Finally,  if file is none,
no file is loaded.


2.3 Initialisation files and goals

Using  commandline  arguments  (see  section  2.4),  SWI-Prolog  can  be
forced to  load files  and execute queries  for initialisation  purposes
or  non-interactive operation.    The  most  commonly used  options  are
-f file or  -s file to  make Prolog load  a file,  -g goal to define  an
initialisation  goal and  -t goal to  define  the toplevel  goal.    The
following  is a  typical example  for starting  an application  directly
from the commandline.

machine% pl -f load.pl -g go -t halt

It tells  SWI-Prolog to load  load.pl, start  the application using  the
entry-point go/0 and ---instead of entering  the interactive toplevel---
exit  after completing  go/0.    The  -q  may  be used  to  supress  all
informational messages.

In  MS-Windows,  the  same  can  be  achieved  using  a  short-cut  with
appropriately  defined  commandline   arguments.     A  typically   seen
alternative  is to  write  a file  run.pl  with content  as  illustrated
below.  Double-clicking run.pl will start the application.

:- [load].                      % load program
:- go.                          % run it
:- halt.                        % and exit

Section  2.10.2.1 discusses  further  scripting  options and  chapter  6
discusses the  generation of runtime executables.   Runtime  executables
are  a mean  to  deliver executables  that  do  not require  the  Prolog
system.


2.4 Command line options

The full set of command line options is given below:

-help
    When  given as  the only  option, it summarises  the most  important
    options.

-v
    When  given as the  only option, it  summarises the version and  the
    architecture identifier.

-arch
    When   given  as  the  only  option,  it  prints   the  architecture
    identifier  (see  current_prolog_flag(arch,  Arch)) and  exits.    See
    also -dump-runtime-variables.

-dump-runtime-variables
    When  given as  the only option,  it prints  a sequence of  variable
    settings  that can  be  used in  shell-scripts to  deal with  Prolog
    parameters.   This feature is  also used by plld (see section  5.7).
    Below is a typical example of using this feature.

    eval `pl -dump-runtime-variables`
    cc -I$PLBASE/include -L$PLBASE/runtime/$PLARCH ...

-q
    Set the prolog-flag  verbose to silent, supressing informational and
    banner messages.

-Lsize[km]
    Give  local stack limit (2 Mbytes default).   Note that there  is no
    space  between the size option  and its argument.   By default,  the
    argument  is interpreted in  Kbytes.   Postfixing the argument  with
    m  causes the argument to be  interpreted in Mbytes.  The  following
    example specifies 32 Mbytes local stack.

    % pl -L32m

    A maximum is  useful to stop buggy programs from claiming all memory
    resources.   -L0 sets the limit to the highest possible value.   See
    section 2.16.

-Gsize[km]
    Give  global  stack limit  (4 Mbytes  default).    See -L  for  more
    details.

-Tsize[km]
    Give  trail  stack  limit  (4  Mbytes  default).     This  limit  is
    relatively  high because trail-stack overflows are not  often caused
    by program bugs.  See -L for more details.

-Asize[km]
    Give  argument stack limit (1 Mbytes  default).  The argument  stack
    limits  the  maximum  nesting of  terms  that  can be  compiled  and
    executed.    SWI-Prolog does `last-argument  optimisation' to  avoid
    many  deeply nested  structure  using this  stack.   Enlarging  this
    limit is only necessary in extreme cases.  See -L for more details.

-c file ...
    Compile files into an `intermediate code file'.  See section 2.10.

-o output
    Used  in combination  with -c  or -b  to determine  output file  for
    compilation.

-O
    Optimised compilation.  See current_prolog_flag/2.

-s file
    Use  file as a  script-file.   The script file  is loaded after  the
    initialisation  file  specified with  the -f file  option.    Unlike
    -f file, using

-s d
    oes not stop Prolog from loaded the personal initialisation file.

-f file
    Use file as  initialisation file instead of the default .plrc (Unix)
    or pl.ini (Windows).   `-f none' stops SWI-Prolog from searching for
    a  startup file.    This option  can be  used as  an alternative  to
    -s file  that stops Prolog from loading the  personal initialisation
    file.  See also section 2.2.

-F script
    Selects  a startup-script from the  SWI-Prolog home directory.   The
    script-file  is  named <script>.rc.    The  default script  name  is
    deduced  from  the  executable,  taking the  leading  alphanumerical
    characters  (letters, digits and underscore) from  the program-name.
    -F none stops looking  for a script.  Intended for simple management
    of  slightly  different  versions.    One  could for  example  write
    a  script  iso.rc  and  then select  ISO  compatibility  mode  using
    pl -F iso or make a link from iso-pl to pl.

-g goal
    Goal  is executed just before entering the top level.  Default  is a
    predicate  which prints the  welcome message.   The welcome  message
    can  thus be suppressed by  giving -g true.   goal can be a  complex
    term.   In this case quotes  are normally needed to protect  it from
    being expanded by the Unix shell.

-t goal
    Use  goal  as  interactive  toplevel instead  of  the  default  goal
    prolog/0.    goal  can be  a complex  term.   If  the toplevel  goal
    succeeds  SWI-Prolog exits  with status  0.   If it  fails the  exit
    status  is  1.    This  flag also  determines  the goal  started  by
    break/0  and abort/0.   If you want to  stop the user from  entering
    interactive  mode  start the  application  with `-g goal'  and  give
    `halt' as toplevel.

-tty
    Unix  only.      Switches  controlling  the  terminal  for  allowing
    single-character  commands to the tracer and  get_single_char/1.   By
    default  manipulating  the terminal  is  enabled unless  the  system
    detects  it is not  connected to a  terminal or it  is running as  a
    GNU-Emacs  inferior process.   This flag  is sometimes required  for
    smooth interaction with other applications.

-x bootfile
    Boot  from bootfile instead  of the system's default  boot file.   A
    bootfile is a  file resulting from a Prolog compilation using the -b
    or -c option or a program saved using qsave_program/[1,2].

-p alias=path1[:path2 ...]
    Define  a path  alias for  file_search_path.    alias is  the name  of
    the  alias, path1  ...   is  a : separated  list of  values for  the
    alias.    A  value is  either a  term of  the  form alias(value)  or
    pathname.    The  computed aliases  are added  to file_search_path/2
    using  asserta/1, so they precede  predefined values for the  alias.
    See  file_search_path/2  for details  on  using  this  file-location
    mechanism.

--
    Stops  scanning for more  arguments, so you  can pass arguments  for
    your  application after this  one.   See current_prolog_flag/2 using
    the flag argv for obtaining the commandline arguments.

The following options  are for system maintenance.   They are given  for
reference only.

-b initfile ...-c file ...
    Boot  compilation.   initfile  ...   are compiled  by the  C-written
    bootstrap  compiler,  file  ...    by  the normal  Prolog  compiler.
    System maintenance only.

-d level
    Set  debug  level to  level.    Only  has effect  if the  system  is
    compiled with the -DO_DEBUG flag.  System maintenance only.


2.5 GNU Emacs Interface

The default  Prolog mode for  GNU-Emacs can be  activated by adding  the
following rules to your Emacs initialisation file:

(setq auto-mode-alist
      (append
       '(("\\.pl" . prolog-mode))
       auto-mode-alist))
(setq prolog-program-name "pl")
(setq prolog-consult-string "[user].\n")
;If you want this.  Indentation is either poor or I don't use
;it as intended.
;(setq prolog-indent-width 8)

Unfortunately the  default Prolog mode  of GNU-Emacs  is not very  good.
An  alternative  prolog.el  file for  GNU-Emacs  20  is  available  from
http://www.freesoft.cz/ pdm/software/emacs/prolog-mode/  and  for   GNU-
Emacs 19 from http://w1.858.telia.com/ u85810764/Prolog-mode/index.html


2.6 Online Help

Online  help  provides a  fast  lookup  and browsing  facility  to  this
manual.   The online  manual can show predicate  definitions as well  as
entire sections of the manual.

The online help  is displayed from the file library('MANUAL').  The file
library(helpidx) provides  an index into this  file.   library('MANUAL')
is created  from the LaTeX  sources with a  modified version of  dvitty,
using overstrike  for printing bold text  and underlining for  rendering
italic  text.    XPCE  is  shipped  with  library(swi_help),  presenting
the  information from  the  online help  in a  hypertext  window.    The
prolog-flag  write_help_with_overstrike controls  whether  or not  help/1
writes  its output  using  overstrike  to realise  bold  and  underlined
output or  not.   If this prolog-flag  is not set  it is initialised  by
the help  library to true  if the TERM variable  equals xterm and  false
otherwise.   If  this default does  not satisfy you,  add the  following
line to your personal startup file (see section 2.2):

:- set_prolog_flag(write_help_with_overstrike, true).


help
    Equivalent to help(help/1).


help(+What)
    Show specified part of the manual.  What is one of:

          <Name>/<Arity> Give help on specified predicate
          <Name>         Give  help on  named  predicate with  any
                         arity or  C interface function  with that
                         name
          <Section>      Display  specified  section.      Section
                         numbers are dash-separated  numbers:  2-3
                         refers  to  section 2.3  of  the  manual.

                         Section   numbers  are   obtained   using
                         apropos/1.

    Examples:

       ?- help(assert).     Give help on predicate assert
       ?- help(3-4).        Display section 3.4 of the manual
       ?- help('PL_retry'). Give    help   on    interface   function
                            PL_retry()

    See   also   apropos/1,    and   the   SWI-Prolog   home   page   at
    http://www.swi.psy.uva.nl/projects/SWI-Prolog/,   which  provides  a
    FAQ, an HTML version  of manual for online browsing and HTML and PDF
    versions for downloading.


apropos(+Pattern)
    Display all predicates,  functions and sections that have Pattern in
    their  name or summary  description.   Lowercase letters in  Pattern
    also match a corresponding uppercase letter.  Example:

        ?- apropos(file).  Display  predicates,  functions and  sec-
                           tions that have  `file' (or `File', etc.)
                           in their summary description.


explain(+ToExplain)
    Give an explanation on  the given `object'.  The argument may be any
    Prolog data object.   If the argument is an atom, a term of the form
    Name/Arity  or a term  of the  form Module:Name/Arity, explain  will
    try to explain the predicate as well as possible references to it.


explain(+ToExplain, -Explanation)
    Unify  Explanation with an explanation for ToExplain.   Backtracking
    yields further explanations.


2.7 Query Substitutions

SWI-Prolog  offers a  query substitution  mechanism similar  to that  of
Unix csh (csh(1)), called  `history'.  The availability of  this feature
is controlled by  set_prolog_flag/2, using  the history prolog-flag.   By
default,  history is  available if  the prolog-flag  readline is  false.
To  enable this  feature,  remembering the  last  50 commands,  put  the
following into your startup file (see section 2.2:

:- set_prolog_flag(history, 50).

The history  system allows the  user to compose  new queries from  those
typed before and  remembered by the system.   It also allows to  correct
queries and  syntax errors.    SWI-Prolog does  not offer  the Unix  csh
capabilities to  include arguments.   This is omitted  as it is  unclear
how the first, second, etc. argument should be defined.

The available history commands are shown in table 2.1.
       __________________________________________________________
       | !!.            |Repeat last query                        |

       | !nr.           |Repeat query numbered <nr>               |
       | !str.          |Repeat last query starting with <str>    |
       | !?str.         |Repeat last query holding <str>          |
       | ^old^new.      |Substitute <old> into <new> in last query |
       | !nr^old^new.   |Substitute in query numbered <nr>        |
       | !str^old^new.  |Substitute in query starting with <str>  |
       | !?str^old^new. |Substitute in query holding <str>        |
       | h.             |Show history list                        |

       |_!h.____________|Show_this_list__________________________ |

                      Table 2.1:  History commands


2.7.1 Limitations of the History System

History expansion  is executed  after raw-reading.   This  is the  first
stage of read_term/2 and friends,  reading the term into a  string while
deleting comment  and canonising blank.   This makes  it hard to use  it
for correcting  syntax errors.   Command-line editing as provided  using
the GNU-readline library is  more suitable for this.   History expansion
is first  of all useful  for executing or  combining commands from  long
ago.


2.8 Reuse of toplevel bindings

Bindings resulting from the successful execution of a  toplevel goal are
asserted in a database.  These values may be  reused in further toplevel
queries as $Var.  Only the latest binding is available.  Example:
1 ?- maplist(plus(1), "hello", X).

X = [105,102,109,109,112]

Yes
2 ?- format('~s~n', [$X]).
ifmmp

Yes

3 ?-

                 Figure 2.1:  Reusing toplevel bindings

Note that variables may be set by executing =/2:

6 ?- X = statistics.

X = statistics

Yes
7 ?- $X.
28.00 seconds cpu time for 183,128 inferences
4,016 atoms, 1,904 functors, 2,042 predicates, 52 modules
55,915 byte codes; 11,239 external references

                      Limit    Allocated       In use
Heap         :                                624,820 Bytes
Local  stack :    2,048,000        8,192          404 Bytes
Global stack :    4,096,000       16,384          968 Bytes
Trail  stack :    4,096,000        8,192          432 Bytes

Yes
8 ?-


2.9 Overview of the Debugger

SWI-Prolog has  a 6-port  tracer, extending the  standard 4-port  tracer
[Clocksin & Melish, 1987]  with two  additional  ports.    The  optional
unify port allows  the user to inspect  the result after unification  of
the head.  The exception port shows exceptions raised  by throw/1 or one
of the built-in predicates.  See section 3.9.

The standard  ports are called call,  exit, redo, fail  and unify.   The
tracer is started  by the trace/0 command,  when a spy point is  reached
and the system is  in debugging mode (see spy/1 and debug/0) or  when an
exception is raised.

The interactive  toplevel goal trace/0 means  ``trace the next  query''.
The  tracer shows  the  port,  displaying  the port  name,  the  current
depth of  the recursion and  the goal.   The goal  is printed using  the
Prolog predicate write_term/2.  The style is defined  by the prolog-flag
debugger_print_options  and can be modified using this flag or  using the
w, p and d commands of the tracer.
1 ?- visible(+all), leash(-exit).

Yes
2 ?- trace, min([3, 2], X).
  Call:  ( 3) min([3, 2], G235) ? creep
  Unify: ( 3) min([3, 2], G235)
  Call:  ( 4) min([2], G244) ? creep
  Unify: ( 4) min([2], 2)
  Exit:  ( 4) min([2], 2)

  Call:  ( 4) min(3, 2, G235) ? creep
  Unify: ( 4) min(3, 2, G235)
  Call:  ( 5) 3 < 2 ? creep
  Fail:  ( 5) 3 < 2 ? creep
  Redo:  ( 4) min(3, 2, G235) ? creep
  Exit:  ( 4) min(3, 2, 2)
  Exit:  ( 3) min([3, 2], 2)

Yes
[trace] 3 ?-

                       Figure 2.2:  Example trace

On leashed  ports (set  with the  predicate leash/1,  default are  call,
exit, redo and  fail) the user is prompted  for an action.  All  actions
are single character  commands which are executed without waiting  for a
return, unless the command line option -tty is active.  Tracer options:

+ (Spy)
    Set a spy point (see spy/1) on the current predicate.

- (No spy)
    Remove the spy point (see nospy/1) from the current predicate.

/ (Find)
    Search  for a port.   After the  `/', the user  can enter a line  to
    specify  the port to  search for.   This line consists  of a set  of
    letters  indicating the  port type,  followed by  an optional  term,
    that  should unify with  the goal run by  the port.   If no term  is
    specified  it is taken as a variable, searching for any port  of the
    specified  type.  If an atom is given, any goal whose  functor has a
    name equal to that atom matches.  Examples:

            /f               Search for any fail port

            /fe solve        Search for a  fail or exit  port of
                             any goal with name solve
            /c solve(a, _)   Search for a call to  solve/2 whose
                             first argument is a variable or the
                             atom a
            /a member(_, _)  Search for  any  port on  member/2.
                             This is equivalent to setting a spy
                             point on member/2.

. (Repeat find)
    Repeat the last find command (see `/').

A (Alternatives)
    Show all goals that have alternatives.

C (Context)
    Toggle  `Show Context'.   If on  the context module  of the goal  is
    displayed between square brackets (see section 4).  Default is off.

L (Listing)
    List the current predicate with listing/1.

a (Abort)
    Abort Prolog execution (see abort/0).

b (Break)
    Enter a Prolog break environment (see break/0).

c (Creep)
    Continue execution, stop at next port.  (Also return, space).

d (Display)
    Set  the max_depth(Depth) option of debugger_print_options,  limiting
    the  depth  to which  terms are  printed.    See also  the  w and  p
    options.

e (Exit)
    Terminate Prolog (see halt/0).

f (Fail)
    Force failure of the current goal.

g (Goals)
    Show the list of  parent goals (the execution stack).  Note that due
    to  tail recursion optimization a  number of parent goals might  not
    exist any more.

h (Help)
    Show available options (also `?').

i (Ignore)
    Ignore the current goal, pretending it succeeded.

l (Leap)
    Continue execution, stop at next spy point.

n (No debug)
    Continue execution in `no debug' mode.

p (Print)
    Set       the       prolog-flag       debugger_print_options       to
    [quoted(true), portray(true), max_depth(10)].        This   is   the
    default.

r (Retry)
    Undo  all actions (except for database and i/o actions) back  to the
    call  port of  the current  goal and  resume execution  at the  call
    port.

s (Skip)
    Continue  execution,  stop  at the  next  port  of this  goal  (thus
    skipping all calls to children of this goal).

u (Up)
    Continue  execution, stop at the next port of the parent  goal (thus
    skipping  this goal and all calls to  children of this goal).   This
    option is useful to stop tracing a failure driven loop.

w (Write)
    Set   the  prolog-flag   debugger_print_options  to   [quoted(true)],
    bypassing portray/1, etc.

The   ideal  4   port  model   as  described   in   many  Prolog   books
[Clocksin & Melish, 1987] is not visible in many  Prolog implementations
because code optimisation  removes part of the choice- and  exit-points.
Backtrack  points   are  not   shown  if  either   the  goal   succeeded
deterministically or its alternatives were removed using the cut.   When
running in  debug mode (debug/0) choice  points are only destroyed  when
removed by  the cut.    In debug  mode, tail  recursion optimisation  is
switched off.

Reference information to  all predicates available for manipulating  the
debugger is in section 3.42.


2.10 Compilation


2.10.1 During program development

During  program   development,  programs   are  normally  loaded   using
consult/1, or the list abbreviation.  It is  common practice to organise
a project  as a  collection of source-files  and a  load-file, a  Prolog
file  containing only  use_module/[1,2]  or ensure_loaded/1  directives,
possibly  with a  definition  of the  entry-point  of the  program,  the
predicate that  is normally used  to start  the program.   This file  is
often  called load.pl.    If the  entry-point is  called go,  a  typical
session starts as:

% pl
<banner>

1 ?- [load].
<compilation messages>

Yes
2 ?- go.
<program interaction>

When  using  Windows,  the  user  may  open  load.pl  from  the  Windows
explorer,  which will cause  plwin.exe to  be started  in the  directory
holding load.pl.  Prolog loads load.pl before entering the toplevel.


2.10.2 For running the result

There are  various options if  you want to make  your program ready  for
real usage.   The best  choice depends on whether  the program is to  be
used only  on machines  holding the SWI-Prolog  development system,  the
size of the program and the operating system (Unix vs. Windows).


2.10.2.1 Using PrologScript

New in  version 4.0.5  is the possibility  to use  a Prolog source  file
directly  as a  Unix  script-file.   the  same  mechanism is  useful  to
specify additional parameters for running a Prolog file on Windows.

If the first  letter of a Prolog file  is #, the first line is  threated
as comment.  To  create a Prolog script, make the first line  start like
this:

    #!/path/to/pl <options> -s

Prolog recognises this  starting sequence and causes the interpreter  to
receive the following argument-list:

    /path/to/pl <options> -s <script> -- <ScriptArguments>

Instead of  -s, the user may  use -f to stop  Prolog from looking for  a
personal initialisation file.

Here is a simple script doing expression evaluation:

#!/usr/bin/pl -q -t main -f

eval :-
        current_prolog_flag(argv, Argv),
        append(_, [--|Args], Argv),
        concat_atom(Args, ' ', SingleArg),
        term_to_atom(Term, SingleArg),
        Val is Term,
        format('~w~n', [Val]).

main :-
        catch(eval, E, (print_message(error, E), fail)),
        halt.
main :-
        halt(1).

And here are two example runs:

% eval 1+2
3
% eval foo
ERROR: Arithmetic: `foo/0' is not a function
%

The  Windows  version supports  the  #!  construct  too,   but  here  it
serves  a rather  different role.    The  Windows shell  already  allows
the  user to  start  Prolog source-files  directly through  the  Windows
file-type  association.   Windows  however makes  it rather  complicated
to  provide  additional parameters,  such  as  the  required  stack-size
for  an  individual Prolog  file.     The #!  line  provides  for  this,
providing a  more flexible approach then  changing the global  defaults.
The  following starts  Prolog  with unlimited  stack-size on  the  given
source-file:

#!/usr/bin/pl -L0 -T0 -G0 -s

....

Note the  use of  /usr/bin/pl, which  specifies the interpreter.    This
argument is ignored in the Windows version, but  required to ensure best
cross-platform compatibility.


2.10.2.2 Creating a shell-script

With  the introduction  of PrologScript  (see section  2.10.2.1),  using
shell-scripts  as explained  in this  section has  become redundant  for
most applications.

Especially  on  Unix systems  and  not-too-large  applications,  writing
a  shell-script  that  simply  loads  your  application  and  calls  the
entry-point is often a good choice.  A skeleton for  the script is given
below, followed by the Prolog code to obtain the program arguments.

#!/bin/sh

base=<absolute-path-to-source>
PL=pl

exec $PL -f none -g "load_files(['$base/load'],[silent(true)])" \
         -t go -- $*

go :-
        current_prolog_flag(argv, Arguments),
        append(_SytemArgs, [--|Args], Arguments), !,
        go(Args).

go(Args) :-
        ...

On Windows  systems, similar  behaviour can  be achieved  by creating  a
shortcut to Prolog, passing the proper options or writing a .bat file.


2.10.2.3 Creating a saved-state

For larger programs,  as well as for  programs that are required run  on
systems that  do not have the  SWI-Prolog development system  installed,
creating a saved state is  the best solution.  A saved state  is created
using qsave_program/[1,2] or  using the linker plld(1).   A saved  state
is a file  containing machine-independent intermediate code in a  format
dedicated for fast loading.  Optionally, the  emulator may be integrated
in  the saved  state,  creating  a single-file,  but  machine-dependent,
executable.  This process is described in chapter 6.


2.10.2.4 Compilation using the -c commandline option

This mechanism loads a series of Prolog source files  and then creates a
saved-state as qsave_program/2 does.  The command syntax is:

% pl [option ...] [-o output] -c file ...

The  options argument  are  options to  qsave_program/2 written  in  the
format below.    The option-names  and their values  are described  with
qsave_program/2.

    --option-name=option-value

For example, to create a stan-alone executable that  starts by executing
main/0  and for  which the  source is  loaded through  load.pl, use  the
command

% pl --goal=main --stand_alone=true -o myprog -c load.pl

This performs exactly the same as executing

% pl
<banner>
?- [load].
?- qsave_program(myprog,
                 [ goal(main),
                   stand_alone(true)
                 ]).
?- halt.


2.11 Environment Control (Prolog flags)

The  predicates  current_prolog_flag/2 and  set_prolog_flag/2 allow  the
user  to examine  and modify  the execution  environment.   It  provides
access  to whether  optional  features are  available on  this  version,
operating  system,  foreign-code  environment,  command-line  arguments,
version,  as well  as runtime  flags to  control  the runtime  behaviour
of  certain  predicates  to  achieve  compatibility  with  other  Prolog
environments.


current_prolog_flag(?Key, -Value)
    The   predicate  current_prolog_flag/2  defines   an  interface   to
    installation   features:    options  compiled  in,  version,   home,
    etc.    With both arguments  unbound, it  will generate all  defined
    prolog-flags.    With  the  `Key' instantiated  it unify  the  value
    of  the  prolog-flag.    Features  come  in three  types:    boolean
    prolog-flags,  prolog-flags  with  an  atom value  and  prolog-flags
    with  an integer  value.    A boolean  prolog-flag is  true iff  the
    prolog-flag  is present and the Value  is the atom true.   Currently
    defined keys:

    arch (atom)
         Identifier for the hardware and operating system  SWI-Prolog is
         running on.  Used  to determine the startup file as well  as to
         select foreign  files for  the right  architecture.   See  also
         section 5.4.

    version (integer)
         The version identifier is an integer with value:

                          10000*Major+ 100*Minor+Patch

         Note that  in  releases upto  2.7.10 this  prolog-flag  yielded
         an  atom   holding  the  three   numbers  separated  by   dots.
         The  current representation  is  much easier  for  implementing
         version-conditional statements.

    home (atom)
         SWI-Prolog's  notion  of   the  home-directory.      SWI-Prolog
         uses  it's  home  directory   to  find  its  startup  file   as
         <home>/startup/startup.<arch> and  to   find  its  library   as
         <home>/library.

    executable (atom)
         Path-name of the  running executable.  Used  by qsave_program/2
         as default emulator.

    argv (list)
         List  is  a   list  of  atoms  representing  the   command-line
         arguments used  to invoke  SWI-Prolog.   Please  note that  all
         arguments are included in the list returned.

    pipe (bool, changeable)
         If  true,  open(pipe(command), mode, Stream),   etc.  are  sup-
         ported.    Can  be  changed to  disable  the use  of  pipes  in
         applications testing this feature.  Not recommended.

    open_shared_object (bool)
         If  true,  open_shared_object/2 and  friends  are  implemented,
         providing access  to shared  libraries (.so  files) or  dynamic
         link libraries (.DLL files).

    shared_object_extension (atom)
         Extension used  by  the operating  system for  shared  objects.
         so  for most  Unix systems  and  dll for  Windows.    Used  for
         locating  files using  the  file_type  executable.    See  also
         absolute_file_name/3.

    dynamic_stacks (bool)
         If true,  the system uses some  form of `sparse-memory  manage-
         ment' to realise the stacks.  If  false, malloc()/realloc() are
         used for  the stacks.   In earlier  days this had  consequenses
         for foreign code.   As  of version 2.5,  this is no longer  the
         case.

         Systems using  `sparse-memory management' are  a bit faster  as
         there is no  stack-shifter, and checking the stack-boundary  is
         often realised  by the hardware  using a  `guard-page'.   Also,
         memory  is actually  returned to  the  system after  a  garbage
         collection or call  to trim_stacks/0 (called by prolog/0  after
         finishing a user-query).

    c_libs (atom)
         Libraries passed  to the C-linker  when SWI-Prolog was  linked.
         May  be  used to  determine  the  libraries  needed  to  create
         statically linked extensions for SWI-Prolog.  See section 5.7.

    c_cc (atom)
         Name of the  C-compiler used to  compile SWI-Prolog.   Normally
         either gcc or cc.  See section 5.7.

    c_ldflags (atom)
         Special  linker  flags  passed   to  link  SWI-Prolog.      See
         section 5.7.

    readline (bool)
         If  true,  SWI-Prolog  is linked  with  the  readline  library.
         This is  done by  default if  you have  this library  installed
         on your  system.    It is  also true  for  the Win32  plwin.exe
         version of SWI-Prolog, which realises a subset  of the readline
         functionality.

    saved_program (bool)
         If  true,   Prolog  is   started  from   a  state  saved   with
         qsave_program/[1,2].

    runtime (bool)
         If true,  SWI-Prolog  is compiled  with  -DO_RUNTIME,  disabling
         various useful development  features (currently the tracer  and
         profiler).

    max_integer (integer)
         Maximum  integer  value.     Most  arithmetic  operations  will
         automatically convert  to floats if  integer values above  this
         are returned.

    min_integer (integer)
         Minimum integer value.

    max_tagged_integer (integer)
         Maximum integer value represented as a `tagged' value.   Tagged
         integers require  4-bytes storage  and are  used for  indexing.
         Larger integers are represented as `indirect data'  and require
         16-bytes  on  the  stacks  (though  a  copy   requires  only  4
         additional bytes).

    min_tagged_integer (integer)
         Start of the tagged-integer value range.

    float_format (atom, changeable)
         C printf() format specification used by write/1  and friends to
         determine how floating point numbers are printed.   The default
         is %g.    The  specified value  is passed  to printf()  without
         further  checking.    For  example,  if you  want  more  digits
         printed, %.12g  will print all floats  using 12 digits  instead
         of the default 6.  See also format/[1,2],  write/1, print/1 and
         portray/1.

    toplevel_print_options (term, changeable)
         This  argument   is  given   as  option-list   to  write_term/2
         for   printing    results   of   queries.         Default    is
         [quoted(true), portray(true), max_depth(10)].

    debugger_print_options (term, changeable)
         This  argument is  given  as  option-list to  write_term/2  for
         printing  goals  by  the  debugger.     Modified  by  the  `w',
         `p'  and  `<N>  d'  commands  of  the  debugger.    Default  is
         [quoted(true), portray(true), max_depth(10)].

    debugger_show_context (bool, changeable)
         If true, show  the context module while printing a  stack-frame
         in the  tracer.   Normally controlled using  the `C' option  of
         the tracer.

    compiled_at (atom)
         Describes when the  system has been  compiled.  Only  available
         if  the C-compiler  used  to  compile SWI-Prolog  provides  the
         __DATE__and __TIME__macros.

    character_escapes (bool, changeable)
         If true  (default),  read/1 interprets  \  escape sequences  in
         quoted atoms and strings.  May be changed.   This flag is local
         to the module in which it is changed.

    double_quotes (codes,chars,atom,string, changeable)
         This flag  determines  how double-quotes  strings are  read  by
         Prolog and is ---like  character_escapes--- maintained for  each
         module.    If codes  (default), a  list  of character-codes  is
         returned,  if chars  a list  of  one-character atoms,  if  atom
         double  quotes  are the  same  as  single-quotes  and  finally,
         string reads the text into a Prolog string  (see section 3.23).
         See also atom_chars/2 and atom_codes/2.

    allow_variable_name_as_functor (bool, changeable)
         If true  (default  is false),  Functor(arg) is  read  as if  it
         was written 'Functor'(arg).   Some applications use the  Prolog
         read/1  predicate for  reading  an application  defined  script
         language.   In these  cases, it is  often difficult to  explain
         none-Prolog  users  of  the  application  that   constants  and
         functions can only  start with a  lowercase letter.   Variables
         can be  turned into atoms  starting with  an uppercase atom  by
         calling read_term/2 using the option variable_names and binding
         the variables to their name.   Using this feature, F(x)  can be
         turned into valid syntax for such script languages.   Suggested
         by Robert van Engelen.  SWI-Prolog specific.

    history (integer, changeable)
         If integer >0,  support Unix csh(1)  like history as described
         in section  2.7.    Otherwise,  only support  reusing  commands
         through the  commandline editor.   The default  is to set  this
         prolog-flag  to 0  if a  commandline  editor is  provided  (see
         prolog-flag readline) and 15 otherwise.

    gc (bool, changeable)
         If true (default), the garbage collector is active.   If false,
         neither garbage-collection,  nor stack-shifts will take  place,
         even not on explicit request.  May be changed.

    agc_margin (integer, changeable)
         If  this amount  of  atoms  has  been created  since  the  last
         atom-garbage collection,  perform  atom garbage  collection  at
         the  first  opportunity.    Initial  value  is  10,000.     May
         be  changed.    A  value  of 0  (zero)  disables  atom  garbage
         collection.  See also PL_register_atom().

    iso (bool, changeable)
         Include some  weird ISO compatibility  that is incompatible  to
         normal SWI-Prolog behaviour.   Currently  it has the  following
         effect:

           o is/2  and  evaluation  under flag/3  do  not  automatically
             convert  floats  to integers  if  the float  represents  an
             integer.

           o The  //2 (float division)  always return a  float, even  if
             applied to integers that can be divided.

           o In  the standard order  of terms (see  section 3.6.1),  all
             floats are before all integers.

           o atom_length/2 yields  an instantiation  error if the  first
             argument is a number.

           o clause/[2,3]  raises  a  permission  error  when  accessing
             static predicates.

           o abolish/[1,2]  raises  a permission  error  when  accessing
             static predicates.

    optimise (bool, changeable)
         If true, compile in optimised mode.  The initial  value is true
         if Prolog was started with the -O commandline option.

         Currently   optimise   compilation   implies   compilation   of
         arithmetic, and  deletion of redundant  true/0 that may  result
         from expand_goal/2.

         Later versions might imply various other optimisations  such as
         integrating small  predicates into  their callers,  eliminating
         constant expressions and other predictable constructs.   Source
         code  optimisation is  never  applied  to predicates  that  are
         declared dynamic (see dynamic/1).

    char_conversion (bool, changeable)
         Determines  whether  character-conversion  takes   place  while
         reading terms.  See also char_conversion/2.

    autoload (bool, changeable)
         If true (default) autoloading of library functions  is enabled.
         See section 2.13.

    verbose_autoload (bool, changeable)
         If  true the  normal  consult  message  will be  printed  if  a
         library is autoloaded.  By default this  message is suppressed.
         Intended to be used for debugging purposes.

    trace_gc (bool, changeable)
         If  true  (false  is  the  default),  garbage  collections  and
         stack-shifts  will  be  reported on  the  terminal.     May  be
         changed.

    max_arity (unbounded)
         ISO  prolog-flag  describing  there  is  no  maximum  arity  to
         compound terms.

    integer_rounding_function (down,toward_zero)
         SO prolog-flag  describing rounding  by //  and rem  arithmetic
         functions.  Value depends on the C-compiler used.

    bounded (true)
         ISO prolog-flag describing  integer representation is bound  by
         min_integer and min_integer.

    tty_control (bool)
         Determines whether  the terminal  is switched to  raw mode  for
         get_single_char/1,  which also  reads the  user-actions for  the
         trace.  May be set.  See also the +/-tty command-line option.

    unknown (fail,warning,error, changeable)
         Determines  the   behaviour  if  an   undefined  procedure   is
         encountered.   If  fail,  the predicates  fails silently.    If
         warn,  a warning  is printed,  and  execution continues  as  if
         the  predicate was  not  defined  and if  error  (default),  an
         existence_error exception is  raised.   This flag  is local  to
         each module.

    debug (bool, changeable)
         Switch on/off  debugging  mode.   If  debug  mode is  activated
         the  system  traps  encountered  spy-points  (see   spy/1)  and
         trace-points  (see  trace/1).     In  addition,  tail-recursion
         optimisation is  disabled and the  system is more  conservative
         in destroying choice-points to simplify debugging.

         Disabling these optimisations can  cause the system to run  out
         of memory on  programs that behave  correctly if debug mode  is
         off.

    tail_recursion_optimisation (bool, changeable)
         Determines  whether  or  not  tail-recursion   optimisation  is
         enabled.     Normally the  value  of  this  flag  is  equal  to
         the  debug  flag.      As  programs   may  run  out  of   stack
         if  tail-recursion optimisation  is  omitted, it  is  sometimes
         necessary to enable it during debugging.

    abort_with_exception (bool, changeable)
         Determines how  abort/0 is realised.    See the description  of
         abort/0 for details.

    debug_on_error (bool, changeable)
         If  true,  start  the  tracer  after   an  error  is  detected.
         Otherwise just continue  execution.   The goal that raised  the
         error  will normally  fail.    See  also fileerrors/2  and  the
         prolog-flag report_error.   May be changed.   Default is  true,
         except for the runtime version.

    report_error (bool, changeable)
         If true, print  error messages, otherwise  suppress them.   May
         be changed.  See also  the debug_on_error prolog-flag.   Default
         is true, except for the runtime version.

    verbose (Atom, changeable)
         This flags is used by print_message/2.  If its value is silent,
         messages of type informational  and banner are supressed.   The
         -q switches the value from the initial normal to silent.

    file_name_variables (bool, changeable)
         If   true  (default   false),   expand   $varname  and   ~   in
         arguments  of  builtin-predicates  that  accept  a   file  name
         (open/3, exists_file/1,  access_file/2,  etc.).   The  predicate
         expand_file_name/2  should  be  used   to  expand   environment
         variables and wildcard patterns.  This  prolog-flag is intended
         for backward compatibility with older versions of SWI-Prolog.

    unix (bool)
         If  true,  the  operating  system  is  some  version  of  Unix.
         Defined  if the  C-compiler used  to  compile this  version  of
         SWI-Prolog either defines __unix__ or unix.

    windows (bool)
         If  true,   the  operating  system  is  an  implementation   of
         Microsoft Windows (3.1, 95, NT, etc.).


set_prolog_flag(+Key, +Value)
    Define  a new prolog-flag or change its value.  Key is an atom.   If
    the  flag is  a system-defined  flag that is  not marked  changeable
    above,  an  attempt to  modify the flag  yields a  permission_error.
    If  the  provided Value  does not  match  the type  of the  flag,  a
    type_error is raised.

    In  addition  to  ISO,  SWI-Prolog allows  for  user-defined  prolog
    flags.   The type of the  flag is determined from the  initial value
    and cannot be changed afterwards.


2.12 An overview of hook predicates

SWI-Prolog provides a large number of hooks, mainly  to control handling
messages, debugging,  startup, shut-down, macro-expansion,  etc.   Below
is  a  summary  of  all  defined  hooks  with  an  indication  of  their
portability.

  o portray/1
    Hook into write_term/3 to alter the way terms are printed (ISO).

  o message_hook/3
    Hook  into  print_message/2 to  alter the  way system  messages  are
    printed (Quintus/SICStus).

  o library_directory/1
    Hook  into absolute_file_name/3 to define  new library  directories.
    (most Prolog system).

  o file_search_path/2
    Hook   into   absolute_file_name/3  to   define   new   search-paths
    (Quintus/SICStus).

  o term_expansion/2
    Hook  into  load_files/1  to  modify  read  terms  before  they  are
    compiled (macro-processing) (most Prolog system).

  o goal_expansion/2
    Same as term_expansion/2 for individual goals (SICStus).

  o prolog_edit:locate/3
    Hook into edit/1 to locate objects (SWI).

  o prolog_edit:edit_source/1
    Hook into edit/1 to call some internal editor (SWI).

  o prolog_edit:edit_command/2
    Hook into edit/1 to define the external editor to use (SWI).

  o prolog_list_goal/1
    Hook  into the tracer  to list the  code associated to a  particular
    goal (SWI).

  o prolog_trace_interception/4
    Hook into the tracer to handle trace-events (SWI).

  o prolog:debug_control_hook/1
    Hook  in spy/1, nospy/1, nospyall/0 and debugging/0 to  extend these
    control-predicates to higher-level libraries.

  o prolog:help_hook/1
    Hook in help/0, help/1 and apropos/1 to extend the help-system.

  o resource/3
    Defines a new resource (not really a hook, but similar) (SWI).

  o exception/3
    Old  attempt  to  a  generic  hook mechanism.     Handles  undefined
    predicates (SWI).


2.13 Automatic loading of libraries

If ---at  runtime--- an undefined predicate  is trapped the system  will
first try to import the predicate from the module's default  module.  If
this fails the auto  loader is activated.  On first activation  an index
to all library files  in all library directories is loaded in  core (see
library_directory/1).  If the undefined predicate can be  located in the
one of the libraries  that library file is automatically loaded  and the
call to  the (previously undefined)  predicate is resumed.   By  default
this  mechanism loads  the  file silently.     The current_prolog_flag/2
verbose_autoload is provided  to get verbose loading.   The  prolog-flag
autoload can be used to enable/disable the entire auto load system.

The auto-loader only  works if the unknown  flag (see unknown/2) is  set
to trace (default).  A more appropriate interaction  with this flag will
be considered.

Autoloading  only handles  (library) source  files that  use the  module
mechanism  described  in  chapter  4.     The  files   are  loaded  with
use_module/2 and only the  trapped undefined predicate will be  imported
to the module  where the undefined predicate  was called.  Each  library
directory  must hold  a file  INDEX.pl  that contains  an index  to  all
library files  in the directory.    This file consists  of lines of  the
following format:

index(Name, Arity, Module, File).

The predicate make/0 scans the autoload libraries and  updates the index
if it  exists, is  writable and out-of-date.   It  is advised to  create
an empty  file called  INDEX.pl in  a library directory  meant for  auto
loading  before doing  anything  else.   This  index  file can  then  be
updated  by running  the prolog  make_library_index/1 (`%'  is the  Unix
prompt):

% mkdir ~/lib/prolog
% cd !$
% pl -g true -t 'make_library_index(.)'

If  there  are  more than  one  library  files  containing  the  desired
predicate the following search schema is followed:

 1. If  there is a  library file  that defines the  module in which  the
    undefined predicate is trapped, this file is used.

 2. Otherwise  library files  are considered  in the  order they  appear
    in  the  library_directory/1  predicate  and  within  the  directory
    alphabetically.


make_library_index(+Directory)
    Create  an index for this  directory.  The  index is written to  the
    file  'INDEX.pl' in the specified directory.   Fails with a  warning
    if the directory does not exist or is write protected.


2.14 Garbage Collection

SWI-Prolog  version  1.4  was  the  first  release  to  support  garbage
collection.     Together with  last-call  optimisation  this  guarantees
forward chaining programs do not waste infinite amounts of memory.


2.15 Syntax Notes

SWI-Prolog uses  standard `Edinburgh'  syntax.   A  description of  this
syntax can be found in the Prolog books referenced  in the introduction.
Below are some  non-standard or non-common constructs that are  accepted
by SWI-Prolog:

  o 0'<char>
    This  construct is not accepted by all Prolog systems that  claim to
    have  Edinburgh compatible syntax.  It describes the ASCII  value of
    <char>.   To  test whether C is  a lower case character  one can use
    between(0'a, 0'z, C).

  o /* .../* ...*/ ...*/
    The  /* ...*/  comment statement  can be  nested.    This is  useful
    if  some  code with  /* ...*/  comment statements  in it  should  be
    commented out.


2.15.1 ISO Syntax Support

SWI-Prolog offers ISO compatible extensions to the Edinburgh syntax.


2.15.1.1 Character Escape Syntax

Within quoted  atoms (using single quotes:   '<atom>'special  characters
are represented  using escape-sequences.    An escape  sequence is  lead
in  by the  backslash  (\) character.    The  list of  escape  sequences
is compatible  with the  ISO standard,  but contains  one extension  and
the interpretation of numerically specified characters  is slightly more
flexible to improve compatibility.

\a
    Alert character.  Normally the ASCII character 7 (beep).

\b
    Backspace character.

\c
    No  output.    All  input  characters  upto but  not  including  the
    first  non-layout  character  are skipped.     This allows  for  the
    specification of pretty-looking  long lines.  For compatibility with
    Quintus Prolog.  Nor supported by ISO. Example:

    format('This is a long line that would look better if it was \c
           split across multiple physical lines in the input')

\<RETURN>
    No  output.   Skips input till the  next non-layout character or  to
    the end of the next line.  Same intention as \c but ISO compatible.

\f
    Form-feed character.

\n
    Next-line character.

\r
    Carriage-return only (i.e. go back to the start of the line).

\t
    Horizontal tab-character.

\v
    Vertical tab-character (ASCII 11).

\x23
    Hexadecimal  specification of a character.   23 is just an  example.
    The  `x'  may be  followed by  a maximum  of  2 hexadecimal  digits.
    The  closing \  is optional.   The  code \xa\3  emits the  character
    10  (hexadecimal `a')  followed by  `3'.   The code  \x201 emits  32
    (hexadecimal  `20') followed by `1'.  According to ISO,  the closing
    \  is  obligatory and  the  number  of digits  is  unlimited.    The
    SWI-Prolog  definition allows for ISO compatible specification,  but
    is compatible with other implementations.

\40
    Octal   character  specification.     The  rules  and   remarks  for
    hexadecimal  specifications apply to  octal specifications too,  but
    the maximum allowed number of octal digits is 3.

\<character>
    Any  character immediately preceded  by a \  and not covered by  the
    above  escape sequences is copied verbatim.   Thus, '\\' is  an atom
    consisting  of a single \ and  '\'' and '''' both describe the  atom
    with a single '.

Character      escaping      is     only      available      if      the
current_prolog_flag(character_escapes, true) is active (default).    See
current_prolog_flag/2.   Character escapes conflict with writef/2  in two
ways:   \40  is interpreted  as decimal  40 by  writef/2, but  character
escapes  handling by  read has  already interpreted  as  32 (40  octal).
Also, \l is translated to  a single `l'.  It is adviced to use  the more
widely supported  format/[2,3] predicate instead.   If you insist  using
writef, either switch  character_escapes to  false, or use double \\,  as
in writef('\\l').


2.15.1.2 Syntax for non-decimal numbers

SWI-Prolog  implements  both  Edinburgh  and  ISO   representations  for
non-decimal numbers.   According to  Edinburgh syntax, such numbers  are
written as <radix>'<number>, where <radix> is  a number between 2 and 36.
ISO defines binary,  octal and hexadecimal numbers using 0[bxo]<number>.
For example:  A is 0b100 \/ 0xf00  is a valid expression.   Such numbers
are always unsigned.


2.16 System limits


2.16.1 Limits on memory areas

SWI-Prolog has  a number of  memory areas which are  only enlarged to  a
certain limit.   The  default sizes for these  areas should suffice  for
most applications, but big  applications may require larger ones.   They
are modified  by command  line options.    The table  below shows  these
areas.   The first column  gives the option name  to modify the size  of
the area.  The option character is immediately followed  by a number and
optionally by  a k or  m.   With k or  no unit  indicator, the value  is
interpreted in Kbytes (1024 bytes), with m, the  value is interpreted in
Mbytes (10241* 024 bytes).

The local-,  global- and  trail-stack are  limited to 128  Mbytes on  32
bit processors,  or more in general to  2 to the power bits-per-long - 5
bytes.

The  PrologScript facility  described  in  section 2.10.2.1  provides  a
mechanism for  specifying options with  the load-file.   On Windows  the
default stack-sizes  are controlled  using the Windows  registry on  the
key  HKEY_CURRENT_USER\Software\SWI\Prolog using  the  names  localSize,
globalSize and trailSize.   The value is a DWORD expressing  the default
stack size  in Kbytes.   A  GUI for modifying  these values of  provided
using the XPCE package.
       ___________________________________________________________
       |_Option_|Default_|Area_name______|Description____________|_||-L||2Mloca||lTstackhe|l||ocal|stack is used

       |        |                        to  store   the  execu- |             |            |
       |        |                        tion  environments   of |             |            |
       |        |                        procedure  invocations. |             |            |
       |        |                        The  space for  an  en- |             |            |
       |        |                        vironment is  reclaimed |             |            |
       |        |                        when  it  fails,  exits |             |            |
       |        |                        without leaving  choice |             |            |
       |        |                        points,   the  alterna- |             |            |
       |        |                        tives are  cut of  with |             |            |
       |        |                                                |             |            |

       |        |                        the  !/0  predicate  or |             |            |
       |        |                        no  choice points  have |             |            |
       |        |                        been created  since the |             |            |
       |        |                        invocation and the last |             |            |
       |        |                        subclause  is   started |             |            |
       |        |                        (tail recursion optimi- |             |            |
       ||       ||      |               |sation).                ||            |            |

       |   -G   | 4M    |global stack   |Theusglobaled stacktois store| terms
       |        |       |               |created during Prolog's |
       |        |       |               |execution.    Terms  on |
       |        |       |               |                        |
       |        |       |               |this stack will  be re- |
       |        |       |               |claimed by backtracking |
       |        |       |               |to a  point before  the |
       |        |       |               |term  was   created  or |
       |        |       |               |by  garbage  collection |

       |        |       |               |(provided  the term  is |
       ||       ||      ||              |no|longer referenced).  ||
       |   -T   | 4M    |trail stack    |Theusetraild stackto isstore|  as-
       |        |       |               |signments during execu- |
       |        |       |               |                        |
       |        |       |               |tion.   Entries on this |
       |        |       |               |stack remain  alive un- |

       |        |       |               |til backtracking before |
       |        |       |               |the  point of  creation |
       |        |       |               |or the  garbage collec- |
       |        |       |               |tor determines they are |
       ||       ||      ||              |nor|needed any longer.  ||

       |   -A   | 1M    |argument stack |Theusargumentestackd isto|  store   one
       |        |       |               |of   the   intermediate |
       |        |       |               |code interpreter's reg- |
       |        |       |               |isters.     The  amount |
       |        |       |               |of space needed on this |
       |        |       |               |stack is determined en- |
       |        |       |               |                        |
       |        |       |               |tirely by the  depth in |
       |        |       |               |which terms  are nested |
       |        |       |               |in  the   clauses  that |

       |        |       |               |constitute the program. |
       |        |       |               |Overflow is most likely |
       |        |       |               |when using long strings |
       |________|_______|_______________|in_a_clause.____________|_

                        Table 2.2:  Memory areas


2.16.1.1 The heap

With  the heap,  we  refer  to the  memory  area  used by  malloc()  and
friends.  SWI-Prolog uses the area to store  atoms, functors, predicates
and their  clauses, records and  other dynamic data.   As of  SWI-Prolog
2.8.5, no limits are  imposed on the addresses returned by  malloc() and
friends.

On  some machines,  the  runtime stacks  described above  are  allocated
using `sparse allocation'.   Virtual space upto the limit is  claimed at
startup and  committed and released  while the  area grows and  shrinks.
On Win32  platform this  is realised using  VirtualAlloc() and  friends.
On Unix systems this is realised using mmap().


2.16.2 Other Limits

Clauses  Currently the  following limitations  apply to  clauses.    The
    arity  may not be more than 1024 and the number of  variables should
    be less than 65536.

Atoms and Strings  SWI-Prolog  has no  limits  on  the  sizes  of  atoms
    and  strings.   read/1  and its derivatives  however normally  limit
    the  number  of newlines  in  an  atom or  string  to 5  to  improve
    error  detection  and recovery.    This  can  be switched  off  with
    style_check/1.

Address space  SWI-Prolog  data  is  packed  in  a 32-bit  word,   which
    contains  both type and value information.  The size of  the various
    memory areas is limited  to 128 Mb for each of the areas, except for
    the program heap, which is not limited.

Integers  Integers are  32-bit  to  the  user,  but  integers  upto  the
    value  of  the max_tagged_integer prolog-flag  are represented  more
    efficiently.

Floats  Floating  point  numbers  are  represented  as  C-native  double
    precision floats, 64 bit IEEE on most machines.


2.16.3 Reserved Names

The boot compiler  (see -b option) does  not support the module  system.
As large parts of  the system are written in Prolog itself we  need some
way to  avoid name clashes  with the  user's predicates, database  keys,
etc.   Like Edinburgh C-Prolog [Pereira, 1986] all predicates,  database
keys, etc. that should  be hidden from the user start with a  dollar ($)
sign (see style_check/1).


Chapter 3.  BUILT-IN PREDICATES


3.1 Notation of Predicate Descriptions

We have  tried to  keep the  predicate descriptions  clear and  concise.
First  the predicate  name is  printed  in bold  face, followed  by  the
arguments in  italics.   Arguments  are preceded by  a `+',  `-' or  `?'
sign.    `+'  indicates the  argument is  input  to the  predicate,  `-'
denotes output and  `?'  denotes `either  input or output'.   Constructs
like `op/3' refer to the predicate `op' with arity `3'.


3.2 Character representation

In  traditional (Edinburgh-)  Prolog, characters  are represented  using
character-codes.   Character codes are  integer indices into a  specific
character set.   Traditionally  the character  set was 7-bits  US-ASCII.
Since a long while  8-bit character sets are allowed,  providing support
for  national character  sets,  of  which  iso-latin-1 (ISO  8859-1)  is
applicable  to many  western  languages.    Text-files are  supposed  to
represent a sequence of character-codes.

ISO Prolog introduces three types, two of which  are used for characters
and one for accessing binary streams (see open/4).  These types are:

  o code
    A  character-code is  an  integer representing  a single  character.
    As  files  may  use  multi-byte encoding  for  supporting  different
    character  sets (utf-8 encoding for example), reading a code  from a
    text-file is in general not the same as reading a byte.

  o char
    Alternatively,  characters  may  be  represented  as  one-character-
    atoms.    This is  a  very natural  representation, hiding  encoding
    problems  from  the  programmer as  well  as providing  much  easier
    debugging.

  o byte
    Bytes are used for accessing binary-streams.

The  current  version  of  SWI-Prolog  does  not   provide  support  for
multi-byte character encoding.  This implies for example  that it is not
capable of  breaking a  multi-byte encoded  atom into characters.    For
SWI-Prolog, bytes  and codes  are the same  and one-character-atoms  are
simple atoms containing one byte.

To  ease  the  pain  of  these  multiple  representations,  SWI-Prolog's
built-in  predicates dealing  with character-data  work  as flexible  as
possible:   they accept  data in  any of these  formats as  long as  the
interpretation  is unambiguous.     In addition,  for  output  arguments
that are  instantiated, the character  is extracted before  unification.
This implies  that the following two  calls are identical, both  testing
whether the next input characters is an a.

peek_code(Stream, a).
peek_code(Stream, 97).

These  multiple-representations  are  handled  by  a   large  number  of
built-in  predicates,   all   of  which   are  ISO-compatible.       For
converting  betweem  code and  character  there  is  char_code/2.     For
breaking  atoms  and  numbers  into  characters  are  are  atom_chars/2,
atom_codes/2,  number_codes/2 and  number_chars/2.    For  character  I/O
on  streams  there  is get_char/[1,2],  get_code/[1,2],  get_byte/[1,2],
peek_char/[1,2],   peek_code/[1,2],   peek_byte/[1,2],   put_code/[1,2],
put_char/[1,2]  and  put_byte/[1,2].     The  prolog-flag  double_quotes
(see current_prolog_flag/2) controls how  text between double-quotes  is
interpreted.


3.3 Loading Prolog source files

This section deals  with loading Prolog source-files.   A Prolog  source
file  is a  text-file (often  referred to  as  ASCII-file) containing  a
Prolog program  or part  thereof.   Prolog  source files  come in  three
flavours:

 A traditional   Prolog  source  file  contains  a  Prolog  clauses  and
    directives,  but no  module-declaration.   They are normally  loaded
    using consult/1 or ensure_loaded/1.

 A module   Prolog source file  starts with a module  declaration.   The
    subsequent Prolog code  is loaded into the specified module and only
    the public predicates  are made available to the context loading the
    module.   Module  files are normally loaded  using use_module/[1,2].
    See chapter 4 for details.

 A include   Prolog source file is loaded using the  include/1 directive
    and normally contains only directives.

Prolog  source-files are  located  using  absolute_file_name/3 with  the
following options:

locate_prolog_file(Spec, Path) :-
        absolute_file_name(Spec,
                           [ file_type(prolog),
                             access(read)
                           ],
                           Path).

The file_type(prolog) option is  used to determine the extension of  the
file using  prolog_file_type/2.    The default extension  is .pl.    Spec
allows  for the  path-alias construct  defined  by absolute_file_name/3.
The  most  commonly  used  path-alias  is  library(LibraryFile).     The
example below loads the library file oset.pl  (containing predicates for
manipulating ordered sets).

:- use_module(library(oset)).

SWI-Prolog   recognises    grammar   rules    (DCG)   as   defined    in
[Clocksin & Melish, 1987].     The  user  may   define  additional  com-
pilation  of  the   source  file  by  defining  the  dynamic   predicate
term_expansion/2.    Transformations  by  this  predicate  overrule  the
systems  grammar  rule transformations.     It  is not  allowed  to  use
assert/1, retract/1 or any other  database predicate in term_expansion/2
other than for local computational purposes.

Directives  may be  placed  anywhere  in a  source  file,  invoking  any
predicate.    They are  executed when  encountered.    If the  directive
fails, a warning is printed.  Directives are specified  by :-/1 or ?-/1.
There is no difference between the two.

SWI-Prolog   does   not   have   a   separate   reconsult/1   predicate.
Reconsulting  is  implied automatically  by  the  fact that  a  file  is
consulted which is already loaded.


load_files(+Files, +Options)
    The  predicate load_files/2 is the parent  of all the other  loading
    predicates.    It  currently supports  a subset  of  the options  of
    Quintus  load_files/2.   Files is  either specifies a  single, or  a
    list  of  source-files.    The specification  for a  source-file  is
    handled  absolute_file_name/2.  See this predicate  for the supported
    expansions.  Options is a list of options using the format

         OptionName(OptionValue)

    The following options are currently supported:

    if(Condition)
         Load the  file only  if the specified  condition is  satisfied.
         The value  true loads the  file unconditionally, changed  loads
         the file  if it  was not loaded  before, or  has been  modified
         since it was loaded the last time, not_loaded loads the file if
         it was not loaded before.

    must_be_module(Bool)
         If true,  raise an  error if  the file  is not  a module  file.
         Used by use_module/[1,2].

    imports(ListOrAll)
         If  all and  the  file is  a  module file,  import  all  public
         predicates.  Otherwise import only the named predicates.   Each
         predicate is refered to  as <name>/<arity>.  This  option has no
         effect if the file is not a module file.

    silent(Bool)
         If  true, load  the  file  without printing  a  message.    The
         specified  value is  the  default for  all  files loaded  as  a
         result of loading the specified files.


consult(+File)
    Read  File as a Prolog  source file.  File  may be a list of  files,
    in  which case all members  are consulted in turn.   File may  start
    with  the csh(1) special sequences ~,   <user>and $<var>.  File  may
    also be library(Name),  in which case the libraries are searched for
    a  file with the specified  name.  See  also library_directory/1 and
    file_search_path/2.  consult/1  may be abbreviated by just  typing a
    number of file names in a list.  Examples:

        ?- consult(load).       % consult load or load.pl
        ?- [library(quintus)].  % load Quintus compatibility library

    Equivalent to load_files(Files, []).


ensure_loaded(+File)
    If the file  is not already loaded, this is equivalent to consult/1.
    Otherwise,   if  the  file  defines  a  module,  import  all  public
    predicates.    Finally,  if the  file is  already loaded,  is not  a
    module  file and the context module  is not the global user  module,
    ensure_loaded/1 will call consult/1.

    With the semantics, we  hope to get as closely possible to the clear
    semantics  without the presence  of a module  system.   Applications
    using modules should consider using use_module/[1,2].

    Equivalent to load_files(Files, [if(changed)]).


include(+File)
    Pretend  the  terms   in  File  are  in  the  source-file  in  which
    :- include(File)  appears.  The include construct is  only honnoured
    if  it appears  as  a directive  in a  source-file.   Normally  File
    contains a sequence of directives.


require(+ListOfNameAndArity)
    Declare  that  this file/module  requires the  specified  predicates
    to  be defined ``with  their commonly accepted  definition''.   This
    predicate originates from  the Prolog portability layer for XPCE. It
    is  intended to  provide a  portable mechanism  for specifying  that
    this module requires the specified predicates.

    The implementation normally  first verifies whether the predicate is
    already defined.   If not, it will search the libraries and load the
    required library.

    SWI-Prolog, having autoloading,  does not load the library.  Instead
    it  creates a procedure  header for the  predicate if this does  not
    exist.    This will flag  the predicate  as `undefined'.   See  also
    check/0 and autoload/0.


make
    Consult  all source  files that  have been changed  since they  were
    consulted.   It checks all  loaded source files:  files loaded  into
    a  compiled state  using pl -c ...  and files  loaded using  consult
    or  one of its  derivatives.  The  predicate make/0 is called  after
    edit/1,  automatically reloading all  modified files.   It the  user
    uses  an external editor (in a separate window), make/0  is normally
    used to update the program after editing.


library_directory(?Atom)
    Dynamic  predicate used  to specify  library directories.    Default
    ./lib,  ~/lib/prolog and  the system's library  (in this order)  are
    defined.    The  user may  add library  directories using  assert/1,
    asserta/1 or remove system defaults using retract/1.


file_search_path(+Alias, ?Path)
    Dynamic  predicate used to specify `path-aliases'.  This  feature is
    best described using an example.  Given the definition

    file_search_path(demo, '/usr/lib/prolog/demo').

    the  file specification demo(myfile)  will be expanded to  /usr/lib/
    prolog/demo/myfile.   The second  argument of file_search_path/2 may
    be another alias.

    Below  is the  initial definition  of the file  search path.    This
    path  implies  swi(<Path>)  refers  to  a  file  in  the  SWI-Prolog
    home   directory.     The  alias  foreign(<Path>) is   intended  for
    storing   shared  libraries  (.so  or   .DLL  files).     See   also
    load_foreign_library/[1,2].

    user:file_search_path(library, X) :-
            library_directory(X).
    user:file_search_path(swi, Home) :-
            current_prolog_flag(home, Home).
    user:file_search_path(foreign, swi(ArchLib)) :-
            current_prolog_flag(arch, Arch),
            atom_concat('lib/', Arch, ArchLib).
    user:file_search_path(foreign, swi(lib)).

    The  file_search_path/2expansion  is used by all loading  predicates
    as well as by absolute_file_name/[2,3].


expand_file_search_path(+Spec, -Path)
    Unifies  Path  will   all  possible  expansions  of  the  file  name
    specification Spec.  See also absolute_file_name/3.


prolog_file_type(?Extension, ?Type)
    This  dynamic multifile predicate defined in module  user determines
    the  extensions considered by file_search_path/2.  Extension is  the
    filename  extension without the leading  dot, Type denotes the  type
    as  used by the file_type(Type) option of  file_search_path/2.   Here
    is the initial definition of prolog_file_type/2:

    user:prolog_file_type(pl,       prolog).
    user:prolog_file_type(Ext,      prolog) :-
            current_prolog_flag(associate, Ext),
            Ext \== pl.
    user:prolog_file_type(qlf,      qlf).
    user:prolog_file_type(Ext,      executable) :-
            current_prolog_flag(shared_object_extension, Ext).

    Users  may  wish to  change  the extension  used for  Prolog  source
    files  to avoid  conflicts (for  example with  perl) as  well as  to
    be  compatible with  some specific  implementation.   The  preferred
    alternative extension is .pro.


source_file(?File)
    Succeeds  if File  is a  loaded Prolog  source file.    File is  the
    absolute and canonical path to the source-file.


source_file(?Pred, ?File)
    Is   true   if  the   predicate  specified   by   Pred  was   loaded
    from   file  File,  where  File  is  an  absolute  path   name  (see
    absolute_file_name/2).  Can be used with  any instantiation pattern,
    but the database  only maintains the source file for each predicate.
    See also clause_property/2.


prolog_load_context(?Key, ?Value)
    Determine loading context.  The following keys are defined:

      ________________________________________________________________
      |_Key___________|Description____________________________________|
      | module        |Module into which file is loaded               |
      | file          |File loaded                                    |
      | stream        |Stream identifier (see current_input/1)        |

      | directory     |Directory in which File lives.                 |
      | term_position |Position of last  term read.  Term of  the form|
      |_______________|'$stream_position'(0,<Line>,0,0,0)_____________|

    Quintus compatibility predicate.  See also source_location/2.


source_location(-File, -Line)
    If  the last term has been read from a physical file (i.e.  not from
    the file user or  a string), unify File with an absolute path to the
    file  and Line with the  line-number in the file.   New code  should
    use prolog_load_context/2.


term_expansion(+Term1, -Term2)
    Dynamic  predicate, normally not defined.  When defined by  the user
    all  terms read during consulting that are given to  this predicate.
    If  the predicate succeeds Prolog will assert Term2 in  the database
    rather  then the read term  (Term1).  Term2 may  be a term of a  the
    form  `?- Goal' or `:- Goal'.  Goal is then treated  as a directive.
    If Term2 is a  list all terms of the list are stored in the database
    or  called (for directives).   If  Term2 is of  the form below,  the
    system  will assert Clause and record the  indicated source-location
    with it.

         '$source_location'(<File>, <Line>):<Clause>

    When compiling a  module (see chapter 4 and the directive module/2),
    expand_term/2  will first try  term_expansion/2 in the module  being
    compiled  to  allow  for  term-expansion rules  that  are  local  to
    a  module.     If  there  is  no  local  definition,  or  the  local
    definition  fails  to translate  the  term,  expand_term/2 will  try
    term_expansion/2  in module user.    For compatibility with  SICStus
    and  Quintus Prolog,  this feature  should not  be used.   See  also
    expand_term/2, goal_expansion/2 and expand_goal/2.


expand_term(+Term1, -Term2)
    This  predicate  is  normally  called by  the  compiler  to  perform
    preprocessing.   First it calls term_expansion/2.   If this predicate
    fails  it performs  a grammar-rule translation.    If this fails  it
    returns the first argument.


goal_expansion(+Goal1, -Goal2)
    Like   term_expansion/2,   goal_expansion/2  provides   for   macro-
    expansion  of Prolog  source-code.   Between  expand_term/2 and  the
    actual  compilation, the body of clauses analysed and the  goals are
    handed  to expand_goal/2, which  uses the goal_expansion/2 hook to do
    user-defined expansion.

    The  predicate goal_expansion/2 is first  called in the module  that
    is being compiled, and then on the user module.

    Only  goals  apearing   in  the  body  of  clauses  when  reading  a
    source-file  are expanded using mechanism,  and only if they  appear
    literally  in the clause, or  as an argument to the  meta-predicates
    not/1, call/1 or  forall/2.  A real predicate definition is required
    to deal with dynamically constructed calls.


expand_goal(+Goal1, -Goal2)
    This  predicate  is  normally  called by  the  compiler  to  perform
    preprocessing.   First it calls goal_expansion/2.  If this  fails it
    returns the first argument.


at_initialization(+Goal)
    Register   Goal   to   be   ran   when   the   system   initialises.
    Initialisation  takes place after  reloading a .qlf (formerly  .wic)
    file  as well as after reloading a  saved-state.  The hooks  are run
    in  the order they were registered.  A warning message is  issued if
    Goal fails, but execution continues.  See also at_halt/1


at_halt(+Goal)
    Register  Goal to be ran when the  system halts.  The hooks  are run
    in  the order they were registered.  Success or failure  executing a
    hook is ignored.  These hooks may not call halt/[0,1].


initialization(+Goal)
    Call  Goal and  register it using  at_initialization/1.   Directives
    that  do  other things  that  creating clauses,  records,  flags  or
    setting  predicate attributes should normally be written  using this
    tag  to ensure the  initialisation is executed  when a saved  system
    starts.  See also qsave_program/[1,2].


compiling
    Succeeds if the  system is compiling source files with the -c option
    into  an  intermediate code  file.    Can be  used to  perform  code
    optimisations in expand_term/2 under this condition.


preprocessor(-Old, +New)
    Read  the input file via a  Unix process that acts as  preprocessor.
    A  preprocessor is specified  as an atom.   The first occurrence  of
    the  string `%f' is replaced by the  name of the file to  be loaded.
    The  resulting atom  is called as  a Unix  command and the  standard
    output  of this command is loaded.   To use the Unix  C preprocessor
    one should define:

    ?- preprocessor(Old, '/lib/cpp -C -P %f'), consult(...).

    Old = none


3.3.1 Quick load files

The features described in this section should be regarded alpha.

As of  version 2.0.0, SWI-Prolog  supports compilation of individual  or
multiple Prolog  sourcefiles into  `Quick Load  Files'.   A `Quick  Load
Files' (.qlf  file) stores  the contents  of the file  in a  precompiled
format.

These files load  considerably faster than sourcefiles and are  normally
more compact.    They are  machine independent  and may  thus be  loaded
on any  implementation of  SWI-Prolog.   Note however  that clauses  are
stored as virtual  machine instructions.   Changes to the compiler  will
generally make old compiled files unusable.

Quick Load Files  are created using qcompile/1.   They are loaded  using
consult/1  or one  of  the other  file-loading predicates  described  in
section 3.3.   If consult is given  the explicit .pl file, it will  load
the Prolog  source.  When  given the .qlf file,  it will load the  file.
When no extension is specified, it will load the  .qlf file when present
and the fileextpl file otherwise.


qcompile(+File)
    Takes  a  single file  specification  like consult/1  (i.e.  accepts
    constructs like library(LibFile)  and creates a Quick Load File from
    File.   The file-extension of this file  is .qlf.  The base  name of
    the Quick Load File is the same as the input file.

    If   the   file   contains   `:- consult(+File)'   or   `:- [+File]'
    statements,  the  referred files  are compiled  into  the same  .qlf
    file.    Other  directives  will  be stored  in  the .qlf  file  and
    executed in the same fashion as when loading the .pl file.

    For  term_expansion/2,  the same rules  as described in section  2.10
    apply.

    Source  references (source_file/2) in the  Quick Load File refer  to
    the Prolog source file from which the compiled code originates.


3.4 Listing and Editor Interface

SWI-Prolog  offers an  extensible  interface which  allows the  user  to
edit objects  of the program:   predicates,  modules, files,  etc.   The
editor interface is  implemented by edit/1 and consists of  three parts:
locating, selecting and starting the editor.

Any of  these parts may  be extended or redefined  by adding clauses  to
various multi-file  (see multifile/1) predicates  defined in the  module
prolog_edit.

The built-in  edit specifications for edit/1  (see prolog_edit:locate/3)
are described below.

   ___________________________________________________________________
   |_____________________Fully_specified_objects______________________|
   | <Module>:<Name>/<Arity>R|efers a predicate                      |
   | module(<Module>)      |Refers to a module                       |
   | file(<Path>)          |Refers to a file                         |

   |_source_file(<Path>)___R|efers_to_a_loaded_source-file___________|_
   |_____________________Ambiguous_specifications_____________________|
   | <Name>/<Arity>        R|efers this predicate in any module      |
   | <Name>                |Refers  to  (1) named  predicate  in  any|
   |                       |module  with any  arity,  (2) a  (source)|
   |_______________________|file_or_(3)_a_module.____________________|_


edit(+Specification)
    First  exploits prolog_edit:locate/3to  translate Specification into
    a  list  of  Locations.    If there  is  more than  one  `hit',  the
    user  is  allows to  select  from the  found  locations.    Finally,
    prolog_edit:edit_source/1 is  used  to invoke  the user's  preferred
    editor.


prolog_edit:locate(+Spec, -FullSpec, -Location)
    Where  Spec is  the  specification provided  through edit/1.    This
    multifile  predicate  is  used to  enumerate  locations at  with  an
    object satisfying the given  Spec can be found.  FullSpec is unified
    with  the complete specification for  the object.  This  distinction
    is  used to  allow for ambiguous  specifications.   For example,  if
    Spec  is  an  atom,  which appears  as  the base-name  of  a  loaded
    file  and as  the name  of a predicate,  FullSpec will  be bound  to
    file(Path) or Name/Arity.

    Location  is a list of attributes  of the location.  Normally,  this
    list  will contain  the term file(File)  and ---if available---  the
    term line(Line).


prolog_edit:locate(+Spec, -Location)
    Same  as prolog_edit:locate/3, but only deals with  fully-sepecified
    objects.


prolog_edit:edit_source(+Location)
    Start  editor on Location.  See  prolog_edit:locate/3for  the format
    of  a location  term.   This  multi-file predicate  is normally  not
    defined.  If it succeeds, edit/1 assumes the editor is started.

    If  it fails,  edit/1 will invoke  an external editor.   The  editor
    to  be invoked  is determined from  the evironment variable  EDITOR,
    which  may  be set  from the  operating system  or  from the  Prolog
    initialisation file using setenv/2.   If no editor is defined, vi is
    the default in Unix systems, and notepad on Windows.

    The  predicate  prolog_edit:edit_command/2 defines  how  the  editor
    will be invoked.


prolog_edit:edit_command(+Editor, -Command)
    Determines  how Editor is  to be invoked using  shell/1.  Editor  is
    the  determined editor  (see edit_source/1), without  the full  path
    specification,  and without  possible (exe) extension.   Command  is
    an  atom describing  the command.   The  pattern %f  is replaced  by
    the  full file-name  of the  location, and  %d by  the line  number.
    If  the  editor can  deal with  starting at  a specified  line,  two
    clauses  should be provided,  one holding only  the %f pattern,  and
    one holding both patterns.

    The  default contains  definitions for vi,  emacs, emacsclient,  vim
    and notepad (latter without line-number version).

    Please contribute your specifications to jan@swi.psy.uva.nl.


prolog_edit:load
    Normally  not-defined  multifile  predicate.    This  predicate  may
    be  defined  to provide  loading hooks  for  user-extensions to  the
    edit  module.   For example,  XPCE provides the  code below to  load
    library(swi_edit),  containing  definitions  to locate  classes  and
    methods  as well as  to bind this  package to the PceEmacs  built-in
    editor.

    :- multifile prolog_edit:load/0.

    prolog_edit:load :-
            ensure_loaded(library(swi_edit)).


listing(+Pred)
    List  specified predicates  (when an  atom is  given all  predicates
    with  this name will  be listed).   The listing  is produced on  the
    basis  of the  internal representation,  thus loosing user's  layout
    and variable name information.  See also portray_clause/1.


listing
    List all predicates of the database using listing/1.


portray_clause(+Clause)
    Pretty  print a  clause.   A clause  should be specified  as a  term
    `<Head> :- <Body>'.  Facts are represented as `<Head> :- true'.


3.5 Verify Type of a Term


var(+Term)
    Succeeds if Term currently is a free variable.


nonvar(+Term)
    Succeeds if Term currently is not a free variable.


integer(+Term)
    Succeeds if Term is bound to an integer.


float(+Term)
    Succeeds if Term is bound to a floating point number.


number(+Term)
    Succeeds if Term is bound to an integer or a floating point number.


atom(+Term)
    Succeeds if Term is bound to an atom.


string(+Term)
    Succeeds if Term is bound to a string.


atomic(+Term)
    Succeeds  if Term is bound to  an atom, string, integer or  floating
    point number.


compound(+Term)
    Succeeds  if Term is bound to a  compound term.  See  also functor/3
    and =../2.


callable(+Term)
    Succeeds  if Term is bound to an atom or a compound term,  so it can
    be handed without type-error to call/1, functor/3 and =../2.


ground(+Term)
    Succeeds if Term holds no free variables.


3.6 Comparison and Unification or Terms


3.6.1 Standard Order of Terms

Comparison and  unification of arbitrary  terms.   Terms are ordered  in
the so called ``standard order''.  This order is defined as follows:

 1. Variables <Atoms <Strings <Numbers <Terms

 2. Old Variable <New Variable

 3. Atoms are compared alphabetically.

 4. Strings are compared alphabetically.

 5. Numbers  are compared  by value.   Integers  and floats are  treated
    identically.

 6. Compound  terms are  first checked  on  their arity,  then on  their
    functor-name  (alphabetically)  and  finally  recursively  on  their
    arguments, leftmost argument first.

If  the  prolog_flag  (see  current_prolog_flag/2)  iso  is  defined,  all
floating point numbers precede all integers.


+Term1 == +Term2
    Succeeds  if Term1  is  equivalent to  Term2.   A  variable is  only
    identical to a sharing variable.


+Term1 \== +Term2
    Equivalent to \+Term1 == Term2.


+Term1 = +Term2
    Unify Term1 with Term2.  Succeeds if the unification succeeds.


unify_with_occurs_check(+Term1, +Term2)
    As  =/2, but  using sound-unification.    That is,  a variable  only
    unifies  to  a term  if  this term  does  not contain  the  variable
    itself.  To illustrate this, consider the two goals below:

    1 ?- A = f(A).

    A = f(f(f(f(f(f(f(f(f(f(...))))))))))
    2 ?- unify_with_occurs_check(A, f(A)).

    No

    I.e.   the  first  creates  a  cyclic-term,  which  is   printed  as
    an   infinitly  nested  f/1  term  (see  the   max_depth   option  of
    write_term/2).  The second executes  logically sound unification and
    thus fails.


+Term1 \= +Term2
    Equivalent to \+Term1 = Term2.


+Term1 =@= +Term2
    Succeeds  if Term1  is `structurally  equal' to Term2.    Structural
    equivalence  is weaker  than equivalence (==/2),  but stronger  than
    unification  (=/2).  Two terms are structurally equal if  their tree
    representation  is identical  and they  have the  same `pattern'  of
    variables.  Examples:

               a  =@=  A       false
               A  =@=  B       true
          x(A,A)  =@=  x(B,C)  false
          x(A,A)  =@=  x(B,B)  true
          x(A,B)  =@=  x(C,D)  true


+Term1 \=@= +Term2
    Equivalent to `\+Term1 =@= Term2'.


+Term1 @< +Term2
    Succeeds if Term1 is before Term2 in the standard order of terms.


+Term1 @=< +Term2
    Succeeds if both terms  are equal (==/2) or Term1 is before Term2 in
    the standard order of terms.


+Term1 @> +Term2
    Succeeds if Term1 is after Term2 in the standard order of terms.


+Term1 @>= +Term2
    Succeeds  if both terms are equal (==/2) or Term1 is after  Term2 in
    the standard order of terms.


compare(?Order, +Term1, +Term2)
    Determine or test  the Order between two terms in the standard order
    of terms.  Order is one of <, >  or =, with the obvious meaning.


3.7 Control Predicates

The predicates of  this section implement control structures.   Normally
these constructs  are translated  into virtual  machine instructions  by
the compiler.   It is still  necessary to implement these constructs  as
true predicates  to support meta-calls,  as demonstrated in the  example
below.  The  predicate finds all currently defined atoms of  1 character
long.   Note that  the cut has no  effect when called  via one of  these
predicates (see !/0).

one_character_atoms(As) :-
        findall(A, (current_atom(A), atom_length(A, 1)), As).


fail
    Always  fail.   The  predicate fail/0  is translated  into a  single
    virtual machine instruction.


true
    Always  succeed.  The predicate  true/0 is translated into a  single
    virtual machine instruction.


repeat
    Always succeed, provide an infinite number of choice points.


!
    Cut.    Discard choice  points of  parent frame  and frames  created
    after the parent frame.   As of SWI-Prolog 3.3, the semantics of the
    cut are compliant with  the ISO standard.  This implies that the cut
    is  transparent to  ;/2, ->/2  and  *->/2.   Cuts  appearing in  the
    condition  part of ->/2 and  *->/2 as well  as in \+/1 are local  to
    the condition.

    As an extension, a  variable goal that is bound the the ! is handled
    as  a true cut.   This is the  only difference between call/1 and  a
    variable appearing as subgoal.

       t1 :- (a, !, fail ; b).        % cuts a/0 and t1/0
       t2 :- (a -> b, !  ; c).        % cuts b/0 and t2/0
       t3(G) :- a, G, fail.           % if `G = !'  cuts a/0 and t1/1
       t4(G) :- a, call(G), fail.     % if `G = !'  cut has no effect
       t5 :- call((a, !, fail ; b)).  % cuts a/0

       t6 :- \+(a, !, fail ; b).      % cuts a/0


+Goal1 , +Goal2
    Conjunction.   Succeeds if both  `Goal1' and `Goal2' can be  proved.
    It  is defined as (this  definition does not lead  to a loop as  the
    second comma is handled by the compiler):

    Goal1, Goal2 :- Goal1, Goal2.


+Goal1 ; +Goal2
    The `or' predicate is defined as:

    Goal1 ; _Goal2 :- Goal1.
    _Goal1 ; Goal2 :- Goal2.


+Goal1 | +Goal2
    Equivalent  to ;/2.    Retained for compatibility  only.   New  code
    should use ;/2.  Still nice though for grammar rules.


+Condition -> +Action
    If-then  and  If-Then-Else.    The  ->/2  construct commits  to  the
    choices  made  at  its  left-hand  side,   destroying  choice-points
    created  inside the  clause (by  ;/2), or  by goals  called by  this
    clause.   Unlike !/0,  the choicepoint of  the predicate as a  whole
    (due  to multiple clauses)  is not destroyed.   The combination  ;/2
    and ->/2  is defines as:

    If -> Then; _Else :- If, !, Then.
    If -> _Then; Else :- !, Else.
    If -> Then :- If, !, Then.

    Note   that  the   operator  precedence  relation   between  ;   and
    ->   ensure  If -> Then ; Else  is  actually  a  term  of  the  form
    ;(->(If, Then), Else).     The  first  two  clauses  belong  to  the
    definition of ;/2), while only the last defines ->/2 .


+Condition *-> +Action ; +Else
    This  construct implements  the so-called `soft-cut'.   The  control
    is  defined as follows:   If Condition succeeds  at least once,  the
    semantics  is the same  as (Condition, Action).   If Condition  does
    not  succeed, the semantics is that of (Condition, Else).   In other
    words,  If Condition succeeds  at least once,  simply behave as  the
    conjunction of Condition and Action, otherwise execute Else.


\+ +Goal
    Succeeds  if  `Goal'  cannot  be  proven (mnemonic:    +  refers  to
    provable  and  the  backslash  (\)  is  normally  used  to  indicate
    negation).


3.8 Meta-Call Predicates

Meta call  predicates are used  to call terms  constructed at run  time.
The basic meta-call mechanism offered by SWI-Prolog is  to use variables
as a  subclause (which  should of  course be bound  to a  valid goal  at
runtime).   A  meta-call is  slower than a  normal call  as it  involves
actually searching the database at runtime for the  predicate, while for
normal calls this search is done at compile time.


call(+Goal)
    Invoke  Goal as a  goal.   Note that clauses  may have variables  as
    subclauses,  which is identical to call/1, except when  the argument
    is bound to the cut.  See !/0.


call(+Goal, +ExtraArg1, ...)
    Append  ExtraArg1, ExtraArg2,  ...   to  the argument  list of  Goal
    and  call the result.   For  example, call(plus(1), 2, X) will  call
    plus/3, binding X to 3.

    The call/[2..]   construct is handled by the compiler, which implies
    that  redefinition as  a predicate has  no effect.   The  predicates
    call/[2-6]  are defined as true  predicates, so they can be  handled
    by interpreted code.


apply(+Term, +List)
    Append  the members of  List to the arguments  of Term and call  the
    resulting  term.   For  example:   apply(plus(1), [2, X]) will  call
    plus(1, 2, X).   apply/2 is  incorporated in the virtual machine  of
    SWI-Prolog.   This implies that the overhead can be compared  to the
    overhead  of call/1.  New code should use call/[2..]   if the length
    of List is  fixed, which is more widely supported and faster because
    there is no need to build and examine the argument list.


not(+Goal)
    Succeeds  when Goal cannot  be proven.   Retained for  compatibility
    only.  New code should use \+/1.


once(+Goal)
    Defined as:

    once(Goal) :-
            Goal, !.

    once/1  can  in  many  cases  be replaced  with  ->/2.     The  only
    difference  is how the  cut behaves  (see !/0).   The following  two
    clauses are identical:

    1) a :- once((b, c)), d.
    2) a :- b, c -> d.


ignore(+Goal)
    Calls  Goal  as once/1,  but succeeds,  regardless  of whether  Goal
    succeeded or not.  Defined as:

    ignore(Goal) :-
            Goal, !.
    ignore(_).


call_with_depth_limit(+Goal, +Limit, -Result)
    If  Goal can be proven  without recursion deeper than Limit  levels,
    call_with_depth_limit/3 succeeds,  binding  Result  to  the  deepest
    recursion  level  used  during the  proof.    Otherwise,  Result  is
    unified  with depth_limit_exceeded  if the limit was exceeded  during
    the  proof,  or the  entire predicate  fails if  Goal fails  without
    exceeding Limit.

    The  depth-limit is guarded by the internal machinery.   This differ
    from  the  depth  computed  based  on a  theoretical  model.     For
    example,  true/0  is  translated  into an  inlined  virtual  machine
    instruction.   Also, repeat/0 is not implemented as below, but  as a
    non-deterministic foreign predicate.

    repeat.
    repeat :-
            repeat.

    As  a  result, call_with_depth_limit/3 may  still loop  inifitly  on
    programs  that should  theoretically finish  in finite time.    This
    problem  can be cured by  using Prolog equivalents to such  built-in
    predicates.

    This   predicate  may  be   used  for  theorem-provers  to   realise
    techniques  like iterrative  deepening.   It  was implemented  after
    discussion with Steve Moyle smoyle@ermine.ox.ac.uk.


3.9 ISO compliant Exception handling

SWI-Prolog defines the predicates catch/3 and throw/1  for ISO compliant
raising  and catching  of exceptions.    In  the current  implementation
(2.9.0), only part of  the built-in predicates generate exceptions.   In
general, exceptions are implemented for I/O and arithmetic.


catch(:Goal, +Catcher, :Recover)
    Behaves  as call/1 if  no exception is  raised when executing  Goal.
    If  a exception  is raised  using throw/1 while  Goal executes,  and
    the  Goal is the innermost goal  for which Catcher unifies with  the
    argument  of throw/1,  all choicepoints generated  by Goal are  cut,
    and Recover is called as in call/1.

    The  overhead of calling a  goal through catch/3 is very  comparable
    to call/1.  Recovery from an exception has a similar overhead.


throw(+Exception)
    Raise an exception.   The system will look for the innermost catch/3
    ancestor  for which Exception unifies  with the Catcher argument  of
    the catch/3 call.  See catch/3 for details.

    If  there is no  catch/3 willing to catch  the error in the  current
    Prolog  context,  the  toplevel  (prolog/0) catches  the  error  and
    prints  a  warning  message.    If  an  exception was  raised  in  a
    callback  from C (see  chapter 5), PL_next_solution() will fail  and
    the exception context can be retrieved using PL_exception().


3.9.1 Debugging and exceptions

Before  the introduction  of exceptions  in SWI-Prolog  a runtime  error
was handled  by printing  an error  message, after  which the  predicate
failed.   If  the prolog_flag (see current_prolog_flag/2) debug_on_error
was in effect  (default), the tracer was  switched on.  The  combination
of the  error message and trace  information is generally sufficient  to
locate the error.

With exception handling,  things are different.   A programmer may  wish
to trap an  exception using catch/3 to avoid  it reaching the user.   If
the  exception is  not handled  by user-code,  the interactive  toplevel
will trap it to prevent termination.

If  we  do  not  take  special  precautions,   the  context  information
associated with  an unexpected exception (i.e.  a programming error)  is
lost.  Therefore,  if an exception is raised, which is not  caught using
catch/3 and the toplevel is running, the error will  be printed, and the
system will enter trace mode.

If the system  is in an non-interactive  callback from foreign code  and
there is no catch/3  active in the current context, it  cannot determine
whether or  not the  exception will  be caught by  the external  routine
calling Prolog.    It will  then base  its behaviour on  the prolog_flag
debug_on_error:

  o current_prolog_flag(debug_on_error, false)
    The  exception does  not trap the  debugger and  is returned to  the
    foreign  routine  calling Prolog,  where it  can  be accessed  using
    PL_exception().  This is the default.

  o current_prolog_flag(debug_on_error, true)
    If the exception is  not caught by Prolog in the current context, it
    will trap the tracer to help analysing the context of the error.

While looking for the  context in which an exception takes place,  it is
adviced to switch on debug mode using the predicate debug/0.


3.9.2 The exception term

Builtin  predicates  generates exceptions  using  a  term  error(Formal,
Context).    The  first argument  is  the  `formal' description  of  the
error,  specifying the class  and generic  defined context  information.
When applicable,  the ISO  error-term definition  is used.   The  second
part  describes some  additional context  to help  the programmer  while
debugging.    In its  most  generic form  this  is a  term of  the  form
context(Name/Arity,  Message), where Name/Arity  describes the  built-in
predicate  that raised  the error,  and Message  provides an  additional
description of the error.  Any part of this structure  may be a variable
if no information was present.


3.9.3 Printing messages

The predicate print_message/2 may be  used to print a message term  in a
human readable  format.   The other predicates  from this section  allow
the user  to refine  and extend  the message system.    The most  common
usage of  print_message/2 is to  print error  messages from  exceptions.
The code below  prints errors encountered during the execution  of Goal,
without  further propagating  the  exception  and without  starting  the
debugger.

        ...,
        catch(Goal, E,
              ( print_message(error, E),
                fail
              )),
        ...

Another common  use is to  defined message_hook/3 for printing  messages
that are normally silent, suppressing messages,  redirecting messages or
make something happen in addition to printing the message.


print_message(+Kind, +Term)
    The  predicate print_message/2 is  used to  print messages,  notably
    from  exceptions  in  a human-readable  format.    Kind  is  one  of
    informational,   banner,  warning,  error,   help  or  silent.     A
    human-readable message is printed to the stream user_error.

    If  the prolog flag  (see current_prolog_flag/2) verbose is  silent,
    messages  with Kind informational, or banner are treated  as silent.
    See -q.

    This  predicate first translates  the Term into  a list of  `message
    lines'  (see print_message_lines/3for  details).  Next it  will call
    the hook  message_hook/3to allow the  user intercepting the message.
    If  message_hook/3 fails it  will print the  message unless Kind  is
    silent.

    The  print_message/2  predicate  and  its  rules  are  in  the  file
    <plhome>/boot/messages.pl,   which  may   be   inspected  for   more
    information on the error messages and related error terms.

    See also message_to_string/2.


print_message_lines(+Stream, +Prefix, +Lines)
    Print a  message (see print_message/2) that has  been translated to a
    list of message elements.  The elements of this list are:

    <Format>-<Args>
         Where Format is an atom and Args is a list  of format argument.
         Handed to format/3.

    flush
         If this  appears as the  last element,  Stream is flushed  (see
         flush_output/1) and no final newline is generated.

    at_same_line
         If this  appears as  first element,  no prefix  is printed  for
         the first line  and the line-position is  not forced to 0  (see
         format/1, ~N).

    <Format>
         Handed to format/3 as format(Stream, Format, []).

    nl
         A new line  is started and if  the message is not complete  the
         Prefix is printed too.

    See also print_message/2 and message_hook/3.


message_hook(+Term, +Kind, +Lines)
    Hook  predicate that may be define  in the module user to  intercept
    messages  from  print_message/2.    Term  and Kind  are the  same  as
    passed to  print_message/2.  Lines is a  list of format statements as
    described with print_message_lines/3.  See also message_to_string/2.

    This  predicate should  be defined  dynamic and  multifile to  allow
    other modules defining clauses for it too.


message_to_string(+Term, -String)
    Translates  a message-term  into a  string object  (see section  ??.
    Primarily  intended  to  write  messages to  Windows  in  XPCE  (see
    section 1.5) or other GUI environments.


3.10 Handling signals

As  of  version  3.1.0,   SWI-Prolog  is  capable  to   handle  software
interrupts  (signals) in  Prolog as  well as  in foreign  (C) code  (see
section 5.6.12).

Signals are used to handle internal errors (execution  of a non-existing
CPU  intruction,  arithmetic  domain  errors,   illegal  memory  access,
resource  overflow,   etc.),  as   well  as  for  dealing   asynchronous
inter-process communication.

Signals  are  defined  by  the Posix  standard  and  part  of  all  Unix
machines.    The  MS-Windows  Win32  provides  a subset  of  the  signal
handling  routines, lacking  the vital  funtionality to  raise a  signal
in  another   thread  for   achieving  asynchronous  inter-process   (or
inter-thread) communication (Unix kill() function).


on_signal(+Signal, -Old, :New)
    Determines  the reaction on  Signal.   Old is  unified with the  old
    behaviour, while the behaviour  is switched to New.  As with similar
    environment-control  predicates,  the  current  value  is  retrieved
    using on_signal(Signal, Current, Current).

    The  action  description  is  an  atom  denoting  the  name  of  the
    predicate  that  will be  called  if Signal  arrives.    on_signal/3
    is  a meta predicate,  which implies that <Module>:<Name>  refers the
    <Name>/1 in the module <Module>.

    Two  predicate-names have  special meaning.    throw implies  Prolog
    will  map  the  signal  onto  a Prolog  exception  as  described  in
    section  3.9.   default resets  the handler  to the settings  active
    before SWI-Prolog manipulated the handler.

    Signals  bound   to  a  foreign  function  through  PL_signal()  are
    reported using the term $foreign_function(Address).

    After  receiving a signal mapped to throw, the exception  raised has
    the structure

         error(signal(<SigName>, <SigNum>), <Context>)

    One possible usage of  this is, for example, to limit the time spent
    on  proving a goal.  This  requires a little C-code for  setting the
    alarm timer (see chapter 5):

    #include <SWI-Prolog.h>
    #include <unistd.h>

    foreign_t
    pl_alarm(term_t time)
    { double t;

      if ( PL_get_float(time, &t) )
      { alarm((long)(t+0.5));

        PL_succeed;
      }

      PL_fail;
    }

    install_t
    install()
    { PL_register_foreign("alarm", 1, pl_alarm, 0);
    }

    Next, we can define the following Prolog code:

    :- load_foreign_library(alarm).

    :- on_signal(alrm, throw).

    :- module_transparent
            call_with_time_limit/2.

    call_with_time_limit(Goal, MaxTime) :-
            alarm(MaxTime),
            catch(Goal, error(signal(alrm, _), _), fail), !,
            alarm(0).
    call_with_time_limit(_, _) :-
            alarm(0),
            fail.

    The  signal names are  defined by the  C-Posix standards as  symbols
    of  the form  SIG_<SIGNAME>.    The Prolog name  for a  signal is the
    lowercase  version of <SIGNAME>.  The predicate current_signal/3 may
    be used to map between names and signals.

    Initially,  some  signals  are  mapped to  throw,  while  all  other
    signals  are default.    The following signals  throw an  exception:
    ill, fpe, segv, pipe, alrm, bus, xcpu, xfsz and vtalrm.


current_signal(?Name, ?Id, ?Handler)
    Enumerate  the  currently defined  signal  handling.   Name  is  the
    signal  name,  Id is  the numerical  identifier and  Handler is  the
    currently defined handler (see on_signal/3).


3.10.1 Notes on signal handling

Before  deciding  to deal  with  signals  in  your  application,  please
consider the following:

  o Portibility
    On MS-Windows, the  signal interface is severely limited.  Different
    Unix  brands support  different sets  of signals,  and the  relation
    between signal name and number may vary.

  o Safety
    Signal   handling   is   not   completely  safe   in   the   current
    implementation,  especially  if throw  is used  in combination  with
    external  foreign  code.    The  system  will use  the  C  longjmp()
    construct  to  direct control  to the  innermost PL_next_solution(),
    thus  forcing an external procedure to be abandoned at  an arbitrary
    moment.   Most likely not all SWI-Prologs own foreign code  is (yet)
    safe too.

  o Garbage Collection
    The  garbage  collector  will block  all  signals that  are  handled
    by  Prolog.    While handling  a  signal, the  garbage-collector  is
    disabled.

  o Time of delivery
    Normally  delivery  is immediate  (or as  defined  by the  operating
    system  used).   Signals are blocked  when the garbage collector  is
    active,  and internally delayed if they occur within in  a `critical
    section'.  The critical sections are generally very short.


3.11 The `block' control-structure

The  block/3 predicate  and  friends  have been  introduced  before  ISO
compatible  catch/3  exception  handling  for  compatibility  with  some
Prolog implementation.    The only  feature not covered  by catch/3  and
throw/1 is the posibility  to execute global cuts.  New code  should use
catch/3 and throw/1 to deal with exceptions.


block(+Label, +Goal, -ExitValue)
    Execute  Goal in a block.   Label is the name  of the block.   Label
    is  normally an  atom, but  the system imposes  no type  constraints
    and  may even be a variable.   ExitValue is normally unified  to the
    second argument of an exit/2 call invoked by Goal.


exit(+Label, +Value)
    Calling  exit/2 makes the innermost block which Label  unifies exit.
    The  block's ExitValue is unified with  Value.  If this  unification
    fails the block fails.


fail(+Label)
    Calling  fail/1 makes the innermost  block which Label unifies  fail
    immediately.  Implemented as

    fail(Label) :- !(Label), fail.


!(+Label)
    Cut  all  choice-points created  since the  entry  of the  innermost
    block which Label unifies.


3.12 DCG Grammar rules

Grammar rules  form a comfortable interface  to difference-lists.   They
are designed  both to support  writing parsers  that build a  parse-tree
from a list as for  generating a flat list from a term.   Unfortunately,
Definite  Clause  Grammar (DCG)  handling  is  not part  of  the  Prolog
standard.   Most Prolog  engines implement DCG,  but the details  differ
slightly.

Grammar rules  look like  ordinary clauses  using -->/2  for  separating
the head  and body rather then  :-/2.   Expanding grammar rules is  done
by expand_term/2, which  adds two additional  argument to each term  for
representing the difference list.   We will illustrate the  behaviour by
defining a rule-set for parsing an integer.

integer(I) -->
        digit(D0),
        digits(D),
        { number_chars(I, [D0|D])
        }.

digits([D|T]) -->
        digit(D), !,
        digits(T).
digits([]) -->
        [].

digit(D) -->
        [D],
        { code_type(D, digit)
        }.

The  body of  a grammar  rule  can contain  three  types of  terms.    A
compound  term interpreted  as a  reference  to a  grammar-rule.    Code
between {...}  is interpreted  as a  reference to  ordinary Prolog  code
and finally,  a list  is interpreted  as a  sequence of literals.    The
Prolog control-constructs (\+/1,  ->/2, ;//2, ,/2  and !/0) can be  used
in grammar rules.

Grammar rule-sets are  called using the builtin predicates phrase/2  and
phrase/3:


phrase(+RuleSet, +InputList)
    Equivalent to phrase(RuleSet, InputList, []).


phrase(+RuleSet, +InputList, -Rest)
    Activate  the rule-set with given name.  `InputList' is the  list of
    tokens to parse,  `Rest' is unified with the remaining tokens if the
    sentence is parsed  correctly.  The example below calls the rule-set
    `integer' defined above.

    ?- phrase(integer(X), "42 times", Rest).

    X = 42
    Rest = [32, 116, 105, 109, 101, 115]


3.13 Database

SWI-Prolog offers  three different database mechanisms.   The first  one
is  the common  assert/retract  mechanism  for manipulating  the  clause
database.    As facts  and clauses  asserted using  assert/1  or one  of
its  derivatives become  part of  the program  these predicates  compile
the term  given to them.    retract/1 and retractall/1  have to unify  a
term and  therefore have to  decompile the program.   For these  reasons
the assert/retract  mechanism is  expensive.   On the  other hand,  once
compiled, queries to the database are faster than  querying the recorded
database discussed below.  See also dynamic/1.

The second way of  storing arbitrary terms in the database is  using the
``recorded database''.   In  this database terms  are associated with  a
key.  A key can be an atom, integer or term.   In the last case only the
functor and  arity determine the  key.   Each key has  a chain of  terms
associated with it.   New terms  can be added either  at the head or  at
the tail  of this  chain.   This mechanism  is considerably faster  than
the assert/retract mechanism as terms are not compiled,  but just copied
into the heap.

The third mechanism is a special purpose one.   It associates an integer
or atom with  a key, which is  an atom, integer or  term.  Each key  can
only have  one atom or integer  associated with it.   It is faster  than
the mechanisms  described above, but  can only be  used to store  simple
status information like counters, etc.


abolish(:PredicateIndicator)
    Removes  all clauses of a  predicate with functor Functor and  arity
    Arity  from  the  database.    All  predicate  attributes  (dynamic,
    multifile,  index, etc.)  are  reset to their defaults.   Abolishing
    an  imported predicate only removes  the import link; the  predicate
    will keep its old definition in its definition module.

    According  to the  ISO standard,  abolish/1 can only  be applied  to
    dynamic  procedures.    This is  odd, as  for  dealing with  dynamic
    procedures  there  is  already  retract/1 and  retractall/1.     The
    abolish/1  predicate has been introduced in DEC-10  Prolog precisely
    for dealing with  static procedures.  In SWI-Prolog, abolish/1 works
    on static procedures, unless the prolog flag iso is set to true.

    It  is adviced  to use  retractall/1 for  erasing all  clauses of  a
    dynamic predicate.


abolish(+Name, +Arity)
    Same  as abolish(Name/Arity).   The predicate abolish/2 conforms  to
    the Edinburgh standard, while abolish/1 is ISO compliant.


redefine_system_predicate(+Head)
    This  directive  may be  used  both in  module  user and  in  normal
    modules to redefine  any system predicate.  If the system definition
    is  redefined in  module user,  the  new definition  is the  default
    definition  for  all sub-modules.    Otherwise  the redefinition  is
    local  to the module.   The system definition remains in the  module
    system.

    Redefining   system   predicate   facilitates  the   definition   of
    compatibility packages.  Use in other context is discouraged.


retract(+Term)
    When  Term  is an  atom  or a  term  it is  unified with  the  first
    unifying  fact or clause  in the database.   The  fact or clause  is
    removed from the database.


retractall(+Head)
    All  facts or  clauses in the  database for  which the head  unifies
    with Head are removed.


assert(+Term)
    Assert  a fact or clause in the  database.  Term is asserted  as the
    last fact or clause of the corresponding predicate.


asserta(+Term)
    Equivalent  to assert/1,  but Term  is asserted as  first clause  or
    fact of the predicate.


assertz(+Term)
    Equivalent to assert/1.


assert(+Term, -Reference)
    Equivalent  to  assert/1, but  Reference is  unified  with a  unique
    reference  to the asserted clause.  This key can later be  used with
    clause/3 or erase/1.


asserta(+Term, -Reference)
    Equivalent  to assert/2,  but Term  is asserted as  first clause  or
    fact of the predicate.


assertz(+Term, -Reference)
    Equivalent to assert/2.


recorda(+Key, +Term, -Reference)
    Assert  Term  in the  recorded  database  under key  Key.    Key  is
    an  integer,  atom or  term.   Reference  is unified  with a  unique
    reference to the record (see erase/1).


recorda(+Key, +Term)
    Equivalent to recorda(Key, Value,  _).


recordz(+Key, +Term, -Reference)
    Equivalent to recorda/3, but  puts the Term at the tail of the terms
    recorded under Key.


recordz(+Key, +Term)
    Equivalent to recordz(Key, Value,  _).


recorded(+Key, -Value, -Reference)
    Unify  Value  with the  first  term recorded  under Key  which  does
    unify.    Reference  is  unified with  the  memory location  of  the
    record.


recorded(+Key, -Value)
    Equivalent to recorded(Key, Value,  _).


erase(+Reference)
    Erase  a  record or  clause from  the  database.   Reference  is  an
    integer  returned by  recorda/3 or  recorded/3, clause/3,  assert/2,
    asserta/2  or assertz/2.    Other integers might  conflict with  the
    internal  consistency of the system.  Erase can only be  called once
    on  a record or clause.  A second call also might  conflict with the
    internal consistency of the system.


flag(+Key, -Old, +New)
    Key  is an  atom, integer or  term.   Unify Old  with the old  value
    associated  with Key.  If the key is used for the first  time Old is
    unified  with the integer  0.   Then store the  value of New,  which
    should  be an integer, float,  atom or arithmetic expression,  under
    Key.   flag/3 is a very  fast mechanism for storing simple  facts in
    the database.  Example:

    :- module_transparent succeeds_n_times/2.

    succeeds_n_times(Goal, Times) :-
            (   flag(succeeds_n_times, Old, 0),
                Goal,
                   flag(succeeds_n_times, N, N+1),
                fail
            ;   flag(succeeds_n_times, Times, Old)
            ).


3.13.1 Update view

Traditionally,  Prolog systems  used  the immediate  update view:    new
clauses  became   visible  to   predicates  backtracking  over   dynamic
predicates   immediately   and  retracted   clauses   became   invisible
immediately.

Starting  with SWI-Prolog  3.3.0  we  adhere the  logical  update  view,
where backtrackable predicates that enter the definition  of a predicate
will not  see any changes  (either caused by  assert/1 or retract/1)  to
the  predicate.    This view  is the  ISO  standard, the  most  commonly
used and  the most  `safe'.   Logical  updates are  realised by  keeping
reference-counts on  predicates and  generation information on  clauses.
Each change  to the database  causes an increment  of the generation  of
the database.  Each  goal is tagged with the generation in which  it was
started.  Each  clause is flagged with the generation it was  created as
well as  the generation  it was  erased.   Only  clauses with  `created'
...`erased' interval  that encloses the generation  of the current  goal
are considered visible.


3.13.2 Indexing databases

By  default,   SWI-Prolog,  as   most  other  implementations,   indexes
predicates  on their  first argument.    SWI-Prolog  allows indexing  on
other and multiple arguments using the declaration index/1.

For advanced database indexing, it defines hash_term/2:


hash_term(+Term, -HashKey)
    If  Term is a ground term (see ground/1), HashKey is unified  with a
    positive integer value that  may be used as a hash-key to the value.
    If  Term is not ground, the predicate succeeds  immediately, leaving
    HashKey an unbound variable.

    This  predicate  may be  used to  build hash-tables  as  well as  to
    exploit argument-indexing to find complex terms more quickly.

    The  hash-key does not rely on temporary information  like addresses
    of  atoms and may be assumed constant over different  invocations of
    SWI-Prolog.


3.14 Declaring predicates properties

This  section  describes  directives  which  manipulate   attributes  of
predicate  definitions.     The  functors  dynamic/1,   multifile/1  and
discontiguous/1  are  operators  of  priority  1150  (see  op/3),  which
implies  the  list of  predicates  they  involve  can just  be  a  comma
separated list:

:- dynamic
        foo/0,
        baz/2.

On SWI-Prolog all  these directives are just  predicates.  This  implies
they can also  be called by a program.   Do not rely on this  feature if
you want to maintain portability to other Prolog implementations.


dynamic +Functor/+Arity, ...
    Informs  the interpreter  that  the definition  of the  predicate(s)
    may  change  during  execution (using  assert/1  and/or  retract/1).
    Currently  dynamic/1  only stops  the interpreter  from  complaining
    about  undefined  predicates  (see  unknown/2).     Future  releases
    might  prohibit  assert/1  and retract/1  for  not-dynamic  declared
    procedures.


multifile +Functor/+Arity, ...
    Informs  the system that the  specified predicate(s) may be  defined
    over  more than one  file.  This  stops consult/1 from redefining  a
    predicate when a new definition is found.


discontiguous +Functor/+Arity, ...
    Informs  the system that the  clauses of the specified  predicate(s)
    might not be together in the source file.  See also style_check/1.


index(+Head)
    Index  the clauses  of the predicate  with the  same name and  arity
    as  Head  on the  specified arguments.    Head is  a  term of  which
    all  arguments  are  either  `1' (denoting  `index  this  argument')
    or  `0'  (denoting `do  not index  this argument').    Indexing  has
    no  implications  for the  semantics of  a  predicate, only  on  its
    performance.    If  indexing is  enabled on  a  predicate a  special
    purpose  algorithm  is used  to select  candidate  clauses based  on
    the  actual arguments of  the goal.   This algorithm checks  whether
    indexed  arguments might  unify in  the clause  head.   Only  atoms,
    integers  and  functors   (e.g.  name  and  arity  of  a  term)  are
    considered.    Indexing  is  very useful  for predicates  with  many
    clauses representing facts.

    Due to the  representation technique used at most 4 arguments can be
    indexed.  All  indexed arguments should be in the first 32 arguments
    of  the predicate.    If  more than  4 arguments  are specified  for
    indexing only the first  4 will be accepted.  Arguments above 32 are
    ignored for indexing.

    By  default  all predicates  with <arity> 1  are indexed  on their
    first  argument.  It is possible to redefine indexing  on predicates
    that  already have clauses attached to  them.  This will initiate  a
    scan through the  predicates clause list to update the index summary
    information stored with each clause.

    If---for  example---one wants to  represents sub-types using a  fact
    list  `sub_type(Sub, Super)' that should be  used both to  determine
    sub- and super types one should declare sub_type/2 as follows:

    :- index(sub_type(1, 1)).

    sub_type(horse, animal).
    ...
    ...


3.15 Examining the program


current_atom(-Atom)
    Successively unifies Atom with  all atoms known to the system.  Note
    that  current_atom/1 always succeeds if  Atom is instantiated to  an
    atom.


current_functor(?Name, ?Arity)
    Successively unifies Name  with the name and Arity with the arity of
    functors known to the system.


current_flag(-FlagKey)
    Successively  unifies  FlagKey with  all keys  used  for flags  (see
    flag/3).


current_key(-Key)
    Successively  unifies  Key  with  all keys  used  for  records  (see
    recorda/3, etc.).


current_predicate(?Name, ?Head)
    Successively  unifies Name  with  the name  of predicates  currently
    defined and Head with  the most general term built from Name and the
    arity of the  predicate.  This predicate succeeds for all predicates
    defined  in the specified module, imported  to it, or in one  of the
    modules from which the predicate will be imported if it is called.


predicate_property(?Head, ?Property)
    Succeeds  if Head refers to a predicate that has  property Property.
    Can  be used  to test whether  a predicate  has a certain  property,
    obtain  all properties known  for Head,  find all predicates  having
    property  or  even obtaining  all  information available  about  the
    current program.  Property is one of:

    interpreted
         Is true if the predicate is defined in Prolog.   We return true
         on this because, although the code is actually  compiled, it is
         completely transparent, just like interpreted code.

    built_in
         Is true  if the predicate  is locked  as a built-in  predicate.
         This implies it  cannot be redefined  in its definition  module
         and it can normally not be seen in the tracer.

    foreign
         Is true if the predicate is defined in the C language.

    dynamic
         Is  true  if  the  predicate  is  declared  dynamic  using  the
         dynamic/1 declaration.

    multifile
         Is  true if  the  predicate  is declared  multifile  using  the
         multifile/1 declaration.

    undefined
         Is  true if  a procedure  definition  block for  the  predicate
         exists, but there are no  clauses in it and it is  not declared
         dynamic.   This is  true if  the predicate occurs  in the  body
         of a  loaded predicate,  an attempt  to call it  has been  made
         via one  of the  meta-call predicates  or the  predicate had  a
         definition in  the past.   See  the library  package check  for
         example usage.

    transparent
         Is true  if the  predicate  is declared  transparent using  the
         module_transparent/1 declaration.

    exported
         Is true if the predicate  is in the public list of  the context
         module.

    imported_from(Module)
         Is true if  the predicate is  imported into the context  module
         from module Module.

    indexed(Head)
         Predicate is indexed (see index/1) according to Head.   Head is
         a term  whose name and  arity are  identical to the  predicate.
         The arguments are unified  with `1' for indexed arguments,  `0'
         otherwise.

    file(FileName)
         Unify FileName  with the name  of the  sourcefile in which  the
         predicate is defined.  See also source_file/2.

    line_count(LineNumber)
         Unify LineNumber with  the line number  of the first clause  of
         the predicate.   Fails if the predicate is not  associated with
         a file.  See also source_file/2.

    number_of_clauses(ClauseCount)
         Unify ClauseCount to the number of clauses  associated with the
         predicate.  Fails for foreign predicates.


dwim_predicate(+Term, -Dwim)
    `Do What I Mean'  (`dwim') support predicate.  Term is a term, which
    name  and arity  are used  as a predicate  specification.   Dwim  is
    instantiated  with the  most general  term built from  Name and  the
    arity  of a defined predicate  that matches the predicate  specified
    by  Term  in the  `Do What  I Mean'  sense.    See dwim_match/2  for
    `Do  What I Mean' string matching.   Internal system predicates  are
    not generated,  unless style_check(+dollar)is active.   Backtracking
    provides all alternative matches.


clause(?Head, ?Body)
    Succeeds  when  Head can  be unified  with a  clause  head and  Body
    with  the  corresponding clause  body.    Gives alternative  clauses
    on  backtracking.     For  facts  Body  is  unified  with  the  atom
    true.   Normally clause/2 is  used to find clause definitions for  a
    predicate,  but it can  also be used to  find clause heads for  some
    body template.


clause(?Head, ?Body, ?Reference)
    Equivalent  to  clause/2,   but  unifies  Reference  with  a  unique
    reference to the  clause (see also assert/2, erase/1).  If Reference
    is  instantiated to a reference the  clause's head and body will  be
    unified with  Head and Body.


nth_clause(?Pred, ?Index, ?Reference)
    Provides  access to  the clauses  of a predicate  using their  index
    number.    Counting  starts at  1.   If  Reference  is specified  it
    unifies  Pred with the  most general term  with the same  name/arity
    as  the predicate  and Index  with the index-number  of the  clause.
    Otherwise  the name  and arity  of Pred  are used  to determine  the
    predicate.    If Index is  provided Reference  will be unified  with
    the  clause  reference.   If  Index  is unbound,  backtracking  will
    yield  both the  indices and the  references of  all clauses of  the
    predicate.  The following example finds the 2nd clause of member/2:

    ?- nth_clause(member(_,_), 2, Ref), clause(Head, Body, Ref).

    Ref = 160088
    Head = system : member(G575, [G578|G579])
    Body = member(G575, G579)


clause_property(+ClauseRef, -Property)
    Queries  properties  of   a  clause.     ClauseRef  is  a  reference
    to   a   clause   as   produced   by   clause/3,   nth_clause/3   or
    prolog_frame_attribute/3.  Property is one of the following:

    file(FileName)
         Unify FileName  with the name  of the  sourcefile in which  the
         clause is defined.  Fails if the clause is not  associated to a
         file.

    line_count(LineNumber)
         Unify LineNumber with the line number of the clause.   Fails if
         the clause is not associated to a file.

    fact
         True if the clause has no body.

    erased
         True if  the  clause has  been erased,  but  not yet  reclaimed
         because it is referenced.


3.16 Input and output

SWI-Prolog provides two  different packages for input  and output.   One
confirms  to  the  Edinburgh  standard.    This  package  has  a  notion
of  `current-input' and  `current-output'.    The  reading  and  writing
predicates implicitly refer  to these streams.   In the second  package,
streams are  opened explicitly and  the resulting handle  is used as  an
argument to the reading  and writing predicate to specify the  source or
destination.   Both packages are fully  integrated; the user may  switch
freely between them.


3.16.1 Input and output using implicit source and destination

The  package  for  implicit input  and  output  destination  is  upwards
compatible to DEC-10 and  C-Prolog.  The reading and  writing predicates
refer to resp.   the current input- and output stream.   Initially these
streams are  connected to the  terminal.  The  current output stream  is
changed using tell/1 or  append/1.  The current input stream  is changed
using see/1.  The streams current value can  be obtained using telling/1
for output- and seeing/1  for input streams.  The table below  shows the
valid stream specifications.  The reserved names user_input, user_output
and user_error are for neat integration with the explicit streams.

           ___________________________________________________
           | user        |This  reserved name  refers to  the|
           |             |terminal                           |
           | user_input  I|nput from the terminal            |

           | user_output O|utput to the terminal             |
           | user_error  U|nix error stream (output only)    |
           | <Atom>      N|ame of a Unix file                |
           |_pipe(<Atom>)N|ame_of_a_Unix_command_____________|_

Source and  destination are  either a file,  one of  the reserved  words
above, or a term  `pipe(Command)'.  In the predicate  descriptions below
we will call the source/destination argument `SrcDest'.   Below are some
examples of source/destination specifications.

       ?- see(data).         % Start reading from file `data'.
       ?- tell(user_error).  % Start writing on the error stream.

       ?- tell(pipe(lpr)).   % Start writing to the printer.

Another example  of using  the pipe/1 construct  is shown  below.   Note
that  the  pipe/1  construct  is  not  part  of  Prolog's  standard  I/O
repertoire.

getwd(Wd) :-
        seeing(Old), see(pipe(pwd)),
        collect_wd(String),
        seen, see(Old),
        atom_codes(Wd, String).

collect_wd([C|R]) :-
        get0(C), C \== -1, !,
        collect_wd(R).
collect_wd([]).


see(+SrcDest)
    Make  SrcDest the  current input  stream.   If  SrcDest was  already
    opened  for  reading  with see/1  and  has  not been  closed  since,
    reading  will be resumed.  Otherwise SrcDest will be opened  and the
    file pointer is positioned at the start of the file.


tell(+SrcDest)
    Make  SrcDest the  current output stream.    If SrcDest was  already
    opened  for writing with tell/1 or append/1 and has not  been closed
    since,  writing will  be resumed.    Otherwise the  file is  created
    or---when existing---truncated.  See also append/1.


append(+File)
    Similar  to tell/1,  but positions the  file pointer  at the end  of
    File  rather than truncating an existing  file.  The pipe  construct
    is not accepted by this predicate.


seeing(?SrcDest)
    Unify the name of the current input stream with SrcDest.


telling(?SrcDest)
    Unify the name of the current output stream with SrcDest.


seen
    Close the current input stream.  The new input stream becomes user.


told
    Close  the current  output stream.   The  new output stream  becomes
    user.


3.16.2 Explicit Input and Output Streams

The predicates  below are  part of the  Quintus compatible  stream-based
I/O package.   In this package streams are explicitly created  using the
predicate open/3.   The resulting stream identifier is then passed  as a
parameter to  the reading and writing  predicates to specify the  source
or destination of the data.


open(+SrcDest, +Mode, -Stream, +Options)
    ISO  compliant predicate  to open  a stream.   SrcDes  is either  an
    atom,  specifying  a  Unix file,  or  a term  `pipe(Command)',  just
    like  see/1 and  tell/1.   Mode  is one  of read,  write, append  or
    update.   Mode  append opens the file  for writing, positioning  the
    file-pointer  at the end.   Mode update opens the file for  writing,
    positioning  the file-pointer at the  beginning of the file  without
    truncating the  file.  See also stream_position/3.   Stream is either
    a variable, in which  case it is bound to an integer identifying the
    stream,  or an  atom, in  which case  this atom will  be the  stream
    identifier.  The Options list can contain the following options:

    type(Type)
         Using type text (default), Prolog will write a  text-file in an
         operating-system compatible way.   Using type binary the  bytes
         will be read  or written without any  translation.  Note  there
         is no difference between the two on Unix systems.

    alias(Atom)
         Gives the  stream a name.   Below  is an example.   Be  careful
         with  this  option  as stream-names  are  global.     See  also
         set_stream/2.

         ?- open(data, read, Fd, [alias(input)]).

                 ...,
                 read(input, Term),
                 ...

    eof_action(Action)
         Defines  what  happens  if the  end  of  the  input  stream  is
         reached.   Action eof_code makes get0/1  and friends return  -1
         and read/1 and friends return the atom end_of_file.  Repetitive
         reading keeps yielding the  same result.  Action error  is like
         eof_code, but repetitive  reading will  raise an error.    With
         action reset,  Prolog will  examine the file  again and  return
         more data if the file has grown.

    buffer(Buffering)
         Defines output  buffering.   The atom  fullf (default)  defines
         full buffering, line buffering  by line, and false implies  the
         stream is  fully unbuffered.   Smaller  buffering is useful  if
         another process or the user is waiting for the output  as it is
         being produced.   See also flush_output/[0,1].  This option  is
         not an ISO option.

    close_on_abort(Bool)
         If  true (default),  the  stream is  closed  on an  abort  (see
         abort/0).   If  false, the  stream is  not closed.    If it  is
         an output  stream,  it will  be flushed  however.   Useful  for
         logfiles and if  the stream is  associated to a process  (using
         the pipe/1 construct).

    The  option reposition is not supported in SWI-Prolog.   All streams
    connected to a file may be repositioned.


open(+SrcDest, +Mode, ?Stream)
    Equivalent to open/4 with an empty option-list.


open_null_stream(?Stream)
    Open  a stream that produces no output.  All counting  functions are
    enabled  on such a stream.   An attempt  to read from a  null-stream
    will  immediately signal  end-of-file.   Similar to Unix  /dev/null.
    Stream can be an atom, giving the null-stream an alias name.


close(+Stream)
    Close the specified stream.   If Stream is not open an error message
    is  displayed.  If the closed stream is the current input  or output
    stream the terminal is made the current input or output.


close(+Stream, +Options)
    Provides  close(Stream, [force(true)]) as the  only option.   Called
    this  way, any resource error  (such as write-errors while  flushing
    the output buffer) are ignored.


stream_property(?Stream, ?StreamProperty)
    ISO  compatible predicate for querying  status of open I/O  streams.
    StreamProperty is one of:

    file_name(Atom)
         If Stream is  associated to a file,  unify Atom to the name  of
         this file.

    mode(IOMode)
         Unify  IOMode to  the  mode given  to  open/4 for  opening  the
         stream.   Values are:  read,  write, append and the  SWI-Prolog
         extension update.

    input
         True if Stream has mode read.

    output
         True if Stream has mode write, append or update.

    alias(Atom)
         If Atom is bound,  test of the stream has the  specified alias.
         Otherwise unify Atom with the first alias of the stream.

    position(Term)
         Unify  Term  with the  current  stream-position.     A  stream-
         position  is  a  term   of  format  $stream_position(CharIndex,
         LineNo, LinePos).  See also term_position/3.

    end_of_stream(E)
         If Stream is  an input stream,  unify E with  one of the  atoms
         not, at or past.  See also at_end_of_stream/[0,1].

    eof_action(A)
         Unify A with one of  eof_code,  reset or error.  See  open/4 for
         details.

    reposition(Bool)
         Unify Bool  with true  if  the position  of the  stream can  be
         set (see seek/4).   It  is assumed the  position can be set  if
         the stream  has a  seek-function and is  not based  on a  POSIX
         file-descriptor that is not associated to a regular file.

    type(T)
         Unify Bool with text or binary.

    file_no(Integer)
         If  the stream  is  associated  with a  POSIX  file-descriptor,
         unify  Integer  with   the  descriptor  number.      SWI-Prolog
         extension used  primarily  for integration  with foreign  code.
         See also Sfileno() from SWI-Stream.h.


current_stream(?Object, ?Mode, ?Stream)
    The  predicate current_stream/3 is  used to access  the status of  a
    stream as well as  to generate all open streams.  Object is the name
    of the file opened  if the stream refers to an open file, an integer
    file-descriptor  if  the  stream  encapsulates  an  operating-system
    stream  or the atom  [] if the stream  refers to some other  object.
    Mode is one of read or write.


set_stream_position(+Stream, +Pos)
    Set  the  current  position  of Stream  to  Pos.    Pos  is  a  term
    as  returned by stream_property/2 using the position(Pos)  property.
    See also seek/4.


seek(+Stream, +Offset, +Method, -NewLocation)
    Reposition the current point  of the given Stream.  Method is one of
    bof,  current or eof, indicating positioning relative to  the start,
    current  point or  end of  the underlying  object.   NewLocation  is
    unified with the new offset, relative to the start of the stream.

    If  the seek  modifies the  current  location, the  line number  and
    character position in the line are set to 0.

    If  the stream cannot be repostioned, a reposition error  is raised.
    The  predicate seek/4 is  compatible to  Quintus Prolog, though  the
    error  conditions  and  signalling  is  ISO compliant.     See  also
    stream_position/3.


set_stream(+Stream, +Attribute)
    Modify  an attribute of  an existing  stream.   Attribute is in  the
    current  implemention  only alias(AliasName)  to  set the  alias  of
    an  already created  stream.   If AliasName  is the name  of one  of
    the  standard  streams is  used,  this stream  is  rebound.    Thus,
    set_stream(S, current_input)  is  the same  as  set_input/1  and  by
    setting the  alias of a stream to user_input,  etc. all user terminal
    input is read from this stream.  See also interactor/0.


3.16.3 Switching Between Implicit and Explicit I/O

The predicates  below can be  used for  switching between the  implicit-
and the explicit stream based I/O predicates.


set_input(+Stream)
    Set  the current input  stream to become Stream.   Thus,  open(file,
    read, Stream), set_input(Stream) is equivalent to see(file).


set_output(+Stream)
    Set the current output stream to become Stream.


current_input(-Stream)
    Get  the current input stream.   Useful to get access to  the status
    predicates associated with streams.


current_output(-Stream)
    Get the current output stream.


3.17 Status of streams


wait_for_input(+ListOfStreams, -ReadyList, +TimeOut)
    Wait  for input on  one of the  streams in ListOfStreams and  return
    a  list  of  streams  on  which input  is  available  in  ReadyList.
    wait_for_input/3 waits  for at most  TimeOut seconds.   Timeout  may
    be  specified as a floating point  number to specify fractions of  a
    second.   If Timeout equals  0, wait_for_input/3waits  indefinitely.
    This  predicate can be used  to implement timeout while reading  and
    to  handle  input from  multiple  sources.   The  following  example
    will  wait for input from the  user and an explicitly opened  second
    terminal.  On return, Inputs may hold user or P4 or both.

    ?- open('/dev/ttyp4', read, P4),
       wait_for_input([user, P4], Inputs, 0).


character_count(+Stream, -Count)
    Unify  Count with the  current character index.   For input  streams
    this  is the number  of characters read since  the open, for  output
    streams  this is the number of characters written.   Counting starts
    at 0.


line_count(+Stream, -Count)
    Unify  Count with the  number of  lines read or  written.   Counting
    starts at 1.


line_position(+Stream, -Count)
    Unify  Count with the position on the current line.  Note  that this
    assumes  the position is 0 after the  open.  Tabs are assumed  to be
    defined on each  8-th character and backspaces are assumed to reduce
    the count by one, provided it is positive.


fileerrors(-Old, +New)
    Define error behaviour  on errors when opening a file for reading or
    writing.   Valid values are the  atoms on (default) and off.   First
    Old  is unified with the current value.   Then the new value  is set
    to New.


3.18 Primitive character I/O

See section 3.2 for an overview of supported character representations.


nl
    Write  a newline character  to the current output  stream.  On  Unix
    systems nl/0 is equivalent to put(10).


nl(+Stream)
    Write a newline to Stream.


put(+Char)
    Write  Char  to  the  current  output  stream,  Char  is  either  an
    integer-expression  evaluating to an ASCII  value (0 Char 255) or
    an atom of one character.


put(+Stream, +Char)
    Write Char to Stream.


put_byte(+Byte)
    Alias for put/1.


put_byte(+Stream, +Byte)
    Alias for put/2


put_char(+Char)
    Alias for put_char/1.


put(+Stream, +Char)
    Alias for put/2


put_code(+Code)
    Alias for put/1.


put_code(+Stream, +Code)
    Alias for put/2


tab(+Amount)
    Writes  Amount  spaces  on  the  current  output  stream.     Amount
    should  be an expression that  evaluates to a positive integer  (see
    section 3.26).


tab(+Stream, +Amount)
    Writes Amount spaces to Stream.


flush_output
    Flush  pending output on current  output stream.   flush_output/0 is
    automatically  generated by  read/1 and derivatives  if the  current
    input stream is user and the cursor is not at the left margin.


flush_output(+Stream)
    Flush  output on the specified stream.  The stream must be  open for
    writing.


ttyflush
    Flush pending output on stream user.  See also flush_output/[0,1].


get_byte(-Byte)
    Read the current input  stream and unify the next byte with Byte (an
    integer between 0 and 255.  Byte is unified with -1 on end of file.


get_byte(+Stream, -Byte)
    Read the next byte from Stream.


get_code(-Code)
    Read  the current  input stream  and unify Code  with the  character
    code  of the  next character.   Char is  unified with  -1 on end  of
    file.  See also get_char/1.


get_code(+Stream, -Code)
    Read the next character-code from Stream.


get_char(-Char)
    Read  the  current  input  stream  and  unify  Char  with  the  next
    character  as a  one-character-atom.   See  also atom_chars/2.    On
    end-of-file, Char is unified to the atom end_of_file.


get_char(+Stream, -Char)
    Unify   Char   with   the   next  character   from   Stream   as   a
    one-character-atom.  See also get_char/2, get_byte/2 and get_code/2.


get0(-Char)
    Edinburgh version of the ISO get_byte/1 predicate.


get0(+Stream, -Char)
    Edinburgh version of the ISO get_byte/2 predicate.


get(-Char)
    Read  the  current   input  stream  and  unify  the  next  non-blank
    character with Char.  Char is unified with -1 on end of file.


get(+Stream, -Char)
    Read the next non-blank character from Stream.


peek_byte(-Byte)
    Reads  the next input byte  like get_byte/1, but does not remove  it
    from the input stream.


peek_byte(+Stream, -Byte)
    Reads  the next input byte  like get_byte/2, but does not remove  it
    from the stream.


peek_code(-Code)
    Reads  the next input code  like get_code/1, but does not remove  it
    from the input stream.


peek_code(+Stream, -Code)
    Reads  the next input code  like get_code/2, but does not remove  it
    from the stream.


peek_char(-Char)
    Reads the  next input character like get_char/1,  but does not remove
    it from the input stream.


peek_char(+Stream, -Char)
    Reads the  next input character like get_char/2,  but does not remove
    it from the stream.


skip(+Char)
    Read the input until  Char or the end of the file is encountered.  A
    subsequent call to get0/1 will read the first character after Char.


skip(+Stream, +Char)
    Skip input (as skip/1) on Stream.


get_single_char(-Char)
    Get  a  single character  from input  stream  `user' (regardless  of
    the  current input stream).   Unlike get0/1 this predicate does  not
    wait  for a  return.   The  character is  not echoed  to the  user's
    terminal.   This predicate is meant for keyboard menu selection etc.
    If SWI-Prolog was  started with the -tty option this predicate reads
    an  entire line of input  and returns the first non-blank  character
    on  this line, or the ASCII code  of the newline (10) if  the entire
    line consisted of blank characters.


at_end_of_stream
    Succeeds  after the last character  of the current input stream  has
    been  read.    Also succeeds  if  there is  no valid  current  input
    stream.


at_end_of_stream(+Stream)
    Succeeds  after the last character of  the named stream is read,  or
    Stream is not a  valid input stream.  The end-of-stream test is only
    available  on buffered  input stream (unbuffered  input streams  are
    rarely used, see open/4).


copy_stream_data(+StreamIn, +StreamOut, +Len)
    Copy Len bytes from stream StreamIn to StreamOut.


copy_stream_data(+StreamIn, +StreamOut)
    Copy data all (remaining) data from stream StreamIn to StreamOut.


3.19 Term reading and writing

This section  describes the basic term  reading and writing  predicates.
The  predicates  term_to_atom/2,  atom_to_term/3  and  sformat/3  provide
means  for translating  atoms and  strings  to terms.    The  predicates
format/[1,2] and writef/2 provide formatted output.

There are  two ways  to manipulate  the output  format.   The  predicate
print/[1,2]  may  be  programmed  using  portray/1.      The  format  of
floating point  numbers may  be manipulated  using the  prolog_flag  (see
current_prolog_flag/2) float_format.

Reading  is  sensitive   to  the  prolog_flag  character_escapes,   which
controls  the interpretation  of the  \ character  in  quoted atoms  and
strings.


write_term(+Term, +Options)
    The  predicate  write_term/2  is  the generic  form  of  all  Prolog
    term-write predicates.  Valid options are:

    quoted(true or false)
         If true, atoms and  functors that needs quotes will  be quoted.
         The default is false.

    character_escapes(true or false)
         If true,  and  quoted(true) is  active,  special characters  in
         quoted atoms and  strings are emitted as ISO  escape-sequences.
         Default is taken from the reference module (see below).

    ignore_ops(true or false)
         If true, the generic term-representation (<functor>(<args> ...))
         will be  used for  all terms,  Otherwise (default),  operators,
         list-notation and  {}/1  will be  written using  their  special
         syntax.

    module(Module)
         Define the reference module  (default user).  This  defines the
         default value for  the character_escapes option as well as  the
         operator definitions to use.  See also op/3.

    numbervars(true or false)
         If true, terms  of the format $VAR(N), where  <N> is a positive
         integer, will be  written as a variable  name.  The default  is
         false.

    portray(true or false)
         If true, the  hook portray/1 is  called before printing a  term
         that is not  a variable.   If portray/1  succeeds, the term  is
         considered printed.  See  also print/1.  The default  is false.
         This option is an extension to the ISO write_term options.

    max_depth(Integer)
         If the term is nested deeper than Integer,  print the remainder
         as eclipse (...).  A 0 (zero) value (default)  imposes no depth
         limit.  This option  also delimits the number of printed  for a
         list.  Example:

         ?- write_term(a(s(s(s(s(0)))), [a,b,c,d,e,f]), [max_depth(3)]).
         a(s(s(...)), [a, b|...])

         Yes

         Used   by  the   toplevel   and   debugger  to   limit   screen
         output.   See also the  prolog-flags toplevel_print_options  and
         debugger_print_options.


write_term(+Stream, +Term, +Options)
    As  write_term/2,  but output  is sent  to  Stream rather  than  the
    current output.


write_canonical(+Term)
    Write  Term on  the current output  stream using standard  parenthe-
    sised prefix notation  (i.e. ignoring operator declarations).  Atoms
    that need quotes are  quoted.  Terms written with this predicate can
    always  be read back,  regardless of current operator  declarations.
    Equivalent to write_term/2 using the options ignore_ops and quoted.


write_canonical(+Stream, +Term)
    Write Term in canonical form on Stream.


write(+Term)
    Write  Term  to the  current output,  using  brackets and  operators
    where  appropriate.     See  current_prolog_flag/2  for  controlling
    floating point output format.


write(+Stream, +Term)
    Write Term to Stream.


writeq(+Term)
    Write  Term  to the  current output,  using  brackets and  operators
    where  appropriate.    Atoms that  need quotes  are quoted.    Terms
    written  with this predicate can  be read back with read/1  provided
    the currently active operator declarations are identical.


writeq(+Stream, +Term)
    Write Term to Stream, inserting quotes.


print(+Term)
    Prints  Term on the  current output stream  similar to write/1,  but
    for each (sub)term  of Term first the dynamic predicate portray/1 is
    called.   If this predicate succeeds print assumes the (sub)term has
    been written.  This allows for user defined term writing.


print(+Stream, +Term)
    Print Term to Stream.


portray(+Term)
    A dynamic predicate, which  can be defined by the user to change the
    behaviour  of print/1 on (sub)terms.   For each subterm  encountered
    that is not  a variable print/1 first calls portray/1 using the term
    as  argument.    For lists  only the  list as  a whole  is given  to
    portray/1.   If portray succeeds  print/1 assumes the term has  been
    written.


read(-Term)
    Read  the next Prolog term from  the current input stream and  unify
    it  with Term.  On a syntax error read/1 displays an  error message,
    attempts  to  skip  the erroneous  term  and  fails.    On  reaching
    end-of-file Term is unified with the atom end_of_file.


read(+Stream, -Term)
    Read Term from Stream.


read_clause(-Term)
    Equivalent  to  read/1,  but  warns  the  user  for  variables  only
    occurring  once in a term  (singleton variables) which do not  start
    with  an underscore  if style_check(singleton) is active  (default).
    Used  to read Prolog source files (see consult/1).  New  code should
    use read_term/2 with the option singletons(warning).


read_clause(+Stream, -Term)
    Read a clause from Stream.  See read_clause/1.


read_term(-Term, +Options)
    Read  a term from the current  input stream and unify the term  with
    Term.    The  reading is  controlled  by options  from the  list  of
    Options.   If this list is  empty, the behaviour is the same  as for
    read/1.   The options are upward compatible to Quintus Prolog.   The
    argument order is  according to the ISO standard.  Syntax-errors are
    always reported using exception-handling (see catch/3).  Options:

    variables(Vars)
         Unify  Vars  with a  list  of  variables  in the  term.     The
         variables appear in  the order they have  been read.  See  also
         free_variables/2.  (ISO).

    variable_names(Vars)
         Unify Vars with a list  of `Name = Var', where Name is  an atom
         describing the variable name and Var is a  variable that shares
         with the corresponding variable in Term.  (ISO).

    singletons(Vars)
         As variable_names,  but only  reports  the variables  occurring
         only  once in  the  Term read.     Variables starting  with  an
         underscore (`_') are not included in this list.  (ISO).

    syntex_errors(Atom)
         If error  (default),  throw and  exception on  a syntax  error.
         Other values  are fail, which  causes a  message to be  printed
         using print_message/2, after which  the predicate fails,  quiet
         which causes  the predicate  to fail silently  and dec10  which
         causes syntax errors to be printed, after which read_term/[2,3]
         continues reading the next term.   Using dec10, read_term/[2,3]
         never fails.  (Quintus, SICStus).

    module(Module)
         Specify  Module  for  operators,   character_escapes   flag  and
         double_quotes flag.  The  value of the latter two is  overruled
         if  the corresponding  read_term/3  option  is provided.     If
         no module  is specified, the  current `source-module' is  used.
         (SWI-Prolog).

    character_escapes(Bool)
         Defines  how  to  read  \  escape-sequences  in  quoted  atoms.
         See the  prolog-flags character_escapes, current_prolog_flag/2.
         (SWI-Prolog).

    double_quotes(Bool)
         Defines  how to  read  "..." strings.    See  the  prolog-flags
         double_quotes, current_prolog_flag/2.  (SWI-Prolog).

    term_position(Pos)
         Unifies Pos with the starting  position of the term read.   Pos
         if of the same format as use by stream_position/3.

    subterm_positions(TermPos)
         Describes the  detailed layout of  the term.   The formats  for
         the various types of terms  if given below.  All  positions are
         character positions.    If  the input  is related  to a  normal
         stream,  these  positions are  relative  to the  start  of  the
         input, when  reading from  the terminal, they  are relative  to
         the start of the term.

         From-To
             Used for primitive types (atoms, numbers, variables).

         string_position(From, To)
             Used  to indicate  the  position of  a string  enclosed  in
             double quotes (").

         brace_term_position(From, To, Arg)
             Term  of  the form  {...},  as used  in  DCG  rules.    Arg
             describes the argument.

         list_position(From, To, Elms, Tail)
             A list.  Elms describes the  positions of the elements.  If
             the list specifies the tail as |<TailTerm>, Tail is unified
             with  the term-position  of the  tail,  otherwise with  the
             atom none.

         term_position(From, To, FFrom, FTo, SubPos)
             Used  for a compound  term not matching  one of the  above.
             FFrom  and  FTo  describe  the  position  of  the  functor.
             SubPos  is a  list,  each element  of  which describes  the
             term-position of the corresponding subterm.


read_term(+Stream, -Term, +Options)
    Read term with options from Stream.  See read_term/2.


read_history(+Show, +Help, +Special, +Prompt, -Term, -Bindings)
    Similar  to read_term/2 using the option variable_names, but  allows
    for history  substitutions.  read_history/6is used  by the top level
    to  read the user's actions.   Show is  the command the user  should
    type  to show  the saved  events.   Help is  the command  to get  an
    overview  of the capabilities.   Special is a list of commands  that
    are  not saved in the  history.  Prompt  is the first prompt  given.
    Continuation  prompts for  more  lines are  determined by  prompt/2.
    A  %w  in the  prompt  is substituted  by  the event  number.    See
    section 2.7 for available substitutions.

    SWI-Prolog calls read_history/6 as follows:

    read_history(h, '!h', [trace], '%w ?- ', Goal, Bindings)


prompt(-Old, +New)
    Set  prompt associated  with read/1  and its  derivatives.   Old  is
    first  unified with the current prompt.  On success the  prompt will
    be  set to New if  this is an atom.   Otherwise an error message  is
    displayed.   A prompt  is printed if one  of the read predicates  is
    called  and the cursor is  at the left margin.   It is also  printed
    whenever  a newline is given and  the term has not been  terminated.
    Prompts are only printed when the current input stream is user.


prompt1(+Prompt)
    Sets  the prompt for the next line  to be read.   Continuation lines
    will be read using the prompt defined by prompt/2.


3.20 Analysing and Constructing Terms


functor(?Term, ?Functor, ?Arity)
    Succeeds  if Term is  a term with  functor Functor and arity  Arity.
    If  Term is a variable  it is unified with  a new term holding  only
    variables.    functor/3 silently  fails on  instantiation faults  If
    Term  is an atom or  number, Functor will  be unified with Term  and
    arity will be unified with the integer 0 (zero).


arg(?Arg, ?Term, ?Value)
    Term  should be instantiated  to a term,  Arg to an integer  between
    1  and  the  arity of  Term.    Value  is  unified with  the  Arg-th
    argument  of Term.   Arg may also  be unbound.   In this case  Value
    will  be unified  with the  successive arguments of  the term.    On
    successful  unification, Arg  is unified  with the argument  number.
    Backtracking  yields alternative  solutions.    The predicate  arg/3
    fails  silently if  Arg= 0 or Arg > arity and raises  the exception
    domain_error(not_less_then_zero, Arg)if Arg <0.


setarg(+Arg, +Term, +Value)
    Extra-logical  predicate.     Assigns the  Arg-th  argument  of  the
    compound  term Term with the given Value.  The assignment  is undone
    if  backtracking brings the  state back into  a position before  the
    setarg/3 call.

    This  predicate may  be used  for destructive  assignment to  terms,
    using them as and extra-logical storage bin.


?Term =.. ?List
    List  is a list which head is the functor of Term and  the remaining
    arguments are the arguments  of the term.  Each of the arguments may
    be  a variable,  but not  both.   This predicate  is called  `Univ'.
    Examples:

    ?- foo(hello, X) =.. List.

    List = [foo, hello, X]

    ?- Term =.. [baz, foo(1)]

    Term = baz(foo(1))


numbervars(+Term, +Functor, +Start, -End)
    Unify  the free variables of Term  with a term constructed from  the
    atom  Functor with one argument.  The argument is the number  of the
    variable.    Counting starts  at Start.    End is  unified with  the
    number that should be given to the next variable.  Example:

    ?- numbervars(foo(A, B, A), this_is_a_variable, 0, End).

    A = this_is_a_variable(0)
    B = this_is_a_variable(1)
    End = 2

    In Edinburgh Prolog the  second argument is missing.  It is fixed to
    be $VAR.


free_variables(+Term, -List)
    Unify  List with  a list of  variables, each  sharing with a  unique
    variable of Term.  For example:

    ?- free_variables(a(X, b(Y, X), Z), L).

    L = [G367, G366, G371]
    X = G367
    Y = G366
    Z = G371


copy_term(+In, -Out)
    Make a copy of  term In and unify the result with Out.  Ground parts
    of  In are  shared by  Out.   Provided In  and Out  have no  sharing
    variables  before  this call  they will  have  no sharing  variables
    afterwards.  copy_term/2 is semantically equivalent to:

    copy_term(In, Out) :-
            recorda(copy_key, In, Ref),
            recorded(copy_key, Out, Ref),
            erase(Ref).


3.21 Analysing and constructing atoms

These predicates  convert between  Prolog constants and  lists of  ASCII
values.   The predicates atom_codes/2, number_codes/2 and name/2  behave
the same  when converting  from a constant  to a  list of ASCII  values.
When  converting the  other way  around, atom_codes/2  will generate  an
atom,  number_codes/2 will generate  a  number or  exception and  name/2
will return a number if possible and an atom otherwise.

The  ISO  standard  defines atom_chars/2  to  describe  the  `broken-up'
atom  as a  list of  one-character atoms  instead of  a  list of  codes.
Upto  version 3.2.x,  SWI-Prolog's atom_chars/2 behaved,  compatible  to
Quintus and  SICStus Prolog,  like atom_codes.    As of 3.3.x  SWI-Prolog
atom_codes/2 and atom_chars/2are compliant to the ISO standard.

To  ease  the pain  of  all  variations in  the  Prolog  community,  all
SWI-Prolog predicates  behave as  flexible as  possible.   This  implies
the  `list-side' accepts  either  a code-list  or  a char-list  and  the
`atom-side' accept all atomic types (atom, number and string).


atom_codes(?Atom, ?String)
    Convert  between an atom  and a list  of ASCII values.   If Atom  is
    instantiated, if will  be translated into a list of ASCII values and
    the  result is unified with String.   If Atom is unbound  and String
    is  a list of  ASCII values, it  will Atom will  be unified with  an
    atom constructed from this list.


atom_chars(?Atom, ?CharList)
    As  atom_codes/2, but  CharList  is a  list of  one-character  atoms
    rather than a list of ASCII values.

    ?- atom_chars(hello, X).

    X = [h, e, l, l, o]


char_code(?Atom, ?ASCII)
    Convert between character and ASCII value for a single character.


number_chars(?Number, ?CharList)
    Similar  to  atom_chars/2,  but  converts between  a number  and  its
    representation  as a  list of  one-character atoms.    Fails with  a
    representation_error  if Number  is unbound  and  CharList does  not
    describe a number.


number_codes(?Number, ?CodeList)
    As  number_chars/2,  but  converts  to a  list  of  character  codes
    (normally  ASCII values) rather  than one-character atoms.   In  the
    mode  -, +, both predicates  behave identically to improve  handling
    of non-ISO source.


name(?AtomOrInt, ?String)
    String  is a  list of ASCII  values describing  Atom.   Each of  the
    arguments may be a  variable, but not both.  When String is bound to
    an  ASCII value list  describing an integer  and Atom is a  variable
    Atom  will be  unified with  the integer value  described by  String
    (e.g.  `name(N, "300"), 400 is N + 100' succeeds).


int_to_atom(+Int, +Base, -Atom)
    Convert  Int to an  ascii representation using  base Base and  unify
    the  result with Atom.   If Base 6=10 the  base will be prepended  to
    Atom.    Base= 0 will try  to interpret  Int as an  ASCII value and
    return  0'<c>.   Otherwise  2 Base 36.    Some examples are  given
    below.

                   int_to_atom(45, 2, A)  -!   A= 20101101
                   int_to_atom(97, 0, A)  -!   A= 00a
                   int_to_atom(56, 10, A) -!   A= 56


int_to_atom(+Int, -Atom)
    Equivalent to int_to_atom(Int, 10, Atom).


term_to_atom(?Term, ?Atom)
    Succeeds  if Atom describes  a term  that unifies with  Term.   When
    Atom  is instantiated Atom is converted and then unified  with Term.
    If  Atom has no  valid syntax,  a syntax_error exception is  raised.
    Otherwise Term is ``written'' on Atom using write/1.


atom_to_term(+Atom, -Term, -Bindings)
    Use  Atom as  input to  read_term/2 using the  option variable_names
    and  return  the read  term in  Term and  the  variable bindings  in
    Bindings.   Bindings is a list of Name =Var couples, thus providing
    access  to the actual  variable names.   See  also read_term/2.   If
    Atom has no valid syntax, a syntax_error exception is raised.


atom_concat(?Atom1, ?Atom2, ?Atom3)
    Atom3  forms the concatenation of Atom1 and Atom2.  At least  two of
    the  arguments must be instantiated  to atoms, integers or  floating
    point  numbers.    For  ISO  compliance,  the  instantiation-pattern
    -, -, +  is allowed  too, non-deterministically  splitting the  3-th
    argument  into two parts  (as append/3  does for lists).   See  also
    string_concat/3.


concat_atom(+List, -Atom)
    List  is  a  list of  atoms,  integers  or floating  point  numbers.
    Succeeds  if  Atom can  be unified  with  the concatenated  elements
    of  List.    If List  has exactly  2 elements  it  is equivalent  to
    atom_concat/3, allowing for variables in the list.


concat_atom(?List, +Separator, ?Atom)
    Creates  an  atom just  like  concat_atom/2, but  inserts  Separator
    between each pair of atoms.  For example:

    ?- concat_atom([gnu, gnat], ', ', A).

    A = 'gnu, gnat'

    This  predicate can  also be  used to split  atoms by  instantiating
    Separator and Atom:

    ?- concat_atom(L, -, 'gnu-gnat').

    L = [gnu, gnat]


atom_length(+Atom, -Length)
    Succeeds  if  Atom is  an atom  of  Length characters  long.    This
    predicate also works  for integers and floats, expressing the number
    of characters output when given to write/1.


atom_prefix(+Atom, +Prefix)
    Succeeds  if  Atom starts  with the  characters from  Prefix.    Its
    behaviour  is equivalent  to ?- concat(Prefix, _, Atom), but  avoids
    the construction of an atom for the `remainder'.


sub_atom(+Atom, ?Before, ?Len, ?After, ?Sub)
    ISO  predicate  for breaking  atoms.    It maintains  the  following
    relation:  Sub is  a sub-atom of Atom that starts at Before, has Len
    characters and Atom contains After characters after the match.

    ?- sub_atom(abc, 1, 1, A, S).

    A = 1, S = b

    The  implementation  minimalises  non-determinism  and  creation  of
    atoms.    This  is a  very flexible  predicate that  can do  search,
    prefix- and suffix-matching, etc.


3.22 Classifying characters

SWI-Prolog   offers  two   comprehensive  predicates   for   classifying
characters and character-codes.  These predicates are  defined as built-
in predicates  to exploit the  C-character classification's handling  of
locale (handling of  local character-sets).  These predicates  are fast,
logical and deterministic if applicable.

In   addition,   there   is   the   library   library(ctype)   providing
compatibility to  some other  Prolog systems.   The  predicates of  this
library are defined in terms of code_type/2.


char_type(?Char, ?Type)
    Tests or generates  alternative Types or Chars.  The character-types
    are inspired by the standard C <ctype.h>  primitives.

    alnum
         Char is a letter (upper- or lowercase) or digit.

    alpha
         Char is a letter (upper- or lowercase).

    csym
         Char  is  a  letter   (upper-  or  lowercase),  digit   or  the
         underscore  (_).     These  are  valid  C-  and  Prolog  symbol
         characters.

    csymf
         Char is a letter  (upper- or lowercase) or the  underscore (_).
         These are valid first characters for C- and Prolog symbols

    ascii
         Char is a 7-bits ASCII character (0..127).

    white
         Char is a space or tab.  E.i. white space inside a line.

    cntrl
         Char is an ASCII control-character (0..31).

    digit
         Char is a digit.

    digit(Weigth)
         Char is a digit with value  Weigth.  I.e. char_type(X, digit(6)
         yields X = '6'.  Useful for parsing numbers.

    xdigit(Weigth)
         Char  is  a  haxe-decimal  digit  with  value  Weigth.     I.e.
         char_type(a, xdigit(X) yields X  = '10'.    Useful for  parsing
         numbers.

    graph
         Char produces  a visible mark  on a  page when printed.    Note
         that the space is not included!

    lower
         Char is a lower-case letter.

    lower(Upper)
         Char is a  lower-case version of Upper.   Only true if Char  is
         lowercase and Upper uppercase.

    to_lower(Upper)
         Char is  a lower-case version  of Upper.   For non-letters,  or
         letter without case, Char and Lower are the same.

    upper
         Char is an upper-case letter.

    upper(Lower)
         Char is an upper-case version  of Lower.  Only true if  Char is
         uppercase and Lower lowercase.

    to_upper(Lower)
         Char is an  upper-case version of Lower.   For non-letters,  or
         letter without case, Char and Lower are the same.

    punct
         Char is  a punctuation character.   This  is a graph  character
         that is not a letter or digit.

    space
         Char  is some  form  of layout  character  (tab,  vertical-tab,
         newline, etc.).

    end_of_file
         Char is -1.

    end_of_line
         Char ends a line (ASCII: 10..13).

    newline
         Char is a the newline character (10).

    period
         Char counts as the end of a sentence (.,!,?).

    quote
         Char is a quote-character (", ', `).

    paren(Close)
         Char is  an  open-parenthesis and  Close is  the  corresponding
         close-parenthesis.


code_type(?Code, ?Type)
    As  char_type/2,  but uses character-codes rather than  one-character
    atoms.     Please note  that  both  predicates are  as  flexible  as
    possible.    They handle  either representation if  the argument  is
    instantiated  and  only will  instantiate with  an  integer code  or
    one-character  atom depending  of the version  used.   See also  the
    prolog-flag double_quotes, atom_chars/2 and atom_codes/2.


3.23 Representing text in strings

SWI-Prolog  supports the  data type  string.   Strings  are  a time  and
space efficient mechanism  to handle text text  in Prolog.  Strings  are
stores as a byte array on the global (term) stack  and thus destroyed on
backtracking and reclaimed by the garbage collector.

Strings were  added to  SWI-Prolog based on  an early  draft of the  ISO
standard, offerring  a mechanism to  represent temporary character  data
efficiently.    As  SWI-Prolog  strings can  handle  0-bytes,  they  are
frequently used through  the foreign language interface (section 5)  for
storing arbitrary byte-sequences.

Starting with version  3.3, SWI-Prolog offers garbage collection  on the
atom-space as well as  representing 0-bytes in atoms.   Although strings
and  atoms still  have  different  features,  new code  should  consider
using  atoms to  avoid too  many  representations for  text as  well  as
for  compatibility to  other Prolog  systems.   Below  are  some of  the
differences:

  o creation
    Creating  strings  is fast,  as the  data is  simply  copied to  the
    global  stack.   Atoms are  unique and  therefore more expensive  in
    terms  of memory  and time to  create.   On the other  hand, if  the
    same  text has  to be  represented  multiple times,  atoms are  more
    efficient.

  o destruction
    Backtracking destroys strings at  no cost.  They are cheap to handle
    by  the garbage  collector, but  it should be  noted that  extensive
    use  of strings will cause many  garbage collections.  Atom  garbage
    collection is generally faster.

See also the prolog-flag double_quotes.


string_to_atom(?String, ?Atom)
    Logical  conversion between  a string  and an atom.    At least  one
    of  the two arguments  must be instantiated.   Atom  can also be  an
    integer or floating point number.


string_to_list(?String, ?List)
    Logical conversion between  a string and a list of ASCII characters.
    At least one of the two arguments must be instantiated.


string_length(+String, -Length)
    Unify  Length  with  the number  of  characters  in String.     This
    predicate  is  functionally  equivalent  to atom_length/2  and  also
    accepts atoms, integers and floats as its first argument.


string_concat(?String1, ?String2, ?String3)
    Similar  to atom_concat/3,  but the unbound argument will  be unified
    with  a string object rather  than an atom.   Also, if both  String1
    and  String2 are  unbound and String3  is bound to  text, it  breaks
    String3,  unifying the start with  String1 and the end with  String2
    as  append does  with lists.   Note  that this  is not  particularly
    fast  on long  strings as  for each redo  the system  has to  create
    two  entirely new strings, while the list equivalent only  creates a
    single new list-cell and moves some pointers around.


sub_string(+String, ?Start, ?Length, ?After, ?Sub)
    Sub  is a substring of String starting at Start, with  length Length
    and  String has After  characters left  after the match.   See  also
    sub_atom/5.


3.24 Operators

Operators  are  defined  to  improve  the  readibility  of  source-code.
For  example, without  operators, to  write  2*3+4*5 one  would have  to
write +(*(2,3),*(4,5)).    In Prolog,  a number of  operators have  been
predefined.   All operators, except for  the comma (,) can be  redefined
by the user.

Some care  has to  be taken  before defining  new operators.    Defining
too many  operators might  make your  source `natural'  looking, but  at
the same  time lead  to hard to  understand the  limits of your  syntax.
To ease  the pain, as  of SWI-Prolog 3.3.0,  operators are local to  the
module  in which  they are  defined.   The  module-table  of the  module
user acts as  default table for all modules.   This global table can  be
modified explictly from inside a module:

:- module(prove,
          [ prove/1
          ]).

:- op(900, xfx, user:(=>)).

Unlike  what many  users  think,  operators  and quoted  atoms  have  no
relation:   defining a atom  as an operator  does not influence  parsing
characters into atoms and  quoting an atom does not stop it  from acting
as an operator.   To stop an atom  acting as an operator, enclose  it in
braces like this:  (myop).


op(+Precedence, +Type, :Name)
    Declare  Name  to  be  an  operator of  type  Type  with  precedence
    Precedence.    Name  can also  be a  list of  names,  in which  case
    all  elements of the  list are declared  to be identical  operators.
    Precedence  is an integer between 0 and 1200.  Precedence  0 removes
    the  declaration.   Type is one  of:   xf, yf, xfx,  xfy, yfx,  yfy,
    fy  or fx.   The `f'  indicates the position  of the functor,  while
    x  and y  indicate the position  of the  arguments.   `y' should  be
    interpreted  as ``on this position  a term with precedence lower  or
    equal to the precedence  of the functor should occur''.  For `x' the
    precedence  of the argument must be strictly lower.   The precedence
    of  a term is  0, unless  its principal functor  is an operator,  in
    which  case the precedence  is the precedence of  this operator.   A
    term enclosed in brackets (...) has precedence 0.

    The  predefined operators are  shown in  table 3.1.   Note that  all
    operators can be redefined by the user.
     ______________________________________________________________
     | 1200 |xfx  |-->, :-                                        |
     | 1200 | fx  |:-, ?-                                         |
     | 1150 | fx  |dynamic, multifile, module_transparent, discon-|
     |      |     |tiguous, volatile, initialization              |
     | 1100 |xfy  |;, |                                           |

     | 1050 |xfy  |->                                             |
     | 1000 |xfy  |,                                              |
     |  954 |xfy  |\                                              |
     |  900 | fy  |\+                                             |
     |  900 | fx  |~                                              |
     |  700 |xfx  |<, =, =.., =@=,  =:=, =<, ==, =\=,  >, >=, @<, |
     |      |     |@=<, @>, @>=, \=, \==, is                      |
     |  600 |xfy  |:                                              |

     |  500 | yfx |+, -, /\, \/, xor                              |
     |  500 | fx  |+, -, ?, \                                     |
     |  400 | yfx |*, /, //, <<, >>, mod, rem                     |
     |  200 |xfx  |**                                             |
     |__200_|xfy__|^______________________________________________|_

                      Table 3.1:  System operators


current_op(?Precedence, ?Type, ?:Name)
    Succeeds when Name  is currently defined as an operator of type Type
    with precedence Precedence.  See also op/3.


3.25 Character Conversion

Although I  wouldn't really know  for what you would  like to use  these
features, they are provided for ISO complicancy.


char_conversion(+CharIn, +CharOut)
    Define  that term-input (see  read_term/3)  maps each character  read
    as  CharIn to the character CharOut.   Character conversion is  only
    executed  if the prolog-flag char_conversion is set to true  and not
    inside  quoted  atoms or  strings.    The  initial table  maps  each
    character onto itself.  See also current_char_conversion/2.


current_char_conversion(?CharIn, ?CharOut)
    Queries    the   current   character   conversion-table.         See
    char_conversion/2 for details.


3.26 Arithmetic

Arithmetic can be  divided into some special purpose integer  predicates
and  a series  of  general predicates  for  floating point  and  integer
arithmetic as  appropriate.  The  integer predicates are as  ``logical''
as possible.  Their usage is recommended  whenever applicable, resulting
in faster and more ``logical'' programs.

The  general arithmetic  predicates  are  optionally compiled  now  (see
set_prolog_flag/2  and  the   -O  command  line   option).      Compiled
arithmetic reduces global  stack requirements and improves  performance.
Unfortunately compiled arithmetic cannot  be traced, which is why  it is
optional.

The  general  arithmetic   predicates  all  handle  expressions.      An
expression is either a simple number or a function.   The arguments of a
function are expressions.  The functions are described in section 3.27.


between(+Low, +High, ?Value)
    Low  and High  are integers, High  Low.   If Value  is an integer,
    Low  Value High.   When Value  is a  variable it  is successively
    bound to all integers between Low and High.


succ(?Int1, ?Int2)
    Succeeds  if Int2= Int1+ 1.  At least one of  the arguments must be
    instantiated to an integer.


plus(?Int1, ?Int2, ?Int3)
    Succeeds if  Int3= Int1+Int2.   At least two of the three arguments
    must be instantiated to integers.


+Expr1 > +Expr2
    Succeeds  when expression  Expr1 evaluates to  a larger number  than
    Expr2.


+Expr1 < +Expr2
    Succeeds  when expression Expr1 evaluates  to a smaller number  than
    Expr2.


+Expr1 =< +Expr2
    Succeeds  when  expression Expr1  evaluates to  a  smaller or  equal
    number to Expr2.


+Expr1 >= +Expr2
    Succeeds  when  expression  Expr1 evaluates  to  a larger  or  equal
    number to Expr2.


+Expr1 =\= +Expr2
    Succeeds  when expression Expr1 evaluates  to a number non-equal  to
    Expr2.


+Expr1 =:= +Expr2
    Succeeds  when  expression  Expr1 evaluates  to  a number  equal  to
    Expr2.


-Number is +Expr
    Succeeds  when  Number   has  successfully  been  unified  with  the
    number  Expr  evaluates to.    If  Expr evaluates  to a  float  that
    can  be represented  using  an integer  (i.e. the  value is  integer
    and  within the  range  that can  be described  by Prolog's  integer
    representation),  Expr is unified with the integer value.

    Note  that normally, is/2  will be used  with unbound left  operand.
    If equality is to be tested, =:=/2 should be used.  For example:

         ?- 1.0 is sin(pi/2).         Fails!.   sin(pi/2) evaluates
                                      to   1.0,   but   is/2   will
                                      represent this as the integer
                                      1,  after  which  unify  will
                                      fail.

         ?- 1.0 is float(sin(pi/2)).  Succeeds,   as   the  float/1
                                      function forces the result to
                                      be float.
         ?- 1.0 =:= sin(pi/2).        Succeeds as expected.


3.27 Arithmetic Functions

Arithmetic functions  are terms  which are evaluated  by the  arithmetic
predicates described  above.   SWI-Prolog tries  to hide the  difference
between  integer  arithmetic  and floating  point  arithmetic  from  the
Prolog  user.   Arithmetic  is done  as integer  arithmetic  as long  as
possible  and converted  to floating  point arithmetic  whenever one  of
the arguments  or the combination of  them requires it.   If a  function
returns  a floating  point  value which  is  whole it  is  automatically
transformed into  an integer.   There  are three  types of arguments  to
functions:

        Expr     Arbitrary   expression,   returning  either   a
                 floating point value or an integer.
        IntExpr  Arbitrary expression that should  evaluate into
                 an integer.
        Int      An integer.

In  case integer  addition, subtraction  and  multiplication would  lead
to  an integer  overflow  the operands  are automatically  converted  to
floating point  numbers.   The floating  point functions (sin/1,  exp/1,
etc.)     form  a  direct  interface  to  the  corresponding  C  library
functions used  to compile SWI-Prolog.   Please refer  to the C  library
documentation for details on precision, error handling, etc.


- +Expr
    Result =-Expr


+Expr1 + +Expr2
    Result =Expr1 +Expr2


+Expr1 - +Expr2
    Result =Expr1 -Expr2


+Expr1 * +Expr2
    Result =Expr1*Expr2


+Expr1 / +Expr2
    Result =Expr1=Expr2


+IntExpr1 mod +IntExpr2
    Modulo:   Result = IntExpr1 - (IntExpr1 // IntExpr2)  * IntExpr2 The
    function  mod/2  is  implemented using  the  C  % operator.     It's
    behaviour with negtive values is illustrated in the table below.

                              2  =   17  mod   5
                              2  =   17  mod  -5
                             -2  =  -17  mod   5
                             -2  =  -17  mod   5


+IntExpr1 rem +IntExpr2
    Remainder of division:  Result = float_fractional_part(IntExpr1/IntExpr2)


+IntExpr1 // +IntExpr2
    Integer division:  Result = truncate(Expr1/Expr2)


abs(+Expr)
    Evaluate Expr and return the absolute value of it.


sign(+Expr)
    Evaluate to -1 if Expr <0, 1 if Expr >0 and 0 if Expr =0.


max(+Expr1, +Expr2)
    Evaluates to the largest of both Expr1 and Expr2.


min(+Expr1, +Expr2)
    Evaluates to the smallest of both Expr1 and Expr2.


.(+Int, [])
    A  list of  one  element evaluates  to the  element.   This  implies
    "a"  evaluates to  the ASCII value  of the  letter `a' (97).    This
    option  is available for  compatibility only.   It will not work  if
    `style_check(+string)'  is active as  "a" will  then be  transformed
    into  a string object.    The recommended way  to specify the  ASCII
    value of the letter `a' is 0'a.


random(+Int)
    Evaluates  to a  random integer i  for which  0 i< Int.   The seed
    of  this random  generator is  determined by the  system clock  when
    SWI-Prolog was started.


round(+Expr)
    Evaluates Expr and rounds the result to the nearest integer.


integer(+Expr)
    Same as round/1 (backward compatibility).


float(+Expr)
    Translate  the result to a floating point number.   Normally, Prolog
    will  use integers  whenever possible.    When used  around the  2nd
    argument  of is/2, the result will  be returned as a floating  point
    number.  In other contexts, the operation has no effect.


float_fractional_part(+Expr)
    Fractional  part of a  floating-point number.   Negative if Expr  is
    negative, 0 if Expr is integer.


float_integer_part(+Expr)
    Integer  part  of  floating-point  number.    Negative  if  Expr  is
    negative, Expr if Expr is integer.


truncate(+Expr)
    Truncate Expr to an integer.  Same as float_integer_part/1.


floor(+Expr)
    Evaluates  Expr and returns the largest integer smaller or  equal to
    the result of the evaluation.


ceiling(+Expr)
    Evaluates  Expr and returns the smallest integer larger or  equal to
    the result of the evaluation.


ceil(+Expr)
    Same as ceiling/1 (backward compatibility).


+IntExpr >> +IntExpr
    Bitwise shift IntExpr1 by IntExpr2 bits to the right.


+IntExpr << +IntExpr
    Bitwise shift IntExpr1 by IntExpr2 bits to the left.


+IntExpr \/ +IntExpr
    Bitwise `or' IntExpr1 and IntExpr2.


+IntExpr /\ +IntExpr
    Bitwise `and' IntExpr1 and IntExpr2.


+IntExpr xor +IntExpr
    Bitwise `exclusive or' IntExpr1 and IntExpr2.


\ +IntExpr
    Bitwise negation.


sqrt(+Expr)
    Result =square root of Expr


sin(+Expr)
    Result =sine of Expr.  Expr is the angle in radians.


cos(+Expr)
    Result =cosine of Expr.  Expr is the angle in radians.


tan(+Expr)
    Result =tangus of Expr.  Expr is the angle in radians.


asin(+Expr)
    Result =inverse sine of Expr.  Result is the angle in radians.


acos(+Expr)
    Result =inverse cosine of Expr.  Result is the angle in radians.


atan(+Expr)
    Result =inverse tangus of Expr.  Result is the angle in radians.


atan(+YExpr, +XExpr)
    Result = inverse tangus of YExpr / XExpr.   Result is  the angle in
    radians.    The return  value is  in the  range [-pi:::pi].   Used  to
    convert between rectangular and polar coordinate system.


log(+Expr)
    Result =natural logarithm of Expr


log10(+Expr)
    Result =10 base logarithm of Expr


exp(+Expr)
    Result =e to the power Expr


+Expr1 ** +Expr2
    Result =Expr1 to the power Expr2


+Expr1 ^ +Expr2
    Same as **/2.  (backward compatibility).


pi
    Evaluates to the mathematical constant pi (3.141593).


e
    Evaluates to the mathematical constant e (2.718282).


cputime
    Evaluates  to a floating  point number expressing  the cpu time  (in
    seconds)  used by Prolog  up till  now.   See also statistics/2  and
    time/1.


3.28 Adding Arithmetic Functions

Prolog predicates  can be given  the role of arithmetic  function.   The
last  argument is  used  to  return the  result,  the  arguments  before
the last  are the  inputs.   Arithmetic  functions are  added using  the
predicate arithmetic_function/1, which takes  the head as its  argument.
Arithmetic  functions  are module  sensitive,  that  is  they  are  only
visible from the module  in which the function is defined  and declared.
Global  arithmetic  functions  should be  defined  and  registered  from
module user.   Global definitions can  be overruled locally in  modules.
The builtin functions described above can be redefined as well.


arithmetic_function(+Head)
    Register  a Prolog  predicate as an  arithmetic function (see  is/2,
    >/2 , etc.).   The Prolog  predicate should  have one more  argument
    than  specified by Head, which it either a term Name/Arity,  an atom
    or  a complex term.   This last argument  is an unbound variable  at
    call  time and  should  be instantiated  to an  integer or  floating
    point  number.    The other  arguments  are the  parameters.    This
    predicate  is  module  sensitive  and will  declare  the  arithmetic
    function  only for the context  module, unless declared from  module
    user.  Example:

    1 ?- [user].
    :- arithmetic_function(mean/2).

    mean(A, B, C) :-
            C is (A+B)/2.
    user compiled, 0.07 sec, 440 bytes.

    Yes
    2 ?- A is mean(4, 5).

    A = 4.500000


current_arithmetic_function(?Head)
    Successively unifies all  arithmetic functions that are visible from
    the context module with Head.


3.29 List Manipulation


is_list(+Term)
    Succeeds  if Term is  bound to the  empty list ([])  or a term  with
    functor `.' and arity 2.


proper_list(+Term)
    Equivalent  to is_list/1,  but also requires the tail of the  list to
    be a list (recursively).  Examples:

    is_list([x|A])          % true
    proper_list([x|A])      % false


append(?List1, ?List2, ?List3)
    Succeeds  when List3  unifies with  the concatenation  of List1  and
    List2.   The  predicate can be  used with any instantiation  pattern
    (even three variables).


member(?Elem, ?List)
    Succeeds  when Elem can be unified with one of the members  of List.
    The predicate can be used with any instantiation pattern.


memberchk(?Elem, +List)
    Equivalent to member/2, but leaves no choice point.


delete(+List1, ?Elem, ?List2)
    Delete all members  of List1 that simultaneously unify with Elem and
    unify the result with List2.


select(?Elem, ?List, ?Rest)
    Select  Elem  from List  leaving  Rest.    It behaves  as  member/2,
    returning  the  remaining  elements in  Rest.    Note  that  besides
    selecting  elements  from a  list, it  can also  be  used to  insert
    elements.


nth0(?Index, ?List, ?Elem)
    Succeeds  when  the  Index-th element  of  List unifies  with  Elem.
    Counting starts at 0.


nth1(?Index, ?List, ?Elem)
    Succeeds  when  the  Index-th element  of  List unifies  with  Elem.
    Counting starts at 1.


last(?Elem, ?List)
    Succeeds  if Elem unifies with  the last element of  List.  If  List
    is  a proper list last/2 is deterministic.   If List has  an unbound
    tail, backtracking will cause List to grow.


reverse(+List1, -List2)
    Reverse  the order  of the elements  in List1  and unify the  result
    with the elements of List2.


flatten(+List1, -List2)
    Transform  List1, possibly holding lists  as elements into a  `flat'
    list by replacing  each list with its elements (recursively).  Unify
    the resulting flat list with List2.  Example:

    ?- flatten([a, [b, [c, d], e]], X).

    X = [a, b, c, d, e]


length(?List, ?Int)
    Succeeds  if Int  represents the  number of elements  of list  List.
    Can be used to create a list holding only variables.


merge(+List1, +List2, -List3)
    List1  and List2 are  lists, sorted to  the standard order of  terms
    (see  section 3.6).    List3 will be  unified with  an ordered  list
    holding  both the elements of List1  and List2.  Duplicates are  not
    removed.


3.30 Set Manipulation


is_set(+Set)
    Succeeds  if  Set  is  a  proper list  (see  proper_list/1)  without
    duplicates.


list_to_set(+List, -Set)
    Unifies  Set  with a  list  holding the  same  elements as  List  in
    the  same order.   If  list contains duplicates,  only the first  is
    retained.  See also sort/2.  Example:

    ?- list_to_set([a,b,a], X)

    X = [a,b]


intersection(+Set1, +Set2, -Set3)
    Succeeds  if Set3 unifies  with the intersection  of Set1 and  Set2.
    Set1  and Set2  are  lists without  duplicates.   They  need not  be
    ordered.


subtract(+Set, +Delete, -Result)
    Delete  all  elements  of set  `Delete'  from  `Set' and  unify  the
    resulting set with `Result'.


union(+Set1, +Set2, -Set3)
    Succeeds if Set3 unifies  with the union of Set1 and Set2.  Set1 and
    Set2 are lists without duplicates.  They need not be ordered.


subset(+Subset, +Set)
    Succeeds if all elements of Subset are elements of Set as well.


merge_set(+Set1, +Set2, -Set3)
    Set1  and Set2 are lists without duplicates, sorted to  the standard
    order  of terms.    Set3  is unified  with an  ordered list  without
    duplicates holding the union of the elements of Set1 and Set2.


3.31 Sorting Lists


sort(+List, -Sorted)
    Succeeds  if Sorted can be unified with a list holding  the elements
    of  List, sorted to the standard  order of terms (see section  3.6).
    Duplicates  are removed.  Implemented by translating the  input list
    into  a temporary  array,  calling the  C-library function  qsort(3)
    using  PL_compare() for  comparing  the  elements, after  which  the
    result is translated into the result list.


msort(+List, -Sorted)
    Equivalent to sort/2, but does not remove duplicates.


keysort(+List, -Sorted)
    List  is a proper list whose elements are Key-Value, that  is, terms
    whose  principal  functor is  (-)/2,  whose  first argument  is  the
    sorting  key, and whose second argument is the satellite data  to be
    carried  along with  the key.   keysort/2  sorts List like  msort/2,
    but  only compares  the keys.    Can be used  to sort  terms not  on
    standard  order, but  on any criterion  that can  be expressed on  a
    multi-dimensional scale.   Sorting on more than one criterion can be
    done  using terms as keys,  putting the first criterion as  argument
    1,  the second as argument 2,  etc.  The order of  multiple elements
    that have the same Key is not changed.


predsort(+Pred, +List, -Sorted)
    Sorts  similar to sort/2, but determines  the order of two terms  by
    calling  Pred(-Delta, +E1, +E2).   This call  must unify Delta  with
    one  of <, const>  or =.  If  built-in predicate compare/3 is  used,
    the result is the same as sort/2.  See also keysort/2.


3.32 Finding all Solutions to a Goal


findall(+Var, +Goal, -Bag)
    Creates  a  list  of the  instantiations  Var gets  successively  on
    backtracking  over Goal and unifies the  result with Bag.   Succeeds
    with  an  empty  list  if Goal  has  no  solutions.    findall/3  is
    equivalent  to  bagof/3  with  all free  variables  bound  with  the
    existence  operator (^), except that bagof/3 fails when goal  has no
    solutions.


bagof(+Var, +Goal, -Bag)
    Unify  Bag with the alternatives of Var, if Goal has  free variables
    besides  the one  sharing  with Var  bagof will  backtrack over  the
    alternatives  of  these  free  variables,   unifying  Bag  with  the
    corresponding  alternatives of Var.   The construct +Var^Goal  tells
    bagof  not to  bind  Var in  Goal.   bagof/3  fails if  Goal has  no
    solutions.

    The  example below  illustrates bagof/3  and the  ^ operator.    The
    variable bindings are printed together on one line to save paper.

    2 ?- listing(foo).

    foo(a, b, c).
    foo(a, b, d).
    foo(b, c, e).
    foo(b, c, f).
    foo(c, c, g).

    Yes
    3 ?- bagof(C, foo(A, B, C), Cs).

    A = a, B = b, C = G308, Cs = [c, d] ;
    A = b, B = c, C = G308, Cs = [e, f] ;
    A = c, B = c, C = G308, Cs = [g] ;

    No
    4 ?- bagof(C, A^foo(A, B, C), Cs).

    A = G324, B = b, C = G326, Cs = [c, d] ;
    A = G324, B = c, C = G326, Cs = [e, f, g] ;

    No
    5 ?-


setof(+Var, +Goal, -Set)
    Equivalent  to bagof/3, but sorts the  result using sort/2 to get  a
    sorted list of alternatives without duplicates.


3.33 Invoking Predicates on all Members of a List

All the predicates in this section call a predicate on  all members of a
list or until the  predicate called fails.  The predicate is  called via
call/[2..], which  implies common arguments can be  put in front of  the
arguments obtained from the list(s).  For example:

?- maplist(plus(1), [0, 1, 2], X).

X = [1, 2, 3]

we will phrase this as ``Predicate is applied on ...''


checklist(+Pred, +List)
    Pred  is applied successively on each element of List until  the end
    of  the list  or Pred fails.    In the latter  case the  checklist/2
    fails.


maplist(+Pred, ?List1, ?List2)
    Apply  Pred  on all  successive  pairs of  elements from  List1  and
    List2.    Fails if  Pred can  not be  applied to a  pair.   See  the
    example above.


sublist(+Pred, +List1, ?List2)
    Unify  List2 with  a list  of all elements  of List1  to which  Pred
    applies.


3.34 Forall


forall(+Cond, +Action)
    For  all alternative bindings  of Cond  Action can be  proven.   The
    example  verifies that all arithmetic  statements in the list L  are
    correct.  It does not say which is wrong if one proves wrong.

    ?- forall(member(Result = Formula, [2 = 1 + 1, 4 = 2 * 2]),
                     Result =:= Formula).


3.35 Formatted Write

The  current  version   of  SWI-Prolog  provides  two  formatted   write
predicates.    The  first  is writef/[1,2],  which  is  compatible  with
Edinburgh C-Prolog.   The  second is  format/[1,2], which is  compatible
with Quintus  Prolog.   We hope  the Prolog  community will once  define
a  standard formatted  write predicate.    If you  want performance  use
format/[1,2] as this predicate is defined in  C. Otherwise compatibility
reasons might tell you which predicate to use.


3.35.1 Writef


write_ln(+Term)
    Equivalent to write(Term), nl.


writef(+Atom)
    Equivalent to writef(Atom, []).


writef(+Format, +Arguments)
    Formatted  write.      Format  is  an  atom  whose  characters  will
    be   printed.     Format  may  contain  certain   special  character
    sequences   which  specify   certain  formatting  and   substitution
    actions.    Arguments then  provides all  the terms  required to  be
    output.

    Escape sequences to generate a single special character:

             __________________________________________________
             | \n   |Output  a  nemline  character  (see  also |
             |      |nl/[0,1])                                 |
             | \l   |Output a line separator (same as \n)      |

             | \r   |Output   a   carriage-return    character |
             |      |(ASCII 13)                                |
             | \t   |Output the ASCII character TAB (9)        |
             | \\   |The character \ is output                 |
             | \%   |The character % is output                 |
             | \nnn |where <nnn>  is an  integer  (1-3 digits) |
             |      |the character  with ASCII  code  <nnn> is |

             |______|output_(NB_:_<nnn>_is_read_as_decimal)____|

    Note  that  \l,   \nnn  and  \\  are  interpreted  differently  when
    character-escapes are in effect.  See section 2.15.1.1.

    Escape  sequences to include  arguments from Arguments.   Each  time
    a  %  escape sequence  is found  in Format  the  next argument  from
    Arguments is formatted according to the specification.

              _________________________________________________%t

              | %w  print/1 the next item (mnemonic:  term)   |    |

              | %q  |write/1the next item                     |

              |     |writeq/1the next item                    |
              | %d  |Write the term,  ignoring operators.  See|
              |     |also  write_term/2.      Mnemonic:    old|
              | %p  |Edinburgh display/1.                     |

              |     |print/1the next item (identical to %t)   |
              | %n  |Put  the next item  as a  character (i.e.|

              |     |it is an ASCII value)                    |
              | %r  |Write the  next item  N times where  N is|
              |     |the second item (an integer)             |
              | %s  |Write the  next item  as a String  (so it|
              |     |must be a list of characters)            |
              | %f  |Perform a ttyflush/0 (no items used)     |
              | %Nc |Write  the   next  item  Centered  in  N |

              |     |columns.                                 |
              | %Nl |Write the next  item Left justified in N |
              |     |columns.                                 |
              | %Nr |Write the next item Right justified in N |
              |     |columns.   N is a decimal number with  at|
              |     |least  one digit.   The  item must  be an|
              |_____|atom,_integer,_float_or_string.__________|_


swritef(-String, +Format, +Arguments)
    Equivalent to writef/2,  but ``writes'' the result on String instead
    of the current output stream.  Example:

    ?- swritef(S, '%15L%w', ['Hello', 'World']).

    S = "Hello          World"


swritef(-String, +Format)
    Equivalent to swritef(String, Format, []).


3.35.2 Format


format(+Format)
    Defined as `format(Format) :- format(Format, []).'


format(+Format, +Arguments)
    Format   is  an   atom,   list  of   ASCII  values,   or  a   Prolog
    string.   Arguments  provides the arguments  required by the  format
    specification.   If  only one argument is  required and this is  not
    a  list of  ASCII values the  argument need  not be put  in a  list.
    Otherwise the arguments are put in a list.

    Special sequences start  with the tilde (~), followed by an optional
    numeric  argument, followed by a character describing the  action to
    be  undertaken.  A numeric argument is either a sequence  of digits,
    representing  a  positive decimal  number, a  sequence `<character>,
    representing  the ASCII value of the character (only useful  for ~t)
    or  a asterisk (*), in when  the numeric argument is taken  from the
    next  argument of  the argument  list,  which should  be a  positive
    integer.  Actions are:

      ~  Output the tilde itself.

      a  Output the  next  argument, which  should  be an  atom.    This
         option is equivalent to w.  Compatibility reasons only.

      c  Output the  next argument  as an ASCII  value.   This  argument
         should be an  integer in the range  [0, ..., 255] (including  0
         and 255).

      d  Output  next argument  as  a decimal  number.    It  should  be
         an integer.    If  a numeric  argument is  specified  a dot  is
         inserted argument  positions from the  right (useful for  doing
         fixed point arithmetic with integers, such as  handling amounts
         of money).

      D  Same as d, but  makes large values easier to read  by inserting
         a comma every three digits left to the dot or right.

      e  Output next argument as a floating point  number in exponential
         notation.    The  numeric  argument  specifies  the  precision.
         Default is  6 digits.   Exact representation  depends on the  C
         library function printf().   This function is invoked  with the
         format %.<precision>e.

      E  Equivalent  to e,  but  outputs a  capital  E to  indicate  the
         exponent.

      f  Floating point  in non-exponential  notation.    See C  library
         function printf().

      g  Floating point in e or f notation, whichever is shorter.

      G  Floating point in E or f notation, whichever is shorter.

      i  Ignore  next argument  of  the  argument  list.    Produces  no
         output.

      k  Give the next argument to displayq/1 (canonical write).

      n  Output a newline character.

      N  Only output  a newline  if the  last character  output on  this
         stream was not a newline.  Not properly implemented yet.

      p  Give the next argument to print/1.

      q  Give the next argument to writeq/1.

      r  Print integer  in radix the  numeric argument  notation.   Thus
         ~16r prints its argument  hexadecimal.  The argument  should be
         in the range [2; :::;36].   Lower case letters are used for  digits
         above 9.

      R  Same as r, but uses upper case letters for digits above 9.

      s  Output a string of  ASCII characters or a string  (see string/1
         and section 3.23) from the next argument.

      t  All remaining space between 2 tabs tops  is distributed equally
         over ~t  statements  between the  tabs  tops.   This  space  is
         padded with  spaces by  default.   If an  argument is  supplied
         this is taken to be  the ASCII value of the character  used for
         padding.   This  can be  used to  do left  or right  alignment,
         centering, distributing, etc.   See also  ~| and ~+ to set  tab
         stops.  A tabs top is assumed at the start of each line.

      |  Set a  tabs top on  the current position.    If an argument  is
         supplied set  a  tabs top  on the  position  of that  argument.
         This  will  cause  all  ~t's  to  be  distributed  between  the
         previous and this tabs top.

      +  Set a tabs top relative  to the current position.   Further the
         same as ~|.

      w  Give the next argument to write/1.

      W  Give the  next two argument  to write_term/2.   This option  is
         SWI-Prolog specific.

    Example:

    simple_statistics :-
        <obtain statistics>         % left to the user
        format('~tStatistics~t~72|~n~n'),
        format('Runtime: ~`.t ~2f~34|  Inferences: ~`.t ~D~72|~n',
                                                [RunT, Inf]),
        ....

    Will output

                                 Statistics

    Runtime: .................. 3.45  Inferences: .......... 60,345


format(+Stream, +Format, +Arguments)
    As format/2, but write the output on the given Stream.


sformat(-String, +Format, +Arguments)
    Equivalent to format/2,  but ``writes'' the result on String instead
    of the current output stream.  Example:

    ?- sformat(S, '~w~t~15|~w', ['Hello', 'World']).

    S = "Hello          World"


sformat(-String, +Format)
    Equivalent to `sformat(String, Format, []).'


3.35.3 Programming Format


format_predicate(+Char, +Head)
    If  a sequence ~c (tilde, followed by some character) is  found, the
    format  derivatives will first check whether the user has  defined a
    predicate  to handle the format.   If  not, the built in  formatting
    rules  described above are  used.   Char is  either an ascii  value,
    or  a one character atom,  specifying the letter to be  (re)defined.
    Head  is a  term, whose  name and  arity are used  to determine  the
    predicate  to call  for  the redefined  formatting character.    The
    first  argument to  the  predicate is  the numeric  argument of  the
    format  command, or the  atom default if  no argument is  specified.
    The  remaining arguments  are filled from  the argument  list.   The
    example  below redefines ~n to produce Arg times return  followed by
    linefeed (so a (Grr.)  DOS machine is happy with the output).

    :- format_predicate(n, dos_newline(_Arg)).

    dos_newline(Arg) :-
            between(1, Ar, _), put(13), put(10), fail ; true.


current_format_predicate(?Code, ?:Head)
    Enumerates  all  user-defined  format  predicates.     Code  is  the
    character  code  of the  format character.    Head  is unified  with
    a  term with  the same  name and  arity as the  predicate.   If  the
    predicate  does not reside  in module user,  Head is qualified  with
    the definition module of the predicate.


3.36 Terminal Control

The  following  predicates  form  a  simple  access   mechanism  to  the
Unix  termcap library  to provide  terminal independent  I/O for  screen
terminals.  These  predicates are only available on Unix machines.   The
SWI-Prolog Windows consoles accepts the ANSI escape sequences.


tty_get_capability(+Name, +Type, -Result)
    Get  the  capability named  Name  from the  termcap  library.    See
    termcap(5)  for the capability  names.   Type specifies the type  of
    the  expected result, and is one of string, number or bool.   String
    results  are returned as  an atom, number  result as an integer  and
    bool  results as the atom on or off.   If an option cannot  be found
    this predicate fails  silently.  The results are only computed once.
    Successive queries on the same capability are fast.


tty_goto(+X, +Y)
    Goto  position  (X, Y) on  the screen.    Note  that the  predicates
    line_count/2  and  line_position/2 will  not  have  a  well  defined
    behaviour while using this predicate.


tty_put(+Atom, +Lines)
    Put  an  atom  via the  termcap  library  function tputs().     This
    function  decodes padding  information  in the  strings returned  by
    tty_get_capability/3 and  should be  used to  output these  strings.
    Lines is the number  of lines affected by the operation, or 1 if not
    applicable (as in almost all cases).


set_tty(-OldStream, +NewStream)
    Set  the  output  stream,  used by  tty_put/2  and tty_goto/2  to  a
    specific stream.  Default is user_output.


tty_size(-Rows, -Columns)
    Determine  the size of the terminal.   If the system provides  ioctl
    calls  for this these are used and  tty_size/2properly  reflects the
    actual  size after a user resize of the window.  As a  fallback, the
    system  uses tty_get_capability/2using  li and co capabilities.   In
    this case the reported  size reflects the size at the first call and
    is not updated after a user-initiated resize of the terminal.


3.37 Operating System Interaction


shell(+Command, -Status)
    Execute  Command on the operating system.   Command is given to  the
    Bourne  shell (/bin/sh).  Status is unified with the exit  status of
    the command.

    On  Win32  systems,  shell/[1,2]  executes  the  command  using  the
    CreateProcess()  API and  waits for the  command to  terminate.   If
    the  command  ends with  a &  sign,  the command  is  handed to  the
    WinExec()  API, which does not wait  for the new task to  terminate.
    See  also  win_exec/2  and  win_shell/2.     Please  note  that  the
    CreateProcess()  API does not imply the Windows  command interpreter
    (command.exe  on  Windows  95/98  and  cmd.exe  on  Windows-NT)  and
    therefore  commands  built-in to  the command-interpreter  can  only
    be   activated  using  the  command  interpreter.      For  example:
    'command.exe /C copy file1.txt file2.txt'


shell(+Command)
    Equivalent to `shell(Command, 0)'.


shell
    Start  an  interactive  Unix  shell.     Default  is  /bin/sh,   the
    environment  variable SHELL overrides this  default.  Not  available
    for Win32 platforms.


win_exec(+Command, +Show)
    Win32  systems only.    Spawns a  Windows task  without waiting  for
    its  completion.  Show is  either iconic or normal and dictates  the
    initial  status of the window.   The iconic option is notably  handy
    to start (DDE) servers.


win_shell(+Operation, +File)
    Win32  systems only.    Opens the  document File  using the  windows
    shell-rules  for doing  so.   Operation  is  one of  open, print  or
    explore  or  another operation  registered with  the  shell for  the
    given document-type.   On modern systems it is also possible to pass
    a  URL as File, opening  the URL in Windows  default browser.   This
    call interfaces to the Win32 API ShellExecute().


win_registry_get_value(+Key, +Name, -Value)
    Win32  systems  only.     Fetches the  value  of  a  Win32  registry
    key.     Key  is  an  atom formed  as  a  path-name  describing  the
    desired  registry key.   Name is the  desired attribute name of  the
    key.    Value  is  unified with  the value.    If  the value  is  of
    type  DWORD, the  value is returned  as an  integer.   If the  value
    is  a string  it is  returned as  a Prolog atom.    Other types  are
    currently  not supported.  The  default `root' is HKEY_CURRENT_USER.
    Other  roots  can  be  specified  explicitely as  HKEY_CLASSES_ROOT,
    HKEY_CURRENT_USER,  HKEY_LOCAL_MACHINE  or  HKEY_USERS.  The  example
    below fetches the  extension to use for Prolog files (see README.TXT
    on the Windows version):

    ?- win_registry_get_value('HKEY_LOCAL_MACHINE/Software/SWI/Prolog',
                              fileExtension,
                              Ext).

    Ext = pl


getenv(+Name, -Value)
    Get  environment variable.    Fails  silently if  the variable  does
    not  exist.     Please  note that  environment  variable  names  are
    case-sensitive on Unix systems and case-insensitive on Windows.


setenv(+Name, +Value)
    Set  environment variable.   Name and  Value should be  instantiated
    to  atoms or  integers.   The  environment variable  will be  passed
    to  shell/[0-2] and  can be  requested using  getenv/2.   They  also
    influence expand_file_name/2.


unsetenv(+Name)
    Remove environment variable from the environment.


unix(+Command)
    This  predicate comes  from  the Quintus  compatibility library  and
    provides  a partial implementation thereof.   It provides access  to
    some operating system  features and unlike the name suggests, is not
    operating  system specific.  Currently  it is the only way to  fetch
    the Prolog command-line arguments.  Defined Command's are below.

    system(+Command)
         Equivalent to calling shell/1.  Use for compatibility only.

    shell(+Command)
         Equivalent to calling shell/1.  Use for compatibility only.

    shell
         Equivalent to calling shell/0.  Use for compatibility only.

    cd
         Equivalent  to  calling   chdir/1  as  chdir(~).      Use   for
         compatibility only.

    cd(+Directory)
         Equivalent to calling chdir/1.  Use for compatibility only.

    argv(-Argv)
         Unify Argv with the  list of commandline arguments provides  to
         this Prolog run.  Please note that  Prolog system-arguments and
         application arguments are separated  by --.  Integer  arguments
         are  passed as  Prolog  integers,  float arguments  and  Prolog
         floating  point  numbers and  all  other  arguments  as  Prolog
         atoms.  New applications should use the prolog-flag argv.

         A  stand-alone program  could  use  the following  skeleton  to
         handle command-line arguments.  See also section 2.10.2.4.

         main :-
                 unix(argv(Argv)),
                 append(_PrologArgs, [--|AppArgs], Argv), !,
                 main(AppArgs).


get_time(-Time)
    Return  the number of  seconds that elapsed  since the epoch of  the
    POSIX,  tim representation:   January  1970,  0 hours.    Time is  a
    floating point number.  The granularity is system dependent.


convert_time(+Time, -Year, -Month, -Day, -Hour, -Minute, -Second, -MilliSeconds)

    Convert   a  time  stamp,   provided  by   get_time/1,   time_file/2,
    etc.   Year is  unified with the year,  Month with the month  number
    (January  is 1), Day  with the day of  the month (starting with  1),
    Hour  with  the hour  of the  day (0--23),  Minute  with the  minute
    (0--59).   Second with the  second (0--59) and MilliSecond with  the
    milliseconds  (0--999).  Note that the latter might not  be accurate
    or  might always be 0, depending  on the timing capabilities of  the
    system.  See also convert_time/2.


convert_time(+Time, -String)
    Convert  a time-stamp as  obtained though get_time/1 into a  textual
    representation  using the C-library function ctime().  The  value is
    returned  as a  SWI-Prolog string object  (see section  3.23).   See
    also convert_time/8.


3.38 File System Interaction


access_file(+File, +Mode)
    Succeeds  if File exists and can be accessed by this  prolog process
    under  mode Mode.   Mode is  one of the  atoms read, write,  append,
    exist,  none or execute.  File may also be the name  of a directory.
    Fails  silently otherwise.   access_file(File, none)simply  succeeds
    without testing anything.

    If  `Mode' is write or append,  this predicate also succeeds if  the
    file  does not exist and the user has write-access to  the directory
    of the specified location.


exists_file(+File)
    Succeeds  when File exists.  This  does not imply the user  has read
    and/or write permission for the file.


file_directory_name(+File, -Directory)
    Extracts  the directory-part of File.  The resulting  Directory name
    ends  with the directory separator character /.  If File is  an atom
    that does not  contain any directory separator characters, the empty
    atom '' is returned.  See also file_base_name/2.


file_base_name(+File, -BaseName)
    Extracts the filename part  from a path specification.  If File does
    not contain any directory separators, File is returned.


same_file(+File1, +File2)
    Succeeds  if both filenames refer to  the same physical file.   That
    is,  if File1  and File2  are the  same string or  both names  exist
    and  point to the  same file (due to  hard or symbolic links  and/or
    relative vs.  absolute paths).


exists_directory(+Directory)
    Succeeds  if Directory exists.    This does not  imply the user  has
    read, search and or write permission for the directory.


delete_file(+File)
    Remove File from the file system.


rename_file(+File1, +File2)
    Rename  File1 into File2.   Currently  files cannot be moved  across
    devices.


size_file(+File, -Size)
    Unify Size with the size of File in characters.


time_file(+File, -Time)
    Unify  the last  modification time of  File with  Time.   Time is  a
    floating  point number expressing the  seconds elapsed since Jan  1,
    1970.  See also convert_time/[2,8] and get_time/1.


absolute_file_name(+File, -Absolute)
    Expand  a local file-name into an absolute path.  The  absolute path
    is  canonised:  references to . and .. are deleted.   This predicate
    ensures  that expanding  a file-name  it returns  the same  absolute
    path  regardless of  how the  file is  addressed.   SWI-Prolog  uses
    absolute  file names  to register  source files  independent of  the
    current  working directory.    See also  absolute_file_name/3.    See
    also absolute_file_name/3 and expand_file_name/2.


absolute_file_name(+Spec, +Options, -Absolute)
    Converts  the  given  file  specification  into  an  absolute  path.
    Option is a list of options to guide the conversion:

    extensions(ListOfExtensions)
         List of  file-extensions to  try.   Default is  ''.   For  each
         extension,  absolute_file_name/3 will first  add the  extension
         and then verify  the conditions imposed  by the other  options.
         If the  condition  fails, the  next extension  of  the list  is
         tried.   Extensions  may be specified  both as  ..ext or  plain
         ext.

    access(Mode)
         Imposes the condition  access_file(File, Mode).   Mode is on  of
         read, write, append, exist or none.  See also access_file/2.

    file_type(Type)
         Defines  extensions.    Current  mapping:   txt  implies  [''],
         prolog  implies ['.pl', ''],  executable  implies  ['.so', ''],
         qlf implies ['.qlf', ''] and directory implies [''].

    file_errors(fail/error)
         If error (default), throw and  existence_error exception  if the
         file cannot be found.  If fail, stay silent.

    solutions(first/all)
         If  first (default),  the  predicates leaves  no  choice-point.
         Otherwise a  choice-point  will be  left and  backtracking  may
         yield more solutions.


is_absolute_file_name(+File)
    True  if File specifies  and absolute path-name.   On Unix  systems,
    this   implies  the  path  starts  with  a  `/'.      For  Microsoft
    based   systems  this  implies  the   path  starts  with  <letter>:.
    This   predicate   is  intended   to  provide   platform-independent
    checking  for  absolute paths.    See also  absolute_file_name/2 and
    prolog_to_os_filename/2.


file_name_extension(?Base, ?Extension, ?Name)
    This  predicate is used to add, remove or test  filename extensions.
    The  main reason  for  its introduction  is to  deal with  different
    filename  properties  in a  portable manner.    If  the file  system
    is   case-insensitive,  testing  for  an  extension  will   be  done
    case-insensitive too.   Extension may be specified with or without a
    leading  dot (.).  If an Extension is generated, it will  not have a
    leading dot.


expand_file_name(+WildCard, -List)
    Unify  List with  a  sorted list  of files  or directories  matching
    WildCard.   The  normal Unix wildcard  constructs `?', `*',  `[...]'
    and  `{...}'  are  recognised.    The interpretation  of  `{...}'  is
    interpreted  slightly different  from  the C  shell (csh(1)).    The
    comma  separated  argument  can  be  arbitrary  patterns,  including
    `{...}'  patterns.  The empty pattern is  legal as well:  `\{.pl,\}'
    matches either `.pl' or the empty string.

    If  the pattern  does contains  wildcard  characters, only  existing
    files  and directories are returned.  Expanding a  `pattern' without
    wildcard  characters returns the argument, regardless on  whether or
    not it exists.

    Before  expanding wildchards, the construct $var is expanded  to the
    value  of the  environment  variable var  and a  possible leading  ~
    character is expanded to the user's home directory..


prolog_to_os_filename(?PrologPath, ?OsPath)
    Converts  between the internal  Prolog pathname conventions and  the
    operating-system  pathname conventions.    The internal  conventions
    are  Unix and this predicates is  equivalent to =/2 (unify) on  Unix
    systems.   On  DOS systems it  will change the  directory-separator,
    limit  the filename length map dots,  except for the last one,  onto
    underscores.


read_link(+File, -Link, -Target)
    If File points to  a symbolic link, unify Link with the value of the
    link and Target to  the file the link is pointing to.  Target points
    to  a file, directory or non-existing entry in the file  system, but
    never  to a link.   Fails if File  is not a link.   Fails always  on
    systems that do not support symbolic links.


tmp_file(+Base, -TmpName)
    Create  a name for a temporary file.  Base is an  identifier for the
    category  of file.  The TmpName is guaranteed to be unique.   If the
    system  halts, it  will automatically  remove all created  temporary
    files.


make_directory(+Directory)
    Create  a  new directory  (folder) on  the filesystem.    Raises  an
    exception  on failure.   On Unix systems,  the directory is  created
    with default permissions (defined by the process umask setting).


delete_directory(+Directory)
    Delete directory (folder)  from the filesystem.  Raises an exception
    on failure.   Please note that in general it will not be possible to
    delete a non-empty directory.


chdir(+Path)
    Change working directory to Path.


3.39 Multi-threading (alpha code)

The features described in this section are only enabled  on Unix systems
providing  POSIX threads  and  if the  system  is configured  using  the
--enable-mt option.   SWI-Prolog multi-theading support is  experimental
and in some areas not safe.

SWI-Prolog multithreading is  based on standard C-language  multithread-
ing support.   It  is not like ParLog  or other paralel  implementations
of the Prolog language.   Prolog threads have their own stacks  and only
share the  Prolog heap:   predicates,  records, flags  and other  global
non-backtrackable data.  SWI-Prolog thread support is  designed with the
following goals in mind.

  o Multi-threaded server applications
    Todays   computing  services  often   focus  on  (internet)   server
    applications.   Such applications often have need  for communication
    between  services  and/or  fast  non-blocking  service  to  multiple
    concurrent  clients.   The shared  heap provides fast  communication
    and  thread  creation  is  relatively cheap  (A  Pentium-II/450  can
    create and join approx.  10,000 threads per second on Linux 2.2).

  o Interactive applications
    Interactive   applications   often   need   to   perform   extensive
    computation.   If  such computations are  executed in a new  thread,
    the  main thread  can process events  and allow  the user to  cancel
    the  ongoing computation.    User interfaces can  also use  multiple
    threads,  each thread dealing  with input from  a distinct group  of
    windows.

  o Natural integration with foreign code
    Each  Prolog thread runs  in a  C-thread, automatically making  them
    cooperate  with  MT-safe foreign-code.    In addition,  any  foreign
    thread  can create its  own Prolog engine  for dealing with  calling
    Prolog from C-code.


thread_create(:Goal, -Id, +Options)
    Create  a new Prolog thread  (and underlying C-thread) and start  it
    by  executing  Goal.   If  the thread  is created  succesfully,  the
    thread-identifier  of the created thread is unified to Id.   Options
    is a list of options.  Currently defined options are:

    local(K-Bytes)
         Set the  limit to  which  the local  stack of  this thread  may
         grow.   If omited,  the limit  of the  calling thread is  used.
         See also the -L commandline option.

    global(K-Bytes)
         Set the  limit to  which the global  stack of  this thread  may
         grow.   If omited,  the limit  of the  calling thread is  used.
         See also the -G commandline option.

    trail(K-Bytes)
         Set the  limit to  which  the trail  stack of  this thread  may
         grow.   If omited,  the limit  of the  calling thread is  used.
         See also the -T commandline option.

    argument(K-Bytes)
         Set the limit  to which the argument  stack of this thread  may
         grow.   If omited,  the limit  of the  calling thread is  used.
         See also the -A commandline option.

    alias(AliasName)
         Associate an `alias-name' with  the thread.  This named  may be
         used to refer to the thread and remains valid  until the thread
         is joined (see thread_join/2).

    detached(Bool)
         If  false  (default),  the  thread  can  be  waited  for  using
         thread_join/2.  thread_join/2 must be called on this  thread to
         reclaim the all resources  associated to the thread.   If true,
         the system will reclaim all associated  resources automatically
         after the thread finishes.  Please not  that thread identifiers
         are freed  for  reuse after  a detached  thread  finishes or  a
         normal thread has been joined.

    The Goal argument is  copied to the new Prolog engine.  This implies
    further  instantiation of this term  in either thread does not  have
    consequences  for the  other thread:   Prolog threads  do not  share
    data from their stacks.


thread_self(-Id)
    Get  the Prolog thread  identifier of  the running thread.   If  the
    thread has an alias, the alias-name is returned.


current_thread(?Id, ?Status)
    Enumerates  identifiers and status  of all currently known  threads.
    Calling  current_thread/2 does not influence any  thread.  See  also
    thread_join/2.   For threads that  have an alias-name, this name  is
    returned  in Id instead of the numerical thread identifier.   Status
    is one of:

    running
         The  thread is  running.    This is  the  initial status  of  a
         thread.   Please note  that threats  waiting for something  are
         considered running too.

    false
         The Goal of the thread has been completed and failed.

    true
         The Goal of the thread has been completed and succeeded.

    exited(Term)
         The Goal of the thread has  been terminated using thread_exit/1
         with Term as argument.

    exception(Term)
         The Goal of the  thread has been terminated due to  an uncaught
         exception (see throw/1 and catch/3).


thread_join(+Id, -Status)
    Wait  for the termination of thread with  given Id.  Then  unify the
    result-status (see  thread_exit/1) of the thread with  Status.  After
    this call, Id  becomes invalid and all resources associated with the
    thread are reclaimed.  See also current_thread/2.

    A thread  that has been completed without thread_join/2 being called
    on  it is  partly reclaimed:   the  Prolog stacks  are released  and
    the  C-thread is destroyed.  A small data-structure  represening the
    exit-status of  the thread is retained until thread_join/2 is called
    on the thread.


thread_exit(+Term)
    Terminates   the  thread   immediately,   leaving  exited(Term)   as
    result-state.  The Prolog stacks and C-thread are reclaimed.


thread_at_exit(:Goal)
    Run  Goal after the execution of  this thread has terminated.   This
    is  to be compared  to at_halt/1, but only  for the current  thread.
    These  hooks are ran regardless of  why the execution of the  thread
    has  been completed.   As  these hooks are  run, the return-code  is
    already available through current_thread/2.


3.39.1 Thread communication

Prolog  threads can  exchange data  using dynamic  predicates,  database
records, and  other globally shared  data.  In  addition, they can  send
messages to each other.   If a threads needs to wait for  another thread
until  that thread  has  produced some  data,  using only  the  database
forces the waiting  thread to poll the  database continuously.   Waiting
for  a message  suspends  the thread  execution  until the  message  has
arrived in its message queue.


thread_send_message(+ThreadId, +Term)
    Place  Term in the message queue of the indicated thread  (which can
    even  be the message queue of itself (see thread_self/1).   Any term
    can  be placed in a message queue, but note that the term  is copied
    to  to receiving thread and variable-bindings  are thus lost.   This
    call returns immediately.


thread_get_message(?Term)
    Examines the thread  message-queue and if necessary blocks execution
    until  a term that unifies  to Term arrives in  the queue.  After  a
    term  from the queue has been unified unified to Term, this  term is
    deleted from the queue and this predicate returns.

    Please  note that not-unifying messages remain in the queue.   After
    the  following has been  executed, thread 1  has the term b(gnu)  in
    its queue and continues execution using A is gnat.

       <thread 1>
       thread_get_message(a(A)),

       <thread 2>
       thread_send_message(b(gnu)),
       thread_send_message(a(gnat)),

    See also thread_peek_message/1.


thread_peek_message(?Term)
    Examines  the thread  message-queue  and compares  the queued  terms
    with   Term  until  one  unifies  or  the  end  of  the   queue  has
    been  reached.    In  the first  case  the call  succeeds  (possibly
    instantiating  Term.   If no term from  the queue unifies this  call
    fails.


thread_signal(+ThreadId, :Goal)
    Make thread ThreadId execute  Goal at the first opportunity.  In the
    current  implementation, this implies at the first pass  through the
    Call-port.   The  predicate thread_signal/2 itself places Goal  into
    the signalled-thread's signal queue and returns immediately.

    Signals  (interrupts)  do  not  cooperate well  with  the  world  of
    multi-threading,  mainly  because the  status of  mutexes cannot  be
    guaranteed  easily.   At the call-port,  the Prolog virtual  machine
    holds no locks and therefore the asynchronous execution is safe.

    Goal  can be any  valid Prolog goal,  including throw/1 to make  the
    receiving thread generate  an exception and trace/0 to start tracing
    the receiving thread.


3.39.2 Thread synchronisation

All  internal Prolog  operations  are thread-safe.    This  implies  two
Prolog  threads  can  operate on  the  same  dynamic  predicate  without
corrupting the consistency  of the predicate.   This section deals  with
user-level  mutexes (called  monitors  in  ADA or  critical-sections  by
Microsoft).   A  mutex is a  MUTual EXclusive device,  which implies  at
most one thread can hold a mutex.

Mutexes are  used to  realise related  updates to  the Prolog  database.
With `related', we refer to the situation where  a `transaction' implies
two  or more  changes to  the Prolog  database.   For  example, we  have
a  predicate address/2,  representing the  address of  a  person and  we
want  to change  the address  by retracting  the old  and asserting  the
new address.    Between these  two operations the  database is  invalid:
this person  has either no  address or two  addresses (depending on  the
assert/retract order).

Here is how to realise a correct update:

:- initialization
        mutex_create(addressbook).

change_address(Id, Address) :-
        mutex_lock(addressbook),
        retractall(address(Id, _)),
        asserta(address(Id, Address)),
        mutex_unlock(addressbook).


mutex_create(?MutexId)
    Create  a mutex.  if MutexId  is an atom, a named mutex  is created.
    If  it is  a variable,  an  anonymous mutex  reference is  returned.
    There is no limit to the number of mutexes that can be created.


mutex_destroy(+MutexId)
    Destroy  a mutex.    After this  call, MutexId  becomes invalid  and
    further references yield an existence_error exception.


mutex_lock(+MutexId)
    Lock  the mutex.   Prolog mutexes are  recursive mutexes:  they  can
    be  locked multiple times by the same thread.  Only  after unlocking
    it  as many times as it  is locked, the mutex becomes  available for
    locking  by other threads.   If another thread has locked the  mutex
    the calling thread is suspended until to mutex is unlocked.

    If  MutexId is  an atom,  and there  is no current  mutex with  that
    name,  the  mutex  is  created automatically  using  mutex_create/1.
    This implies named mutexes need not be declared explicitly.

    Please  note that  locking and  unlocking mutexes  should be  paired
    carefully.    Especially make  sure to  unlock mutexes  even if  the
    protected  code  fails or  raises an  exception.    For most  common
    cases  use  with_mutex/2,  wich  provides a  safer way  for  handling
    prolog-level mutexes.


mutex_trylock(+MutexId)
    As  mutex_lock/1,  but if the mutex  is held by another thread,  this
    predicates fails immediately.


mutex_unlock(+MutexId)
    Unlock  the mutex.  This can only be called if the mutex  is held by
    the  calling thread.   If this is  not the case,  a permission_error
    exception is raised.


mutex_unlock_all
    Unlock  all  mutexes held  by the  current  thread.   This  call  is
    especially  useful  to handle  thread-termination using  abort/0  or
    exceptions.  See also thread_signal/2.


current_mutex(?MutexId, ?ThreadId, ?Count)
    Enumerates  all existing  mutexes.   If the  mutex is  held by  some
    thread,  ThreadId  is  unified with  the  identifier of  te  holding
    thread and Count with  the recursive count of the mutex.  Otherwise,
    ThreadId is [] and Count is 0.


with_mutex(+MutexId, :Goal)
    Execute  Goal while holding MutexId.  If Goal  leaves choicepointes,
    these  are  destroyed  (as  in  once/1).    The  mutex  is  unlocked
    regardless  of whether Goal succeeds, fails or raises  an exception.
    An  exception thrown by Goal is  re-thrown after the mutex has  been
    successfully unlocked.  See also mutex_create/2.

    Although  described in  the thread-section,  this predicate is  also
    available  in the single-threaded  version, where it behaves  simply
    as once/1.


3.39.3 Thread-support library(threadutil)

This library  defines a  couple of useful  predicates for  demonstrating
and debugging  multi-threaded applications.   This library is  certainly
not complete.


threads
    Lists  all  current threads  and their  status.   In  addition,  all
    `zombie'  threads  (finished  threads that  are  not  detached,  nor
    waited for) are joined to reclaim their resources.


interactor
    Create  a  new console  and  run the  Prolog  toplevel in  this  new
    console.  See also attach_console/0.


attach_console
    If  the current thread has no  console attached yet, attach one  and
    redirect  the user  streams (input,  output, and error)  to the  new
    console  window.  The console is an xterm application.   For this to
    work,  you should be  running X-windows and  your xterm should  know
    the -Sccn.

    This  predicate has  a couple  of useful applications.    One is  to
    separate (debugging) I/O  of different threads.  Another is to start
    debugging  a thread that  is running in the  background.  If  thread
    10  is running,  the following  sequence starts the  tracer on  this
    thread:

    ?- thread_signal(10, (attach_console, trace)).


3.39.4 Status of the thread implementation

It is assumed that  the basic Prolog execution is thread-safe.   Various
problems  are  to  be  expected  though,  both  dead-locks  as  well  as
not-thread-safe code in builtin-predicates.


3.40 User Toplevel Manipulation


break
    Recursively  start a new  Prolog top level.   This Prolog top  level
    has its own  stacks, but shares the heap with all break environments
    and  the top level.  Debugging  is switched off on entering  a break
    and  restored on leaving one.   The break environment is  terminated
    by  typing the  system's end-of-file character  (control-D). If  the
    -t toplevel  command  line  option is  given  this goal  is  started
    instead of entering the default interactive top level (prolog/0).


abort
    Abort  the Prolog  execution  and restart  the top  level.   If  the
    -t toplevel  command  line options  is given  this  goal is  started
    instead of entering the default interactive top level.

    There  are two  implementations of abort/0.    The default one  uses
    the  exception  mechanism  (see  throw/1),  throwing  the  exception
    $aborted.   The other one uses the C-construct longjmp()  to discard
    the  entire environment  and rebuild a  new one.   Using  exceptions
    allows  for  proper recovery  of predicates  exploiting  exceptions.
    Rebuilding  the  environment  is  safer if  the  Prolog  stacks  are
    corrupt.    Therefore the  system will use  the rebuild-strategy  if
    the  abort was generated  by an internal  consistency check and  the
    exception  mechanism otherwise.   Prolog  can be  forced to use  the
    rebuild-strategy  setting  the prolog  flag  abort_with_exception  to
    false.


halt
    Terminate  Prolog  execution.   Open  files are  closed  and if  the
    command  line option  -tty is  not active the  terminal status  (see
    Unix  stty(1)) is restored.  Hooks may be registered both  in Prolog
    and  in foreign code.  Prolog hooks  are registered using at_halt/1.
    halt/0 is equivalent to halt(0).


halt(+Status)
    Terminate  Prolog  execution  with  given  status.    Status  is  an
    integer.  See also halt/0.


prolog
    This  goal starts the  default interactive top  level.  Queries  are
    read  from the stream user_input.   See also the history prolog_flag
    (current_prolog_flag/2).    The  prolog/0  predicate  is  terminated
    (succeeds)  by typing  the  end-of-file character  (On most  systems
    control-D).

The following  two hooks allow  for expanding  queries and handling  the
result  of a  query.   These  hooks are  used by  the toplevel  variable
expansion mechanism described in section 2.8.


expand_query(+Query, -Expanded, +Bindings, -ExpandedBindings)
    Hook  in module  user, normally  not defined.    Query and  Bindings
    represents  the  query read  from  the user  and  the names  of  the
    free  variables as obtained  using read_term/3.   If this  predicate
    succeeds, it should  bind Expanded and ExpandedBindings to the query
    and  bindings to be  executed by  the toplevel.   This predicate  is
    used  by  the toplevel  (prolog/0).   See  also expand_answer/2  and
    term_expansion/2.


expand_answer(+Bindings, -ExpandedBindings)
    Hook  in module user,  normally not defined.   Expand the result  of
    a  successfully executed  toplevel  query.   Bindings  is the  query
    <Name>= <Value>binding list  from the query.  ExpandedBindings must
    be unified with the bindings the toplevel should print.


3.41 Creating a Protocol of the User Interaction

SWI-Prolog offers the  possibility to log the interaction with  the user
on  a file.    All  Prolog interaction,  including warnings  and  tracer
output, are written on the protocol file.


protocol(+File)
    Start  protocolling on file  File.  If  there is already a  protocol
    file open then close it first.  If File exists it is truncated.


protocola(+File)
    Equivalent  to protocol/1,  but  does not  truncate the  File if  it
    exists.


noprotocol
    Stop  making a protocol of the user interaction.  Pending  output is
    flushed on the file.


protocolling(-File)
    Succeeds  if a protocol was  started with protocol/1 or  protocola/1
    and unifies File with the current protocol output file.


3.42 Debugging and Tracing Programs

This section is a  reference to the debugger interaction predicates.   A
more use-oriented overview of the debugger is in section 2.9.

If you have  installed XPCE, you can  use the graphical frontend of  the
tracer.  This frontend is installed using the predicate guitracer/0.


trace
    Start  the tracer.   trace/0  itself cannot be  seen in the  tracer.
    Note  that  the Prolog  toplevel treats  trace/0 special;  it  means
    `trace the next goal'.


tracing
    Succeeds  when  the tracer  is  currently switched  on.    tracing/0
    itself can not be seen in the tracer.


notrace
    Stop the tracer.  notrace/0 itself cannot be seen in the tracer.


guitracer
    Installs  hooks  (see prolog_trace_interception/4) into  the  system
    that  redirects  tracing information  to  a GUI  frontend  providing
    structured  access to variable-bindings,  graphical overview of  the
    stack and highlighting of relevant source-code.


noguitracer
    Reverts back to the textual tracer.


trace(+Pred)
    Equivalent to trace(Pred, +all).


trace(+Pred, +Ports)
    Put  a  trace-point  on  all  predicates  satisfying  the  predicate
    specification  Pred.   Ports  is a  list of  portnames (call,  redo,
    exit,  fail).   The atom all refers  to all ports.   If the port  is
    preceded  by a - sign the trace-point  is cleared for the port.   If
    it is preceded by a + the trace-point is set.

    The  predicate trace/2  activates debug  mode (see debug/0).    Each
    time  a port (of the 4-port model) is passed that has  a trace-point
    set  the goal is printed as  with trace/0.  Unlike trace/0  however,
    the  execution is continued without asking for  further information.
    Examples:

        ?- trace(hello).         Trace all  ports of hello  with any
                                 arity in any module.
        ?- trace(foo/2, +fail).  Trace  failures  of  foo/2  in  any
                                 module.
        ?- trace(bar/1, -all).   Stop tracing bar/1.

    The predicate debugging/0 shows all currently defined trace-points.


notrace(+Goal)
    Call  Goal,  but  suspend  the  debugger while  Goal  is  executing.
    The  current  implementation cuts  the  choicepoints of  Goal  after
    successful completion.   See once/1.  Later implementations may have
    the same semantics as call/1.


debug
    Start  debugger.     In  debug  mode,   Prolog  stops  at  spy-  and
    trace-points,  disables tail-recursion  optimisation and  aggressive
    destruction   of   choice-points  to   make  debugging   information
    accessible.  Implemented by the Prolog flag debug.


nodebug
    Stop  debugger.  Implementated by the  prolog flag debug.   See also
    debug/0.


debugging
    Print  debug status and  spy points on current  output stream.   See
    also the prolog flag debug.


spy(+Pred)
    Put   a  spy   point  on  all   predicates  meeting  the   predicate
    specification Pred.  See section 3.4.


nospy(+Pred)
    Remove   spy  point  from  all  predicates  meeting   the  predicate
    specification Pred.


nospyall
    Remove all spy points from the entire program.


leash(?Ports)
    Set/query leashing (ports  which allow for user interaction).  Ports
    is  one of +Name, -Name,  ?Name or a list  of these.  +Name  enables
    leashing  on that  port,  -Name disables  it and  ?Name succeeds  or
    fails  according to  the  current setting.    Recognised ports  are:
    call, redo, exit,  fail and unify.  The special shorthand all refers
    to  all ports, full  refers to all ports  except for the unify  port
    (default).  half refers to the call, redo and fail port.


visible(+Ports)
    Set the ports shown  by the debugger.  See leash/1 for a description
    of the port specification.  Default is full.


unknown(-Old, +New)
    Edinburgh-prolog  compatibility predicate,  interfacing  to the  ISO
    prolog  flag unknown.   Values are  trace (meaning error) and  fail.
    If  the unknown flag is set to warning, unknown/2 reports  the value
    as trace.


style_check(+Spec)
    Set  style checking options.   Spec  is either  +<option>, -<option>,
    ?<option>  or a  list  of such  options.    +<option> sets  a  style
    checking option,  -<option>clears it and ?<option> succeeds or fails
    according to the  current setting.  consult/1 and derivatives resets
    the  style checking options to their value before loading  the file.
    If---for  example---a file  containing long atoms  should be  loaded
    the user can start the file with:

    :- style_check(-atom).

    Currently available options are:

    ____________________________________________________________________
    |_Name__________|Default_|Description_______________________________||singletonon|  ||

    |                         read_clause/1 (used by  consult/1)  warns |               ||
    |                         on variables  only  appearing once  in  a |               ||
    |                         term  (clause)  which  have  a  name  not |               ||

    ||atom          |  on    |starting with an underscore.              ||              ||

    |               |        |read/1 and  derivatives will  produce  an |
    |               |        |error message on quoted atoms or  strings |
    |               |        |longer than 5 lines.                      |
    | dollar        |  off   |Accept dollar as a lower case  character, |
    |               |        |thus avoiding the need for quoting  atoms |
    |               |        |with dollar  signs.   System  maintenance |
    |               |        |use only.                                 |
    | discontiguous |  on    |Warn if the clauses  for a predicate  are |

    |               |        |not together in the same source file.     |
    | string        |  off   |Backward    compatibility.            See |
    |               |        |the       prolog-flag       double_quotes |
    |_______________|________|(current_prolog_flag/2).__________________|


3.43 Obtaining Runtime Statistics


statistics(+Key, -Value)
    Unify system statistics  determined by Key with Value.  The possible
    keys are given in the table 3.2.
   __________________________________________________________________
   | agc             |Number of atom garbage-collections performed  |
   | agc_gained      |Number of atoms removed                       |
   | agc_time        |Time spent in atom garbage-collections        |
   | cputime         |(User) cpu  time since  Prolog was  started in|
   |                 |seconds                                       |

   | inferences      |Total number of  passes via the  call and redo|
   |                 |ports since Prolog was started.               |
   | heap            |Estimated  total   size   of  the   heap  (see|
   |                 |section 2.16.1.1)                             |
   | heapused        |Bytes heap in use by Prolog.                  |
   | heaplimit       |Maximum   size   of   the   heap   (see   sec-|
   |                 |tion 2.16.1.1)                                |
   | local           |Allocated size of the local stack in bytes.   |

   | localused       |Number of bytes in use on the local stack.    |
   | locallimit      |Size to  which the  local stack  is allowed to|
   |                 |grow                                          |
   | global          |Allocated size of the global stack in bytes.  |
   | globalused      |Number of bytes in use on the global stack.   |
   | globallimit     |Size to which  the global stack  is allowed to|
   |                 |grow                                          |

   | trail           |Allocated size of the trail stack in bytes.   |
   | trailused       |Number of bytes in use on the trail stack.    |
   | traillimit      |Size to  which the  trail stack  is allowed to|
   |                 |grow                                          |
   | atoms           |Total number of defined atoms.                |
   | functors        |Total number of defined name/arity pairs.     |
   | predicates      |Total number of predicate definitions.        |
   | modules         |Total number of module definitions.           |

   | codes           |Total amount of byte codes in all clauses.    |
   | threads         |MT-version:  number of active threads         |
   | threads_created |MT-version:  number of created threads        |
   | threads_cputime |MT-version:  seconds CPU time used by finished|
   |_________________|threads_______________________________________|_

                   Table 3.2:  Keys for statistics/2


statistics
    Display a table of system statistics on the current output stream.


time(+Goal)
    Execute  Goal  just like  once/1 (i.e.  leaving  no choice  points),
    but  print used time, number  of logical inferences and the  average
    number  of  lips  (logical  inferences  per  second).     Note  that
    SWI-Prolog  counts the actual  executed number of inferences  rather
    than  the number of passes through  the call- and redo ports of  the
    theoretical 4-port model.


3.44 Finding Performance Bottlenecks

SWI-Prolog  offers  a  statistical  program  profiler  similar  to  Unix
prof(1) for C  and some other languages.   A profiler is used as  an aid
to find performance  pigs in programs.   It provides information on  the
time spent in the various Prolog predicates.

The  profiler  is  based  on the  assumption  that  if  we  monitor  the
functions on  the execution stack  on time  intervals not correlated  to
the program's execution the  number of times we find a procedure  on the
environment stack  is a  measure of  the time spent  in this  procedure.
It  is implemented  by calling  a procedure  each time  slice Prolog  is
active.   This procedure  scans the local stack  and either just  counts
the procedure on top  of this stack (plain profiling) or  all procedures
on  the stack  (cumulative profiling).    To get  accurate results  each
procedure  one is  interested  in should  have  a reasonable  number  of
counts.    Typically  a  minute runtime  will  suffice  to get  a  rough
overview of the most expensive procedures.


profile(+Goal, +Style, +Number)
    Execute  Goal  just  like  time/1.    Collect  profiling  statistics
    according  to  style  (see  profiler/2)  and  show  the  top  Number
    procedures  on  the  current  output  stream  (see  show_profile/1).
    The  results  are kept  in  the database  until reset_profiler/0  or
    profile/3 is  called and can be displayed again with show_profile/1.
    profile/3 is the normal  way to invoke the profiler.  The predicates
    below are low-level predicates that can be used for special cases.


show_profile(+Number)
    Show  the collected  results of the  profiler.   Stops the  profiler
    first  to avoid interference from show_profile/1.  It shows  the top
    Number predicates according the percentage cpu-time used.


profiler(-Old, +New)
    Query  or change  the status of  the profiler.   The  status is  one
    of  off,  plain or  cumulative.    plain implies  the  time used  by
    children  of a predicate is not added to the time of  the predicate.
    For  status cumulative  the time  of children is  added (except  for
    recursive  calls).     Cumulative profiling  implies  the  stack  is
    scanned  up  to the  top  on  each time  slice  to find  all  active
    predicates.    This implies the  overhead grows  with the number  of
    active  frames on the stack.  Cumulative profiling  starts debugging
    mode  to disable tail recursion optimisation, which  would otherwise
    remove  the necessary  parent environments.   Switching status  from
    plain  to cumulative  resets the profiler.    Switching to and  from
    status  off does not reset  the collected statistics, thus  allowing
    to suspend profiling for certain parts of the program.


reset_profiler
    Switches the profiler to off and clears all collected statistics.


profile_count(+Head, -Calls, -Promilage)
    Obtain profile statistics  of the predicate specified by Head.  Head
    is  an atom  for predicates with  arity 0  or a term  with the  same
    name and  arity as the predicate required (see current_predicate/2).
    Calls  is unified with the number  of `calls' and `redos' while  the
    profiler was active.   Promilage is unified with the relative number
    of  counts the predicate  was active (cumulative)  or on top of  the
    stack (plain).  Promilage is an integer between 0 and 1000.


3.45 Memory Management

Note:    limit_stack/2 and  trim_stacks/0 have  no  effect  on  machines
that do  not offer  dynamic stack expansion.    On these machines  these
predicates simply succeed to improve portability.


garbage_collect
    Invoke  the global-  and trail  stack garbage collector.    Normally
    the  garbage   collector  is  invoked  automatically  if  necessary.
    Explicit  invocation  might   be  useful  to  reduce  the  need  for
    garbage  collections in time critical segments  of the code.   After
    the  garbage  collection  trim_stacks/0 is  invoked to  release  the
    collected memory resources.


garbage_collect_atoms
    Reclaim  unused atoms.  Normally invoked  after agc_margin  (a prolog
    flag) atoms have been created.


limit_stack(+Key, +Kbytes)
    Limit one of the  stack areas to the specified value.  Key is one of
    local,  global or trail.   The limit  is an integer, expressing  the
    desired  stack limit in K  bytes.  If  the desired limit is  smaller
    than  the currently  used value,  the limit  is set  to the  nearest
    legal  value above the currently used  value.  If the desired  value
    is  larger than the maximum, the maximum is taken.  Finally,  if the
    desired value is either  0 or the atom unlimited the limit is set to
    its  maximum.   The maximum and initial  limit is determined by  the
    command line options -L, -G and -T.


trim_stacks
    Release  stack memory resources that are not in use at  this moment,
    returning them to the  operating system.  Trim stack is a relatively
    cheap  call.    It can  be used  to release  memory  resources in  a
    backtracking  loop, where the  iterations require typically  seconds
    of execution time  and very different, potentially large, amounts of
    stack space.  Such a loop should be written as follows:

    loop :-
            generator,
                trim_stacks,
                potentially_expensive_operation,
            stop_condition, !.

    The  prolog top level  loop is written  this way, reclaiming  memory
    resources after every user query.


stack_parameter(+Stack, +Key, -Old, +New)
    Query/set  a parameter  for the  runtime stacks.   Stack  is one  of
    local,  global, trail or  argument.   The table below describes  the
    Key/Value pairs.  Old is first unified with the current value.

        ____________________________________________________________
        | limit    |Maximum size of the stack in bytes               |
        |_min_free_|Minimum_free_space_at_entry_of_foreign_predicate_|

    This predicate is  currently only available on versions that use the
    stack-shifter  to enlarge the runtime  stacks when necessary.   It's
    definition is subject to change.


3.46 Windows DDE interface

The  predicates in  this  section  deal with  MS-Windows  `Dynamic  Data
Exchange'  or DDE  protocol.    A Windows  DDE  conversation is  a  form
of interprocess  communication based  on sending reserved  window-events
between the communicating processes.

See also section 5.4 for loading Windows DLL's into SWI-Prolog.


3.46.1 DDE client interface

The DDE client interface  allows Prolog to talk to DDE  server programs.
We  will demonstrate  the use  of the  DDE interface  using the  Windows
PROGMAN (Program Manager) application:

1 ?- open_dde_conversation(progman, progman, C).

C = 0
2 ?- dde_request(0, groups, X)

--> Unifies X with description of groups

3 ?- dde_execute(0, '[CreateGroup("DDE Demo")]').

Yes

4 ?- close_dde_conversation(0).

Yes

For  details   on  interacting  with   progman,   use  the  SDK   online
manual  section on  the  Shell  DDE interface.     See also  the  Prolog
library(progman),  which  may be  used  to  write simple  Windows  setup
scripts in Prolog.


open_dde_conversation(+Service, +Topic, -Handle)
    Open a conversation  with a server supporting the given service name
    and  topic (atoms).    If successful,  Handle  may be  used to  send
    transactions  to the server.    If no willing  server is found  this
    predicate fails silently.


close_dde_conversation(+Handle)
    Close  the  conversation   associated  with  Handle.     All  opened
    conversations  should  be  closed  when they're  no  longer  needed,
    although  the system  will  close any  that remain  open on  process
    termination.


dde_request(+Handle, +Item, -Value)
    Request  a value from the server.   Item is an atom  that identifies
    the  requested  data,  and  Value will  be  a string  (CF_TEXT  data
    in  DDE  parlance)  representing  that  data,   if  the  request  is
    successful.    If unsuccessful, Value  will be  unified with a  term
    of  form error(<Reason>),  identifying the problem.   This call  uses
    SWI-Prolog  string objects  to return  the value  rather then  atoms
    to  reduce the  load on  the atom-space.    See section  3.23 for  a
    discussion on this data type.


dde_execute(+Handle, +Command)
    Request  the  DDE   server  to  execute  the  given  command-string.
    Succeeds  if the  command  could be  executed and  fails with  error
    message otherwise.


dde_poke(+Handle, +Item, +Command)
    Issue  a POKE command to the server on the specified Item.   Command
    is passed as data of type CF_TEXT.


3.46.2 DDE server mode

The (autoload)  library(dde) defines  primitives to  realise simple  DDE
server applications in  SWI-Prolog.  These  features are provided as  of
version 2.0.6 and should be regarded prototypes.  The  C-part of the DDE
server can  handle some  more primitives,  so if you  need features  not
provided by this interface, please study library(dde).


dde_register_service(+Template, +Goal)
    Register  a server  to handle  DDE request or  DDE execute  requests
    from  other applications.  To register a service for a  DDE request,
    Template is of the form:

         +Service(+Topic, +Item, +Value)

    Service  is the name  of the DDE  service provided (like progman  in
    the  client example  above).   Topic is either  an atom,  indicating
    Goal  only handles requests  on this topic  or a variable that  also
    appears  in Goal.  Item and Value are variables that also  appear in
    Goal.  Item represents the request data as a Prolog atom.

    The   example  below  registers   the  Prolog  current_prolog_flag/2
    predicate  to be accessible  from other applications.   The  request
    may  be  given  from  the  same  Prolog  as  well  as  from  another
    application.

    ?- dde_register_service(prolog(current_prolog_flag, F, V),
                            current_prolog_flag(F, V)).

    ?- open_dde_conversation(prolog, current_prolog_flag, Handle),
       dde_request(Handle, home, Home),
       close_dde_conversation(Handle).

    Home = '/usr/local/lib/pl-2.0.6/'

    Handling  DDE execute requests  is very similar.   In this case  the
    template is of the form:

         +Service(+Topic, +Item)

    Passing  a Value argument is  not needed as execute requests  either
    succeed  or fail.  If Goal  fails, a `not processed' is  passed back
    to the caller of the DDE request.


dde_unregister_service(+Service)
    Stop  responding  to  Service.     If  Prolog  is  halted,  it  will
    automatically call this on all open services.


dde_current_service(-Service, -Topic)
    Find currently registered services and the topics served on them.


dde_current_connection(-Service, -Topic)
    Find currently open conversations.


3.47 Miscellaneous


dwim_match(+Atom1, +Atom2)
    Succeeds  if Atom1 matches Atom2  in `Do What I  Mean' sense.   Both
    Atom1  and Atom2  may also be  integers or  floats.   The two  atoms
    match if:

      o  They are identical

      o  They differ by one character (spy  spu)

      o  One character is inserted/deleted (debug  deug)

      o  Two characters are transposed (trace  tarce)

      o  `Sub-words' are glued  differently (existsfile   existsFile
         exists_file)

      o  Two   adjacent  sub   words  are   transposed   (existsFile
         fileExists)


dwim_match(+Atom1, +Atom2, -Difference)
    Equivalent  to  dwim_match/2,  but unifies  Difference  with an  atom
    identifying the the  difference between Atom1 and Atom2.  The return
    values  are (in the same  order as above):   equal, mismatched_char,
    inserted_char, transposed_char, separated and transposed_word.


wildcard_match(+Pattern, +String)
    Succeeds  if String matches the  wildcard pattern Pattern.   Pattern
    is very similar the  the Unix csh pattern matcher.  The patterns are
    given below:

     ?      Matches one arbitrary character.
     *      Matches any number of arbitrary characters.
     [...]  Matches one of the characters specified between the brackets.  <char1>-<char2>indicates a range.
     {...}  Matches any of the patterns of the comma separated list between the braces.

    Example:

    ?- wildcard_match('[a-z]*.{pro,pl}[%~]', 'a_hello.pl%').

    Yes


gensym(+Base, -Unique)
    Generate  a unique  atom from base  Base and  unify it with  Unique.
    Base  should be an  atom.  The  first call will  return <base>1,  the
    next  <base>2, etc.   Note that this is  no warrant that the atom  is
    unique in the system.


sleep(+Time)
    Suspend  execution Time seconds.   Time  is either a floating  point
    number  or an  integer.   Granularity is  dependent on the  system's
    timer  granularity.   A  negative time  causes the  timer to  return
    immediately.   On  most non-realtime operating  systems we can  only
    ensure execution is suspended for at least Time seconds.


Chapter 4.  USING MODULES


4.1 Why Using Modules?

In  traditional Prolog  systems the  predicate  space was  flat.    This
approach   is  not   very  suitable   for  the   development  of   large
applications, certainly not if these applications are  developed by more
than  one programmer.     In many  cases,  the  definition of  a  Prolog
predicate requires  sub-predicates that  are intended  only to  complete
the definition of the main predicate.  With a  flat and global predicate
space these support predicates will be visible from the entire program.

For this reason,  it is desirable that  each source module has it's  own
predicate space.    A module consists  of a  declaration for it's  name,
it's public  predicates and the  predicates themselves.   This  approach
allow the programmer  to use short (local) names for  support predicates
without worrying  about name  conflicts with the  support predicates  of
other  modules.    The  module  declaration also  makes  explicit  which
predicates are  meant for public usage  and which for private  purposes.
Finally,  using the  module information,  cross  reference programs  can
indicate possible problems much better.


4.2 Name-based versus Predicate-based Modules

Two approaches to  realize a module system  are commonly used in  Prolog
and other  languages.  The  first one is  the name based module  system.
In these  systems, each  atom read  is tagged  (normally prefixed)  with
the module  name, with  the exception  of those atoms  that are  defined
public.   In the  second approach, each  module actually implements  its
own predicate space.

A  critical  problem with  using  modules  in Prolog  is  introduced  by
the  meta-predicates  that  transform between  Prolog  data  and  Prolog
predicates.  Consider the case where we write:

:- module(extend, [add_extension/3]).

add_extension(Extension, Plain, Extended) :-
        maplist(extend_atom(Extension), Plain, Extended).

extend_atom(Extension, Plain, Extended) :-
        concat(Plain, Extension, Extended).

In this case  we would like maplist to  call extend_atom/3 in the  module
extend.   A name based module  system will do this  correctly.  It  will
tag the atom  extend_atom  with the module and  maplist will use this  to
construct the tagged  term extend_atom/3.   A name based module  however,
will not only tag the  atoms that will eventually be used to refer  to a
predicate, but all atoms that are not declared public.   So, with a name
based module system also  data is local to the module.   This introduces
another serious problem:

:- module(action, [action/3]).

action(Object, sleep, Arg) :- ....
action(Object, awake, Arg) :- ....

:- module(process, [awake_process/2]).

awake_process(Process, Arg) :-
        action(Process, awake, Arg).

This code  uses a simple  object-oriented implementation technique  were
atoms are used as  method selectors.  Using a name based  module system,
this code will  not work, unless we  declare the selectors public  atoms
in all  modules that use them.   Predicate  based module systems do  not
require particular precautions for handling this case.

It  appears  we  have to  choose  either  to  have  local  data,  or  to
have  trouble with  meta-predicates.    Probably it  is best  to  choose
for  the  predicate  based approach  as  novice  users  will  not  often
write  generic meta-predicates  that  have to  be used  across  multiple
modules, but are likely  to write programs that pass data  around across
modules.   Experienced Prolog  programmers should be  able to deal  with
the complexities of meta-predicates in a predicate based module system.


4.3 Defining a Module

Modules normally are  created by loading a module  file.  A module  file
is a file holding a module/2 directive as its first term.   The module/2
directive declares  the name  and the public  (i.e. externally  visible)
predicates of  the module.   The  rest of  the file is  loaded into  the
module.  Below is an example of a module file, defining reverse/2.

:- module(reverse, [reverse/2]).

reverse(List1, List2) :-
        rev(List1, [], List2).

rev([], List, List).
rev([Head|List1], List2, List3) :-
        rev(List1, [Head|List2], List3).


4.4 Importing Predicates into a Module

As  explained  before,  in  the  predicate  based  approach  adapted  by
SWI-Prolog, each  module has it's own predicate  space.  In  SWI-Prolog,
a module  initially is  completely empty.   Predicates  can be added  to
a  module by  loading a  module  file as  demonstrated in  the  previous
section, using assert or by importing them from another module.

Two mechanisms for  importing predicates explicitly from another  module
exist.   The use_module/[1,2] predicates load  a module file and  import
(part of  the) public predicates  of the file.   The import/1  predicate
imports any predicate from any module.


use_module(+File)
    Load  the  file(s) specified  with File  just like  ensure_loaded/1.
    The  files should  all be  module files.    All exported  predicates
    from  the  loaded  files  are  imported  into  the  context  module.
    The  difference between  this predicate and  ensure_loaded/1 becomes
    apparent  if the file  is already  loaded into another  module.   In
    this  case ensure_loaded/1 does nothing;  use_module will import  all
    public predicates of the module into the current context module.


use_module(+File, +ImportList)
    Load  the file  specified  with File  (only one  file is  accepted).
    File  should be a module file.   ImportList is a list  of name/arity
    pairs  specifying the  predicates that should  be imported from  the
    loaded  module.   If a predicate is  specified that is not  exported
    from  the loaded module  a warning will be  printed.  The  predicate
    will nevertheless be imported to simplify debugging.


import(+Head)
    Import predicate Head  into the current context module.  Head should
    specify the  source module using the <module>:<term>construct.  Note
    that  predicates are normally imported  using one of the  directives
    use_module/[1,2].    import/1  is meant  for handling  imports  into
    dynamically created modules.

It  would  be rather  inconvenient  to  have to  import  each  predicate
referred to by  the module, including the  system predicates.  For  this
reason each  module is  assigned a default  module.   All predicates  in
the default  module are  available without  extra declarations.    Their
definition however can be overruled in the local module.   This schedule
is implemented  by the exception handling  mechanism of SWI-Prolog:   if
an  undefined predicate  exception is  raised for  a  predicate in  some
module, the exception  handler first tries to import the  predicate from
the module's default module.  On success, normal execution is resumed.


4.4.1 Reserved Modules

SWI-Prolog contains two  special modules.   The first one is the  module
system.    This module  contains all  built-in  predicates described  in
this  manual.   Module  system has  no default  module  assigned to  it.
The second  special module is the  module user.   This module forms  the
initial  working space  of the  user.    Initially  it is  empty.    The
default module of module  user is system, making all  built-in predicate
definitions available  as defaults.    Built-in predicates  thus can  be
overruled by defining them in module user before they are used.

All other  modules default to module  user.   This implies they can  use
all predicates imported into user without explicitly importing them.


4.5 Using the Module System

The  current structure  of  the module  system  has been  designed  with
some specific  organisations for  large programs  in mind.   Many  large
programs  define a  basic  library layer  on  top  of which  the  actual
program itself  is defined.    The module  user, acting  as the  default
module for all  other modules of the  program can be used to  distribute
these definitions over  all program module without introducing the  need
to  import this  common layer  each time  explicitly.   It  can also  be
used to  redefine built-in predicates  if this  is required to  maintain
compatibility  to some  other Prolog  implementation.    Typically,  the
loadfile of a large application looks like this:

:- use_module(compatibility).   % load XYZ prolog compatibility

:- use_module(                  % load generic parts
        [ error                 % errors and warnings
        , goodies               % general goodies (library extensions)
        , debug                 % application specific debugging
        , virtual_machine       % virtual machine of application
        , ...                   % more generic stuff
        ]).

:- ensure_loaded(
        [ ...                   % the application itself
        ]).

The  `use_module' declarations  will  import the  public predicates  from
the  generic  modules  into  the  user  module.     The  `ensure_loaded'
directive  loads the  modules that  constitute  the actual  application.
It is  assumed these  modules import  predicates from  each other  using
use_module/[1,2] as far as necessary.

In combination  with the  object-oriented schema described  below it  is
possible  to define  a neat  modular  architecture.   The  generic  code
defines general utilities  and the message passing predicates  (invoke/3
in the  example below).    The application modules  define classes  that
communicate using the message passing predicates.


4.5.1 Object Oriented Programming

Another typical  way to use  the module system  is for defining  classes
within  an object  oriented  paradigm.    The  class structure  and  the
methods  of  a class  can  be  defined  in a  module  and  the  explicit
module-boundary overruling  describes in  section 4.6.2 can  by used  by
the message passing  code to invoke the behaviour.   An outline of  this
mechanism is given below.

%       Define class point

:- module(point, []).           % class point, no exports

%        name           type,           default access
%                                       value

variable(x,             integer,        0,      both).
variable(y,             integer,        0,      both).

%         method name   predicate name  arguments

behaviour(mirror,       mirror,         []).

mirror(P) :-
        fetch(P, x, X),
        fetch(P, y, Y),
        store(P, y, X),
        store(P, x, Y).

The predicates fetch/3  and store/3 are predicates that change  instance
variables of instances.  The figure below indicates  how message passing
can easily be implemented:

%       invoke(+Instance, +Selector, ?ArgumentList)
%       send a message to an instance

invoke(I, S, Args) :-
        class_of_instance(I, Class),
        Class:behaviour(S, P, ArgCheck), !,
        convert_arguments(ArgCheck, Args, ConvArgs),
        Goal =.. [P|ConvArgs],
        Class:Goal.

The construct  <Module>:<Goal> explicitly  calls Goal  in module  Module.
It is discussed in more detail in section 3.8.


4.6 Meta-Predicates in Modules

As indicated  in the introduction,  the problem  with a predicate  based
module system lies in the difficulty to find the  correct predicate from
a Prolog  term.   The predicate `solution(Solution)'  can exist in  more
than one  module, but `assert(solution(4))'  in some module is  supposed
to refer to the correct version of solution/1.

Various  approaches are  possible to  solve this  problem.    One is  to
add an extra  argument to all predicates (e.g. `assert(Module,  Term)').
Another is to tag  the term somehow to indicate which module  is desired
(e.g. `assert(Module:Term)').   Both approaches are not very  attractive
as  they make  the user  responsible for  choosing  the correct  module,
inviting  unclear  programming by  asserting  in  other modules.     The
predicate assert/1  is supposed  to assert  in the module  it is  called
from and should do  so without being told explicitly.  For  this reason,
the notion context module has been introduced.


4.6.1 Definition and Context Module

Each  predicate  of  the program  is  assigned  a  module,  called  it's
definition module.   The definition module of a predicate is  always the
module in which the predicate was originally defined.   Each active goal
in the Prolog system has a context module assigned to it.

The  context module  is used  to  find predicates  from a  Prolog  term.
By  default, this  module  is the  definition  module of  the  predicate
running the  goal.   For meta-predicates  however, this  is the  context
module of the  goal that invoked them.   We call this module_transparent
in SWI-Prolog.    In the `using  maplist' example  above, the  predicate
maplist/3  is declared  module_transparent.    This  implies the  context
module remains extend, the  context module of add_extension/3.   This way
maplist/3 can decide to call extend_atom in module extend rather than in
it's own definition module.

All built-in predicates that  refer to predicates via a Prolog  term are
declared module_transparent.  Below is the code defining maplist.

:- module(maplist, maplist/3).

:- module_transparent maplist/3.

%       maplist(+Goal, +List1, ?List2)
%       True if Goal can successfully be applied to all succes-
sive pairs
%       of elements of List1 and List2.

maplist(_, [], []).
maplist(Goal, [Elem1|Tail1], [Elem2|Tail2]) :-
        apply(Goal, [Elem1, Elem2]),
        maplist(Goal, Tail1, Tail2).


4.6.2 Overruling Module Boundaries

The  mechanism  above  is sufficient  to  create  an  acceptable  module
system.   There are however cases in which  we would like to be  able to
overrule this  schema and  explicitly call  a predicate  in some  module
or assert  explicitly in some  module.   The first  is useful to  invoke
goals  in  some module  from  the  user's  toplevel or  to  implement  a
object-oriented system (see above).  The latter is  useful to create and
modify dynamic modules (see section 4.7).

For  this purpose,  the reserved  term :/2  has  been introduced.    All
built-in predicates  that transform  a term into  a predicate  reference
will check whether  this term is of  the form `<Module>:<Term>'.  If  so,
the predicate is  searched for in Module  instead of the goal's  context
module.   The :  operator may be  nested, in  which case the  inner-most
module is used.

The special  calling construct  <Module>:<Goal>pretends  Goal is  called
from Module instead of the context module.  Examples:

?- assert(world:done).  % asserts done/0 into module world
?- world:assert(done).  % the same
?- world:done.          % calls done/0 in module world


4.7 Dynamic Modules

So  far,   we  discussed  modules  that   were  created  by  loading   a
module-file.     These  modules  have  been   introduced  on  facilitate
the  development  of  large  applications.     The   modules  are  fully
defined at  load-time of the  application and  normally will not  change
during  execution.   Having  the  notion of  a set  of  predicates as  a
self-contained world can be attractive for other purposes as  well.  For
example, assume  an application that  can reason about multiple  worlds.
It is attractive  to store the data of  a particular world in a  module,
so we extract information from a world simply by  invoking goals in this
world.

Dynamic modules  can easily  be created.   Any  built-in predicate  that
tries  to locate  a predicate  in  a specific  module will  create  this
module as a side-effect if it did not yet exist.  Example:

?- assert(world_a:consistent),
   world_a:unknown(_, fail).

These  calls  create  a  module  called  `world_a'  and  make  the  call
`world_a:consistent'  succeed.  Undefined  predicates will not start  the
tracer or autoloader for this module (see unknown/2).

Import and  export from dynamically  created world  is arranged via  the
predicates import/1 and export/1:

?- world_b:export(solve(_,_)).          % exports solve/2 from world_b
?- world_c:import(world_b:solve(_,_)).  % and import it to world_c


4.8 Module Handling Predicates

This section gives the predicate definitions for  the remaining built-in
predicates that handle modules.


:- module(+Module, +PublicList)
    This directive can only  be used as the first term of a source file.
    It  declares  the file  to be  a module  file,  defining Module  and
    exporting  the predicates of  PublicList.   PublicList is a list  of
    name/arity pairs.


module_transparent +Preds
    Preds   is  a  comma  separated  list  of  name/arity   pairs  (like
    dynamic/1).     Each goal  associated  with a  transparent  declared
    predicate will inherit the context module from its parent goal.


meta_predicate +Heads
    This  predicate  is  defined  in  library(quintus)  and  provides  a
    partial  emulation of the Quintus predicate.  See section  4.9.1 for
    details.


current_module(-Module)
    Generates all currently known modules.


current_module(?Module, ?File)
    Is  true if File is the file from which Module was loaded.   File is
    the internal canonical filename.  See also source_file/[1,2].


context_module(-Module)
    Unify   Module  with  the  context  module  of  the   current  goal.
    context_module/1 itself is transparent.


export(+Head)
    Add  a predicate to  the public list  of the context  module.   This
    implies  the predicate will be imported into another module  if this
    module is  imported with use_module/[1,2].   Note that predicates are
    normally  exported using the directive module/2.  export/1  is meant
    to handle export from dynamically created modules.


export_list(+Module, ?Exports)
    Unifies  Exports with a list of terms.   Each term has the  name and
    arity  of a public predicate of Module.   The order of the  terms in
    Exports is not defined.  See also predicate_property/2.


default_module(+Module, -Default)
    Succesively unifies Default  with the module names from which a call
    in  Module attempts to  use the  definition.   For the module  user,
    this  will generate user  and system.   For  any other module,  this
    will generate the module itself, followed by user and system.


module(+Module)
    The  call module(Module) may be  used to switch the default  working
    module  for the interactive  toplevel (see prolog/0).   This may  be
    used  to when  debugging  a module.    The example  below lists  the
    clauses of file_of_label/2 in the module tex.

    1 ?- module(tex).

    Yes
    tex: 2 ?- listing(file_of_label/2).
    ...


4.9 Compatibility of the Module System

The  principles behind  the  module system  of  SWI-Prolog differ  in  a
number of aspects from the Quintus Prolog module system.

  o The  SWI-Prolog module  system allows  the user  to redefine  system
    predicates.

  o All  predicates that are  available in the  system and user  modules
    are visible in all other modules as well.

  o Quintus  has the `meta_predicate/1' declaration were SWI-Prolog  has
    the module_transparent/1 declaration.

The meta_predicate/1 declaration  causes the  compiler to tag  arguments
that pass  module sensitive information  with the  module using the  :/2
operator.  This approach has some disadvantages:

  o Changing   a  meta_predicate   declaration  implies  all   predicates
    calling  the predicate need to be reloaded.  This can  cause serious
    consistency problems.

  o It  does not help for dynamically defined predicates  calling module
    sensitive predicates.

  o It   slows  down   the  compiler   (at  least   in  the   SWI-Prolog
    architecture).

  o At  least within the  SWI-Prolog architecture the run-time  overhead
    is   larger  than  the   overhead  introduced  by  the   transparent
    mechanism.

Unfortunately  the   transparent  predicate   approach  also  has   some
disadvantages.   If a  predicate A passes  module sensitive information
to a  predicate B, passing  the same information to  a module sensitive
system predicate both  A and B  should be declared transparent.   Using
the Quintus approach only A  needs to be treated special (i.e. declared
with  meta_predicate/1).    A  second problem  arises if  the  body of  a
transparent  predicate uses  module sensitive  predicates  for which  it
wants to refer to its  own module.  Suppose we want to  define findall/3
using assert/1  and retract/1.    The example  in figure  4.1 gives  the
solution.
:- module(findall, [findall/3]).

:- dynamic

        solution/1.

:- module_transparent
        findall/3,
        store/2.

findall(Var, Goal, Bag) :-

        assert(findall:solution('$mark')),
        store(Var, Goal),
        collect(Bag).

store(Var, Goal) :-
        Goal,                   % refers to context module of
                                % caller of findall/3
        assert(findall:solution(Var)),

        fail.
store(_, _).

collect(Bag) :-
        ...,

                  Figure 4.1:  findall/3 using modules


4.9.1 Emulating meta_predicate/1

The Quintus  meta_predicate/1 directive can  in many  cases be  replaced
by  the   transparent  declaration.      Below  is  the  definition   of
meta_predicate/1 as available from library(quintus).

:- op(1150, fx, (meta_predicate)).

meta_predicate((Head, More)) :- !,
        meta_predicate1(Head),
        meta_predicate(More).
meta_predicate(Head) :-
        meta_predicate1(Head).

meta_predicate1(Head) :-
        Head =.. [Name|Arguments],
        member(Arg, Arguments),
        module_expansion_argument(Arg), !,
        functor(Head, Name, Arity),
        module_transparent(Name/Arity).
meta_predicate1(_).             % just a mode declaration

module_expansion_argument(:).
module_expansion_argument(N) :- integer(N).

The discussion above  about the problems with the transparent  mechanism
show the two cases in which this simple transformation does not work.


Chapter 5.  FOREIGN LANGUAGE INTERFACE

SWI-Prolog      offers      a     powerful      interface      to      C
[Kernighan & Ritchie, 1978].     The  main  design   objectives  of  the
foreign language interface  are flexibility and performance.   A foreign
predicate  is a  C-function that  has the  same number  of arguments  as
the predicate  represented.   C-functions  are provided  to analyse  the
passed terms,  convert them to basic C-types  as well as to  instantiate
arguments using unification.   Non-deterministic foreign predicates  are
supported,  providing the  foreign  function with  a handle  to  control
backtracking.

C can  call Prolog  predicates, providing  both an  query interface  and
an interface  to extract  multiple solutions  from an  non-deterministic
Prolog predicate.   There is no limit  to the nesting of Prolog  calling
C, calling Prolog,  etc.  It is also  possible to write the `main'  in C
and use Prolog as an embedded logical engine.


5.1 Overview of the Interface

A special include file called SWI-Prolog.h should be included  with each
C-source file  that is  to be  loaded via the  foreign interface.    The
installation process installs this file in the directory  include in the
SWI-Prolog home  directory (?- current_prolog_flag(home, Home).).   This
C-header file defines various data types, macros and  functions that can
be used  to communicate with  SWI-Prolog.  Functions  and macros can  be
divided into the following categories:

  o Analysing Prolog terms

  o Constructing new terms

  o Unifying terms

  o Returning control information to Prolog

  o Registering foreign predicates with Prolog

  o Calling Prolog from C

  o Recorded database interactions

  o Global actions on Prolog (halt, break, abort, etc.)


5.2 Linking Foreign Modules

Foreign modules  may be linked to  Prolog in three  ways.  Using  static
linking,  the  extensions,  a  small  description  file  and  the  basic
SWI-Prolog object  file are linked  together to  form a new  executable.
Using dynamic  linking, the  extensions are linked  to a shared  library
(.so file on  most Unix systems) or  dynamic-link library (.DLL file  on
Microsoft platforms) and loaded into the the running Prolog process..


5.2.1 What linking is provided?

The static  linking schema can  be used on  all versions of  SWI-Prolog.
Whether or  not dynamic  linking is  supported can be  deduced from  the
prolog-flag  open_shared_object  (see current_prolog_flag/2).    If  this
prolog-flag yields true, open_shared_object/2 and related predicates are
defined.  See  section 5.4 for a suitable high-level interface  to these
predicates.


5.2.2 What kind of loading should I be using?

All  described  approaches  have  their  advantages  and  disadvantages.
Static linking  is portable and allows  for debugging on all  platforms.
It is relatively  cumbersome and the libraries  you need to pass to  the
linker may vary from  system to system, though the utility  program plld
described in section 5.7 often hides these problems from the user.

Loading  shared objects  (DLL  files on  Windows) provides  sharing  and
protection  and is  generally the  best choice.    If  a saved-state  is
created using qsave_program/[1,2], an initialization/1 directive  may be
used to load the appropriate library at startup.

Note  that  the  definition of  the  foreign  predicates  is  the  same,
regardless of the linking type used.


5.3 Dynamic Linking of shared libraries

The interface  defined in this  section allows the  user to load  shared
libraries  (.so files  on most  Unix systems,  .dll  files on  Windows).
This  interface is  portable to  Windows  as well  as to  Unix  machines
providing  dlopen(2)  (Solaris,  Linux,  FreeBSD, Irix  and  many  more)
or  shl_open(2) (HP/UX).  It  is  advised to  use  the  predicates  from
section 5.4 in your application.


open_shared_object(+File, -Handle)
    File  is the name of a  .so file (see your C programmers  documenta-
    tion  on how to create  a .so file).   This file is attached to  the
    current  process and Handle is unified  with a handle to the  shared
    object.    Equivalent to open_shared_object(File, [global], Handle).
    See also load_foreign_library/[1,2].

    On  errors, an  exception shared_object(Action, Message) is  raised.
    Message is the return value from dlerror().


open_shared_object(+File, +Options, -Handle)
    As  open_shared_object/2, but  allows  for  additional flags  to  be
    passed.   Options is a list of  atoms.  now implies the  symbols are
    resolved  immediately rather  than lazy (default).   global  implies
    symbols of the  loaded object are visible while loading other shared
    objects (by default they  are local).  Note that these flags may not
    be  supported by your operating system.  Check the  documentation of
    dlopen() or equivalent  on your operating system.  Unsupported flags
    are silently ignored.


close_shared_object(+Handle)
    Detach the shared object identified by Handle.


call_shared_object_function(+Handle, +Function)
    Call the named function  in the loaded shared library.  The function
    is  called  without  arguments  and  the  return-value  is  ignored.
    Normally  this function installs  foreign language predicates  using
    calls to PL_register_foreign().


5.4 Using the library shlib for .DLL and .so files

This  section discusses  the  functionality  of the  (autoload)  library
shlib.pl, providing an interface to shared libraries.   This library can
only be used if the prolog-flag open_shared_object is enabled.


load_foreign_library(+Lib, +Entry)
    Search  for the  given foreign library  and link  it to the  current
    SWI-Prolog  instance.  The library may be specified with  or without
    the  extension.    First,  absolute_file_name/3 is  used  to  locate
    the  file.    If  this succeeds,  the  full path  is passed  to  the
    low-level  function  to open  the library.    Otherwise,  the  plain
    library  name  is passed,  exploiting  the operating-system  defined
    search  mechanism for  the shared  library.   The file_search_path/2
    alias  mechanism  defines the  alias foreign,  which  refers to  the
    directories <plhome>/lib/<arch>and <plhome>/lib, in this order.

    If  the library  can be loaded,  the function  called Entry will  be
    called  without arguments.    The return  value of  the function  is
    ignored.

    The  Entry  function  will  normally  call PL_register_foreign()  to
    declare functions in the library as foreign predicates.


load_foreign_library(+Lib)
    Equivalent  to load_foreign_library/2.   For  the entry-point,  this
    function  first identifies the `base-name' of the library,  which is
    defined  to be the file-name with path nor extension.  It  will then
    try  the  entry-point install-<base>.    On  failure it  will try  to
    function install().  Otherwise no install function will be called.


unload_foreign_library(+Lib)
    If  the foreign  library defines  the function  uninstall_<base>() or
    uninstall(),  this  function will  be called  without arguments  and
    its  return value is  ignored.   Next, abolish/2  is used to  remove
    all  known foreign predicates defined in  the library.  Finally  the
    library itself is detached from the process.


current_foreign_library(-Lib, -Predicates)
    Query  the currently loaded foreign libraries and  their predicates.
    Predicates  is  a  list  with  elements  of  the  form  Module:Head,
    indicating  the predicates installed with PL_register_foreign() when
    the entry-point of the library was called.

Figure  5.1 connects  a Windows  message-box using  a foreign  function.
This example was tested using Windows NT and Microsoft Visual C++ 2.0.

#include <windows.h>
#include <SWI-Prolog.h>

static foreign_t
pl_say_hello(term_t to)
{ char *a;

  if ( PL_get_atom_chars(to, &a) )
  { MessageBox(NULL, a, "DLL test", MB_OK|MB_TASKMODAL);

    PL_succeed;
  }

  PL_fail;
}

install_t
install()
{ PL_register_foreign("say_hello", 1, pl_say_hello, 0);
}

            Figure 5.1:  MessageBox() example in Windows NT


5.4.1 Static Linking

Below  is an  outline of  the files  structure  required for  statically
linking  SWI-Prolog  with  foreign extensions.     \ldots/pl  refers  to
the  SWI-Prolog  home  directory (see  current_prolog_flag/2).    <arch>
refers  to  the  architecture identifier  that  may  be  obtained  using
current_prolog_flag/2.

     .../pl/runtime/<arch>/libpl.a   SWI-Library
     \ldots/pl/include/SWI-Prolog.h  Include file
     \ldots/pl/include/SWI-Stream.h  Stream I/O include file
     \ldots/pl/include/SWI-Exports   Export declarations (AIX only)
     \ldots/pl/include/stub.c        Extension stub

The  definition   of  the  foreign  predicates   is  the  same  as   for
dynamic  linking.   Unlike  with dynamic  linking however,  there is  no
initialisation  function.   Instead,  the file  \ldots/pl/include/stub.c
may  be copied  to  your project  and  modified  to define  the  foreign
extensions.   Below is  stub.c, modified to  link the lowercase  example
described later in this chapter:

/*  Copyright (c) 1991 Jan Wielemaker. All rights reserved.
    jan@swi.psy.uva.nl

    Purpose: Skeleton for extensions
*/

#include <stdio.h>
#include <SWI-Prolog.h>

extern foreign_t pl_lowercase(term, term);

PL_extension predicates[] =
{
/*{ "name",      arity,  function,      PL_FA_<flags> },*/

  { "lowercase", 2       pl_lowercase,  0 },
  { NULL,        0,      NULL,          0 }     /* terminating line */
};

int
main(int argc, char **argv)
{ PL_register_extensions(predicates);

  if ( !PL_initialise(argc, argv) )
    PL_halt(1);

  PL_install_readline();                /* delete if not required */

  PL_halt(PL_toplevel() ? 0 : 1);
}

Now, a new executable may be created by compiling  this file and linking
it to libpl.a from  the runtime directory and the libraries  required by
both the  extensions and the  SWI-Prolog kernel.   This  may be done  by
hand, or using the plld utility described in secrefplld.


5.5 Interface Data types


5.5.1 Type term_t:  a reference to a Prolog term

The principal  data-type is term_t.   Type term_t is what Quintus  calls
QP_term_ref.   This name indicates  better what the type represents:   it
is a handle for a  term rather than the term itself.  Terms can  only be
represented and manipulated  using this type, as  this is the only  safe
way to  ensure the  Prolog kernel is  aware of  all terms referenced  by
foreign code  and thus allows the  kernel to perform  garbage-collection
and/or stack-shifts while foreign  code is active, for example  during a
callback from C.

A term  reference is  a C unsigned  long, representing  the offset of  a
variable on the Prolog environment-stack.  A foreign  function is passed
term references for the predicate-arguments, one for each  argument.  If
references for intermediate  results are needed, such references  may be
created using PL_new_term_ref() or PL_new_term_refs().   These references
normally live till the foreign function returns control  back to Prolog.
Their scope can  be explicitly limited using PL_open_foreign_frame() and
PL_close_foreign_frame()/PL_discard_foreign_frame().

A term_t  always refers to a valid Prolog term (variable,  atom, integer,
float  or compound  term).    A  term  lives either  until  backtracking
takes  us back  to a  point before  the term  was created,  the  garbage
collector  has collected  the  term or  the  term  was created  after  a
PL_open_foreign_frame()and PL_discard_foreign_frame()has been called.

The  foreign-interface functions  can either  read,  unify or  write  to
term-references.   In the  this document we  use the following  notation
for arguments of type term_t:

     term_t +t  Accessed   in  read-mode.       The   `+'
                indicates the argument is `input'.
     term_t -t  Accessed in write-mode.
     term_t ?t  Accessed in unify-mode.

Term references are obtained in any of the following ways.

  o Passed as argument
    The  C-functions implementing  foreign predicates  are passed  their
    arguments  as term-references.    These  references may  be read  or
    unified.  Writing to these variables causes undefined behaviour.

  o Created by PL_new_term_ref()
    A  term  created  by  PL_new_term_ref() is normally  used  to  build
    temporary  terms or be  written by one  of the interface  functions.
    For  example, PL_get_arg() writes a  reference to the  term-argument
    in its last argument.

  o Created by PL_new_term_refs(int n)
    This   function  returns  a   set  of  term   refs  with  the   same
    characteristics as PL_new_term_ref().  See PL_open_query().

  o Created by PL_copy_term_ref(term_t t)
    Creates a new term-reference  to the same term as the argument.  The
    term may be written to.  See figure 5.3.

Term-references  can safely  be  copied  to other  C-variables  of  type
term_t, but all copies will always refer to the same term.


term_t PL_new_term_ref()
    Return  a fresh reference  to a  term.   The reference is  allocated
    on  the local  stack.   Allocating  a term-reference  may trigger  a
    stack-shift  on machines  that cannot  use sparse-memory  management
    for allocation the  Prolog stacks.  The returned reference describes
    a variable.


term_t PL_new_term_refs(int n)
    Return  n  new  term  references.     The  first  term-reference  is
    returned.   The others are t +1, t +2, etc.   There are two reasons
    for  using this function.   PL_open_query()expects the  arguments as
    a  set of  consecutive term references  and very time-critical  code
    requiring a number of term-references can be written as:

    pl_mypredicate(term_t a0, term_t a1)
    { term_t t0 = PL_new_term_refs(2);
      term_t t1 = t0+1;

      ...
    }


term_t PL_copy_term_ref(term_t from)
    Create a new term  reference and make it point initially to the same
    term  as from.  This function  is commonly used to copy  a predicate
    argument to a term reference that may be written.


void PL_reset_term_refs(term_t after)
    Destroy  all term  references that  have been  created after  after,
    including  after itself.    Any  reference to  the invalidated  term
    references after this call results in undefined behaviour.

    Note  that  returning  from  the  foreign  context  to  Prolog  will
    reclaim  all references  used in  the foreign  context.   This  call
    is  only necessary  if  references are  created inside  a loop  that
    never  exits  back to  Prolog.    See  also  PL_open_foreign_frame(),
    PL_close_foreign_frame() and PL_discard_foreign_frame().


5.5.1.1 Interaction with the garbage collector and stack-shifter

Prolog implements two  mechanisms for avoiding stack overflow:   garbage
collection and stack expansion.   On machines that allow for  it, Prolog
will use virtual  memory management to detect stack overflow  and expand
the  runtime stacks.    On  other machines  Prolog will  reallocate  the
stacks and update all pointers to them.  To do so,  Prolog needs to know
which data is  referenced by C-code.  As  all Prolog data known by  C is
referenced through term references (term_t), Prolog has  all information
necessary to perform  its memory management without special  precautions
from the C-programmer.


5.5.2 Other foreign interface types

atom_t An atom in Prologs  internal representation.  Atoms  are pointers
    to  an  opaque structure.    They are  a  unique representation  for
    represented  text, which  implies that  atom A  represents the same
    text as atom B  if-and-only-if A and B are the same pointer.

    Atoms  are  the  central  representation for  textual  constants  in
    Prolog  The  transformation  of C  a  character  string to  an  atom
    implies  a hash-table lookup.  If the same atom is needed  often, it
    is  advised to  store its reference  in a  global variable to  avoid
    repeated lookup.

functor_t A  functor is  the  internal  representation of  a  name/arity
    pair.   They are used to find the name and arity of  a compound term
    as  well as to construct new compound  terms.  Like atoms  they live
    for the whole Prolog session and are unique.

predicate_t Handle  to a  Prolog  predicate.    Predicate  handles  live
    forever (although they can loose their definition).

qid_t Query         Identifier.                          Used         by
    PL_open_query()/PL_next_solution()/PL_close_query() to  handle  back-
    tracking from C.

fid_t Frame         Identifier.                          Used         by
    PL_open_foreign_frame()/PL_close_foreign_frame().

module_t A module is  a unique handle to a  Prolog module.  Modules  are
    used only to call predicates in a specific module.

foreign_t Return type for a C-function implementing a Prolog predicate.

control_t Passed  as additional  argument to  non-deterministic  foreign
    functions.  See PL_retry*() and PL_foreign_context*().

install_t Type for the install() and uninstall() functions  of shared or
    dynamic link libraries.  See secrefshlib.


5.6 The Foreign Include File


5.6.1 Argument Passing and Control

If  Prolog encounters  a foreign  predicate  at run  time it  will  call
a  function  specified  in  the  predicate  definition  of  the  foreign
predicate.   The arguments 1;:::; <arity>pass the  Prolog arguments to the
goal as  Prolog terms.   Foreign  functions should  be declared of  type
foreign_t.   Deterministic foreign  functions have  two alternatives  to
return control back to Prolog:


void PL_succeed()
    Succeed deterministically.  PL_succeed is defined as return TRUE.


void PL_fail()
    Fail  and  start  Prolog  backtracking.     PL_fail  is  defined  as
    return FALSE.


5.6.1.1 Non-deterministic Foreign Predicates

By  default   foreign  predicates   are  deterministic.      Using   the
PL_FA_NONDETERMINISTIC   attribute  (see   PL_register_foreign())  it   is
possible  to register  a  predicate  as a  non-deterministic  predicate.
Writing   non-deterministic   foreign  predicates   is   slightly   more
complicated  as  the  foreign function  needs  context  information  for
generating  the next  solution.   Note  that the  same foreign  function
should  be  prepared  to be  simultaneously  active  in  more  than  one
goal.     Suppose  the  natural_number_below_n/2  is  a  non-deterministic
foreign predicate, backtracking over all natural numbers  lower than the
first argument.  Now consider the following predicate:

quotient_below_n(Q, N) :-
        natural_number_below_n(N, N1),
        natural_number_below_n(N, N2),
        Q =:= N1 / N2, !.

In  this predicate  the function  natural_number_below_n/2  simultaneously
generates solutions for both its invocations.

Non-deterministic foreign functions  should be prepared to handle  three
different calls from Prolog:

  o Initial call (PL_FIRST_CALL)
    Prolog  has just created a frame  for the foreign function and  asks
    it to produce the first answer.

  o Redo call (PL_REDO)
    The previous invocation  of the foreign function associated with the
    current  goal indicated it was possible  to backtrack.  The  foreign
    function should produce the next solution.

  o Terminate call (PL_CUTTED)
    The choice point  left by the foreign function has been destroyed by
    a  cut.  The foreign function is given the opportunity to  clean the
    environment.

Both  the  context  information  and  the  type  of   call  is  provided
by  an  argument  of  type  control_t  appended  to  the  argument  list
for  deterministic foreign  functions.   The  macro PL_foreign_control()
extracts  the   type  of  call   from  the  control   argument.      The
foreign  function  can  pass  a context  handle  using  the  PL_retry*()
macros  and  extract  the handle  from  the  extra  argument  using  the
PL_foreign_context*() macro.


void PL_retry(long)
    The  foreign function  succeeds while leaving  a choice  point.   On
    backtracking  over this  goal the  foreign function  will be  called
    again,  but the control argument now  indicates it is a `Redo'  call
    and  the macro  PL_foreign_context() will return  the handle  passed
    via  PL_retry().   This  handle is a 30  bits signed value (two  bits
    are used for status indication).


void PL_retry_address(void *)
    As  PL_retry(),  but ensures  an address as  returned by malloc()  is
    correctly recovered by PL_foreign_context_address().


int PL_foreign_control(control_t)
    Extracts  the type of  call from the control  argument.  The  return
    values  are  described above.    Note that  the  function should  be
    prepared  to handle the PL_CUTTED case and should be aware  that the
    other arguments are not valid in this case.


long PL_foreign_context(control_t)
    Extracts  the context from the context  argument.  In the call  type
    is  PL_FIRST_CALL the context value is 0L. Otherwise it  is the value
    returned  by the last PL_retry() associated with this goal  (both if
    the call type is PL_REDO as PL_CUTTED).


void * PL_foreign_context_address(control_t)
    Extracts an address as passed in by PL_retry_address().

Note:  If a  non-deterministic foreign function returns using PL_succeed
or  PL_fail,   Prolog  assumes  the  foreign  function  has  cleaned  its
environment.  No call with control argument PL_CUTTED will follow.

The code of figure 5.2 shows a skeleton for  a non-deterministic foreign
predicate definition.

typedef struct                  /* define a context structure */
{ ...
} context;

foreign_t
my_function(term_t a0, term_t a1, foreign_t handle)
{ struct context * ctxt;

  switch( PL_foreign_control(handle) )
  { case PL_FIRST_CALL:
        ctxt = malloc(sizeof(struct context));
        ...
        PL_retry_address(ctxt);
    case PL_REDO:

        ctxt = PL_foreign_context_address(handle);
        ...
        PL_retry_address(ctxt);
    case PL_CUTTED:
        free(ctxt);
        PL_succeed;
  }
}

     Figure 5.2:  Skeleton for non-deterministic foreign functions


5.6.2 Atoms and functors

The  following  functions  provide for  communication  using  atoms  and
functors.


atom_t PL_new_atom(const char *)
    Return an atom handle  for the given C-string.  This function always
    succeeds.    The returned handle  is valid  as long as  the atom  is
    referenced (see section 5.6.2.1).


const char* PL_atom_chars(atom_t atom)
    Return  a C-string for the text represented by the given atom.   The
    returned  text will not be changed by Prolog.  It is  not allowed to
    modify  the contents, not even `temporary' as the string  may reside
    in read-only memory.


functor_t PL_new_functor(atom_t name, int arity)
    Returns  a functor  identifier, a  handle for  the name/arity  pair.
    The returned handle is valid for the entire Prolog session.


atom_t PL_functor_name(functor_t f)
    Return an atom representing the name of the given functor.


int PL_functor_arity(functor_t f)
    Return the arity of the given functor.


5.6.2.1 Atoms and atom-garbage collection

With  the introduction  of  atom-garbage  collection in  version  3.3.0,
atoms  no longer  have live  as long  as the  process.   Instead,  their
lifetime is  guaranteed only as  long as  they are referenced.   In  the
single-threaded version,  atom garbage collections  are only invoked  at
the call-port.   In the multi-threaded  version (see section 3.39,  they
appear asynchronously, except for the invoking thread.

For dealing with  atom garbage collection, two additional  functions are
provided:


void PL_register_atom(atom_t atom)
    Increment  the reference count  of the  atom by one.   PL_new_atom()
    performs  this automatically,  returning  an atom  with a  reference
    count of at least one.


void PL_unregister_atom(atom_t atom)
    Decrement  the reference count of the atom.  If  the reference-count
    drops below zero, an assertion error is raised.

Please note that the  following two calls are different with  respect to
atom garbage collection:

PL_unify_atom_chars(t, "text");
PL_unify_atom(t, PL_new_atom("text"));

The  latter increments  the  reference count  of  the atom  text,  which
effectively ensures the atom will never be collected.   It is adviced to
use the *_chars() or *_nchars() functions whenever applicable.


5.6.3 Analysing Terms via the Foreign Interface

Each argument  of a foreign function  (except for the control  argument)
is of type term_t, an opaque  handle to a Prolog term.  Three  groups of
functions are  available for  the analysis  of terms.    The first  just
validates the type,  like the Prolog  predicates var/1, atom/1, etc  and
are  called PL_is_*().    The second  group  attempts to  translate  the
argument into a C primitive type.   These predicates take a term_t and a
pointer to the appropriate C-type and return TRUE or  FALSE depending on
successful or unsuccessful translation.   If the translation fails,  the
pointed-to data is never modified.


5.6.3.1 Testing the type of a term


int PL_term_type(term_t)
    Obtain  the type  of a  term,  which should  be a  term returned  by
    one  of the  other interface  predicates or passed  as an  argument.
    The  function  returns the  type  of  the Prolog  term.    The  type
    identifiers  are listed below.   Note that the extraction  functions
    PL_ge_t*() also validate  the type and  thus the two sections  below
    are equivalent.

            if ( PL_is_atom(t) )
            { char *s;

              PL_get_atom_chars(t, &s);
              ...;
            }

    or

            char *s;
            if ( PL_get_atom_chars(t, &s) )
            { ...;
            }

    ___________________________________________________________________
    | PL_VARIABLE            |An unbound variable.   The value  of term|
    |                        |as such  is a  unique identifier  for the|

    |                        |variable.                                |
    | PL_ATOM                |A Prolog atom.                           |
    | PL_STRING              |A Prolog string.                         |
    | PL_INTEGER             |A Prolog integer.                        |
    | PL_FLOAT               |A Prolog floating point number.          |
    | PL_TERM                |A compound term.   Note that a  list is a|
    |________________________|compound_term_./2._______________________|

The functions PL_is_<type> are an alternative to PL_term_type().  The test
PL_is_variable(term) is equivalent to  PL_term_type(term)== PL_VARIABLE,
but  the first  is considerably  faster.   On  the other  hand, using  a
switch over  PL_term_type() is faster  and more readable  then using  an
if-then-else using  the functions  below.   All  these functions  return
either TRUE or FALSE.


int PL_is_variable(term_t)
    Returns non-zero if term is a variable.


int PL_is_atom(term_t)
    Returns non-zero if term is an atom.


int PL_is_string(term_t)
    Returns non-zero if term is a string.


int PL_is_integer(term_t)
    Returns non-zero if term is an integer.


int PL_is_float(term_t)
    Returns non-zero if term is a float.


int PL_is_compound(term_t)
    Returns non-zero if term is a compound term.


int PL_is_functor(term_t, functor_t)
    Returns  non-zero if term  is compound and  its functor is  functor.
    This  test is  equivalent to  PL_get_functor(),  followed by  testing
    the functor, but easier to write and faster.


int PL_is_list(term_t)
    Returns non-zero if term  is a compound term with functor ./2 or the
    atom [].


int PL_is_atomic(term_t)
    Returns non-zero if term is atomic (not variable or compound).


int PL_is_number(term_t)
    Returns non-zero if term is an integer or float.


5.6.3.2 Reading data from a term

The functions PL_get_*() read information from  a Prolog term.  Most  of
them take two arguments.  The first is the input  term and the second is
a pointer to the output value or a term-reference.


int PL_get_atom(term_t +t, atom_t *a)
    If t is an atom,  store the unique atom identifier over a.  See also
    PL_atom_chars() and PL_new_atom().   If there is  no need to  access
    the data (characters)  of an atom, it is advised to manipulate atoms
    using  their handle.  As the  atom is referenced by t, it  will live
    at  least as long as t does.   If longer live-time is required,  the
    atom should be locked using PL_register_atom().


int PL_get_atom_chars(term_t +t, char **s)
    If  t is an atom, store a  pointer to a 0-terminated C-string  in s.
    It is explicitly  not allowed to modify the contents of this string.
    Some  built-in  atoms may  have the  string  allocated in  read-only
    memory, so `temporary manipulation' can cause an error.


int PL_get_string_chars(term_t +t, char **s, int *len)
    If  t  is  a  string  object,  store a  pointer  to  a  0-terminated
    C-string  in s  and the  length of  the string in  len.   Note  that
    this pointer  is invalidated by backtracking, garbage-collection and
    stack-shifts,  so generally the only save operations are to  pass it
    immediately to a C-function that doesn't involve Prolog.


int PL_get_chars(term_t +t, char **s, unsigned flags)
    Convert  the argument term t to  a 0-terminated C-string.  flags  is
    a  bitwise disjunction  from two  groups of  constants.   The  first
    specifies  which term-types should converted and the second  how the
    argument  is stored.  Below  is a specification of these  constants.
    BUF_RING implies, if  the data is not static (as from an  atom), the
    data  is copied to the next buffer from a ring of 16 buffers.   This
    is  a convenient way  of converting multiple  arguments passed to  a
    foreign  predicate to C-strings.    If BUF_MALLOC  is used, the  data
    must be freed using free() when not needed any longer.
    ___________________________________________________________________
    | CVT_ATOM               |Convert if term is an atom               |
    | CVT_STRING             |Convert if term is a string              |
    | CVT_LIST               |Convert  if term  is a  list of  integers|
    |                        |between 1 and 255                        |

    | CVT_INTEGER            |Convert if term is an integer (using %d) |
    | CVT_FLOAT              |Convert if term is a float (using %f)    |
    | CVT_NUMBER             |Convert if term is a integer or float    |
    | CVT_ATOMIC             |Convert if term is atomic                |
    | CVT_VARIABLE           |Convert variable to print-name           |
    | CVT_WRITE              |Convert any  term that  is not  converted|
    |                        |by any of the  other flags using write/1.|
    |                        |If  no  BUF_*  is  provided,  BUF_RING  is|

    |                        |implied.                                 |
    | CVT_ALL                |Convert  if term  is  any  of the  above,|
    |________________________|except_for_CVT_VARIABLE_and_CVT_WRITE____|
    | BUF_DISCARDABLE        |Data must copied immediately             |
    | BUF_RING               |Data is stored in a ring of buffers      |
    | BUF_MALLOC             |Data is  copied to a new  buffer returned|
    |________________________|by_malloc(3)_____________________________|


int PL_get_list_chars(+term_t l, char **s, unsigned flags)
    Same  as PL_get_chars(l, s, CVT_LIST_flags), provided flags  contains
    no of the CVT_* flags.


int PL_get_integer(+term_t t, int *i)
    If  t is  a Prolog  integer, assign  its value over  i.   On  32-bit
    machines,  this is the  same as PL_get_long(), but avoids a  warning
    from the compiler.  See also PL_get_long().


int PL_get_long(term_t +t, long *i)
    If  t is  a Prolog  integer, assign  its value over  i.   Note  that
    Prolog integers have limited  value-range.  If t is a floating point
    number that can  be represented as a long, this function succeeds as
    well.


int PL_get_pointer(term_t +t, void **ptr)
    In   the  current  system,   pointers  are  represented  by   Prolog
    integers,   but  need  some  manipulation  to  make  sure   they  do
    not  get  truncated   due  to  the  limited  Prolog  integer  range.
    PL_put_pointer()/PL_get_pointer() guarantees pointers  in the  range
    of malloc() are handled without truncating.


int PL_get_float(term_t +t, double *f)
    If t is a float or integer, its value is assigned over f.


int PL_get_functor(term_t +t, functor_t *f)
    If  t  is  compound  or  an  atom,   the  Prolog  representation  of
    the   name-arity  pair  will  be  assigned   over  f.     See   also
    PL_get_name_arity() and PL_is_functor().


int PL_get_name_arity(term_t +t, atom_t *name, int *arity)
    If  t is  compound or  an atom,  the functor-name  will be  assigned
    over  name and the arity over arity.   See also PL_get_functor() and
    PL_is_functor().


int PL_get_module(term_t +t, module_t *module)
    If t is an  atom, the system will lookup or create the corresponding
    module and assign an opaque pointer to it over module,.


int PL_get_arg(int index, term_t +t, term_t -a)
    If  t is  compound and  index is  between 1  and arity  (including),
    assign a with a term-reference to the argument.


int _PL_get_arg(int index, term_t +t, term_t -a)
    Same  as PL_get_arg(),  but no checking  is performed, nor whether  t
    is actually a term, nor whether index is a valid argument-index.


5.6.3.3 Exchanging text using length and string

All  internal text-representation  of  SWI-Prolog is  represented  using
char * plus length and  allow for 0-bytes in them.  The  foreign library
supports this by implementing  a *_nchars() function for  each applicable
*_chars()  function.   Below we briefly present  the signatures of  these
functions.  For full documentation consult the *_chars() function.


int PL_get_atom_nchars(term_t t, unsigned int len, char **s)


int PL_get_list_nchars(term_t t, unsigned int len, char **s)


int PL_get_nchars(term_t t, unsigned int len, char **s, unsigned int flags)


int PL_put_atom_nchars(term_t t, unsigned int len, const char *s)


int PL_put_string_nchars(term_t t, unsigned int len, const char *s)


int PL_put_list_ncodes(term_t t, unsigned int len, const char *s)


int PL_put_list_nchars(term_t t, unsigned int len, const char *s)


int PL_unify_atom_nchars(term_t t, unsigned int len, const char *s)


int PL_unify_string_nchars(term_t t, unsigned int len, const char *s)


int PL_unify_list_ncodes(term_t t, unsigned int len, const char *s)


int PL_unify_list_nchars(term_t t, unsigned int len, const char *s)

In  addition, the  following functions  are available  for creating  and
inspecting atoms:


atom_t PL_new_atom_nchars(unsigned int len, const char *s)
    Create a new atom as PL_new_atom(), but from length and characters.


const char * PL_atom_nchars(atom_t a, unsigned int *len)
    Extract text and length of an atom.


5.6.3.4 Reading a list

The functions from this section are intended to read  a Prolog list from
C. Suppose we expect a list of atoms, the following  code will print the
atoms, each on a line:

foreign_t
pl_write_atoms(term_t l)
{ term_t head = PL_new_term_ref();      /* variable for the elements */
  term_t list = PL_copy_term_ref(l);    /* copy as we need to write */

  while( PL_get_list(list, head, list) )
  { char *s;

    if ( PL_get_atom_chars(head, &s) )
      Sprintf("%s\n", s);
    else
      PL_fail;
  }

  return PL_get_nil(list);              /* test end for [] */
}


int PL_get_list(term_t +l, term_t -h, term_t -t)
    If  l is a list and not [] assign a term-reference to the  head to h
    and to the tail to t.


int PL_get_head(term_t +l, term_t -h)
    If l is a list and not [] assign a term-reference to the head to h.


int PL_get_tail(term_t +l, term_t -t)
    If l is a list and not [] assign a term-reference to the tail to t.


int PL_get_nil(term_t +l)
    Succeeds if  represents the atom [].


5.6.3.5 An example:  defining write/1 in C

Figure  5.3 shows  a  simplified  definition of  write/1  to  illustrate
the described  functions.   This simplified version  does not deal  with
operators.    It is  called  display/1, because  it mimics  closely  the
behaviour of this Edinburgh predicate.
foreign_t
pl_display(term_t t)

{ functor_t functor;
  int arity, len, n;
  char *s;

  switch( PL_term_type(t) )
  { case PL_VARIABLE:
    case PL_ATOM:

    case PL_INTEGER:
    case PL_FLOAT:
      PL_get_chars(t, &s, CVT_ALL);
      Sprintf("%s", s);
      break;
    case PL_STRING:
      PL_get_string_chars(t, &s, &len);
      Sprintf("\"%s\"", s);

      break;
    case PL_TERM:
    { term_t a = PL_new_term_ref();

      PL_get_name_arity(t, &name, &arity);
      Sprintf("%s(", PL_atom_chars(name));
      for(n=1; n<=arity; n++)

      { PL_get_arg(n, t, a);
        if ( n > 1 )
          Sprintf(", ");
        pl_display(a);
      }
      Sprintf(")");
      break;
    default:

      PL_fail;                          /* should not happen */
  }

  PL_succeed;
}

             Figure 5.3:  A Foreign definition of display/1


5.6.4 Constructing Terms

Terms  can  be  constructed  using  functions from  the  PL_put_*()  and
PL_cons_*()  families.    This  approach  builds the  term  `inside-out',
starting  at  the  leaves  and  subsequently  creating  compound  terms.
Alternatively,   terms  may  be  created   `top-down',  first   creating
a  compound  holding  only  variables  and   subsequently  unifying  the
arguments.   This section  discusses functions  for the first  approach.
This approach  is generally  used for  creating arguments  for PL_call()
and PL_open_query.


void PL_put_variable(term_t -t)
    Put  a fresh variable in  the term.   The new variable lives on  the
    global  stack.   Note that the initial  variable lives on the  local
    stack  and is  lost after  a write to  the term-references.    After
    using this function, the variable will continue to live.


void PL_put_atom(term_t -t, atom_t a)
    Put  an  atom  in the  term  reference from  a  handle.    See  also
    PL_new_atom() and PL_atom_chars().


void PL_put_atom_chars(term_t -t, const char *chars)
    Put an atom  in the term-reference constructed from the 0-terminated
    string.  The  string itself will never be references by Prolog after
    this function.


void PL_put_string_chars(term_t -t, const char *chars)
    Put  a zero-terminated string in the term-reference.  The  data will
    be copied.  See also PL_put_string_nchars().


void PL_put_string_nchars(term_t -t, unsigned int len, const char *chars)

    Put  a string,  represented by  a length/start pointer  pair in  the
    term-reference.   The data will be copied.  This interface  can deal
    with 0-bytes in the string.  See also section 5.6.18.


void PL_put_list_chars(term_t -t, const char *chars)
    Put a list of ASCII values in the term-reference.


void PL_put_integer(term_t -t, long i)
    Put a Prolog integer in the term reference.


void PL_put_pointer(term_t -t, void *ptr)
    Put a Prolog integer  in the term-reference.  Provided ptr is in the
    `malloc()-area', PL_get_pointer() will get the pointer back.


void PL_put_float(term_t -t, double f)
    Put a floating-point value in the term-reference.


void PL_put_functor(term_t -t, functor_t functor)
    Create  a new compound  term from functor and  bind t to this  term.
    All arguments of the  term will be variables.  To create a term with
    instantiated  arguments, either instantiate the arguments  using the
    PL_unify_*() functions or use PL_cons_functor().


void PL_put_list(term_t -l)
    Same as PL_put_functor(l, PL_new_functor(PL_new_atom("."), 2)).


void PL_put_nil(term_t -l)
    Same as PL_put_atom_chars("[]").


void PL_put_term(term_t -t1, term_t +t2)
    Make t1 point to the same term as t2.


void PL_cons_functor(term_t -h, functor_t f, ...)
    Create  a term,  whose arguments are  filled from variable  argument
    list  holding the same number of term_t objects as the arity  of the
    functor.  To create the term animal(gnu, 50), use:

            term_t a1 = PL_new_term_ref();
            term_t a2 = PL_new_term_ref();
            term_t t  = PL_new_term_ref();

            PL_put_atom_chars(a1, "gnu");
            PL_put_integer(a2, 50);
            PL_cons_functor(t, PL_new_functor(PL_new_atom("animal"), 2),
                            a1, a2);

    After  this sequence, the term-references a1 and a2 may be  used for
    other purposes.


void PL_cons_functor_v(term_t -h, functor_t f, term_t a0)
    Creates  a compound term like PL_cons_functor(), but a0 is  an array
    of  term references as  returned by PL_new_term_refs().   The  length
    of  this array should match the number of arguments required  by the
    functor.


void PL_cons_list(term_t -l, term_t +h, term_t +t)
    Create  a list (cons-) cell in l from  the head and tail.   The code
    below  creates a list of  atoms from a char **.   The list is  built
    tail-to-head.   The  PL_unify_*()  functions can be  used to build  a
    list head-to-tail.

    void
    put_list(term_t l, int n, char **words)
    { term_t a = PL_new_term_ref();

      PL_put_nil(l);
      while( --n >= 0 )
      { PL_put_atom_chars(a, words[n]);
        PL_cons_list(l, a, l);
      }
    }

    Note  that l can  be redefined  within a PL_cons_list call as  shown
    here because operationally  its old value is consumed before its new
    value is set.


5.6.5 Unifying data

The  functions  of  this  sections  unify  terms  with  other  terms  or
translated C-data structures.   Except for PL_unify(), the functions  of
this section  are specific  to SWI-Prolog.   They  have been  introduced
to make translation  of old code easier,  but also because they  provide
for a faster mechanism  for returning data to Prolog that  requires less
term-references.  Consider the case where we want  a foreign function to
return the  host name of the  machine Prolog is running  on.  Using  the
PL_get_*() and PL_put_*() functions, the code becomes:

foreign_t
pl_hostname(term_t name)
{ char buf[100];

  if ( gethostname(buf, sizeof(buf)) )
  { term_t tmp = PL_new_term_ref();

    PL_put_atom_chars(tmp, buf);
    return PL_unify(name, buf);
  }

  PL_fail;
}

Using PL_unify_atom_chars(), this becomes:

foreign_t
pl_hostname(term_t name)
{ char buf[100];

  if ( gethostname(buf, sizeof(buf)) )
    return PL_unify_atom_chars(name, buf);

  PL_fail;
}


int PL_unify(term_t ?t1, term_t ?t2)
    Unify two Prolog terms and return non-zero on success.


int PL_unify_atom(term_t ?t, atom_t a)
    Unify t with the atom a and return non-zero on success.


int PL_unify_atom_chars(term_t ?t, const char *chars)
    Unify  t with  an atom  created from  chars and  return non-zero  on
    success.


int PL_unify_list_chars(term_t ?t, const char *chars)
    Unify t with a list of ASCII characters constructed from chars.


void PL_unify_string_chars(term_t ?t, const char *chars)
    Unify  t  with  a  Prolog  string  object  created  from  the  zero-
    terminated  string  chars.   The  data will  be copied.    See  also
    PL_unify_string_nchars().


void PL_unify_string_nchars(term_t ?t, unsigned int len, const char *chars)

    Unify   t   with   a  Prolog   string   object  created   from   the
    string  created from the len/chars pair.   The data will be  copied.
    This  interface can  deal  with 0-bytes  in the  string.   See  also
    section 5.6.18.


int PL_unify_integer(term_t ?t, long n)
    Unify t with a Prolog integer from n.


int PL_unify_float(term_t ?t, double f)
    Unify t with a Prolog float from f.


int PL_unify_pointer(term_t ?t, void *ptr)
    Unify  t with a  Prolog integer  describing the pointer.   See  also
    PL_put_pointer() and PL_get_pointer().


int PL_unify_functor(term_t ?t, functor_t f)
    If  t is a compound term with  the given functor, just succeed.   If
    it  is unbound,  create a term  and bind the  variable, else  fails.
    Not  that this function  does not create a  term if the argument  is
    already instantiated.


int PL_unify_list(term_t ?l, term_t -h, term_t -t)
    Unify  l with a list-cell (./2).   If successful, write  a reference
    to  the head of  the list to h  and a reference  to the tail of  the
    list  in t.   This  reference may  be used for  subsequent calls  to
    this  function.   Suppose we  want to  return a list  of atoms  from
    a  char **.   We could  use the example  described by PL_put_list(),
    followed by  a call to PL_unify(), or we can use  the code below.  If
    the  predicate argument is unbound,  the difference is minimal  (the
    code  based on PL_put_list() is probably slightly  faster).  If  the
    argument  is bound, the code below may fail before reaching  the end
    of  the word-list, but even  if the unification succeeds, this  code
    avoids a duplicate (garbage) list and a deep unification.

    foreign_t
    pl_get_environ(term_t env)
    { term_t l = PL_copy_term_ref(env);
      term_t a = PL_new_term_ref();
      extern char **environ;
      char **e;

      for(e = environ; *e; e++)
      { if ( !PL_unify_list(l, a, l) ||
             !PL_unify_atom_chars(a, *e) )
          PL_fail;
      }

      return PL_unify_nil(l);
    }


int PL_unify_nil(term_t ?l)
    Unify l with the atom [].


int PL_unify_arg(int index, term_t ?t, term_t ?a)
    Unifies the index-th argument (1-based) of t with a.


int PL_unify_term(term_t ?t, ...)
    Unify  t with a (normally) compound  term.  The remaining  arguments
    is  a  sequence  of a  type  identifier,  followed by  the  required
    arguments.      This  predicate  is  an  extension  to  the  Quintus
    and  SICStus foreign  interface  from which  the SWI-Prolog  foreign
    interface  has been  derived, but has  proved to  be a powerful  and
    comfortable  way to create compound terms from C. Due to  the vararg
    packing/unpacking and  the required type-switching this interface is
    slightly  slower than using the primitives.   Please note that  some
    bad  C-compilers have fairly low  limits on the number of  arguments
    that may be passed to a function.

    The type identifiers are:

    PL_VARIABLE none
         No op.  Used in arguments of PL_FUNCTOR.

    PL_ATOM atom_t
         Unify the argument with an atom, as in PL_unify_atom().

    PL_INTEGER long
         Unify the argument with an integer, as in PL_unify_integer().

    PL_FLOAT double
         Unify  the  argument  with a  float,  as  in  PL_unify_float().
         Note  that,  as the  argument  is  passed using  the  C  vararg
         conventions, a float must be casted to a double explicitly.

    PL_STRING const char *
         Unify   the   argument   with   a   string   object,    as   in
         PL_unify_string_chars().

    PL_TERM term_t
         Unify  a  subterm.    Note  this  may the  return  value  of  a
         PL_new_term_ref()call to get access to a variable.

    PL_CHARS const char *
         Unify the argument  with an atom,  constructed from the C  char
         *, as in PL_unify_atom_chars().

    PL_FUNCTOR functor_t, ...
         Unify the argument  with a compound  term.  This  specification
         should be  followed by  exactly as many  specifications as  the
         number of arguments of the compound term.

    PL_LIST int length, ...
         Create  a  list  of  the  indicated  length.     The  following
         arguments contain the elements of the list.

    For  example, to  unify an argument  with the term  language(dutch),
    the following skeleton may be used:

    static functor_t FUNCTOR_language1;

    static void
    init_constants()
    { FUNCTOR_language1 = PL_new_functor(PL_new_atom("language"), 1);
    }

    foreign_t
    pl_get_lang(term_t r)
    { return PL_unify_term(r,
                           PL_FUNCTOR, FUNCTOR_language1,
                               PL_CHARS, "dutch");
    }

    install_t
    install()
    { PL_register_foreign("get_lang", 1, pl_get_lang, 0);
      init_constants();
    }


int PL_chars_to_term(const char *chars, term_t -t)
    Parse  the string chars  and put the  resulting Prolog term into  t.
    chars  may or may  not be  closed using a  Prolog full-stop (i.e.  a
    dot  followed by  a blank).   Returns  FALSE if a  syntax error  was
    encountered  and TRUE after successful  completion.  In addition  to
    returning  FALSE, the exception-term  is returned in  t on a  syntax
    error.  See also term_to_atom/2.

    The following example build a goal-term from a string and calls it.

    int
    call_chars(const char *goal)
    { fid_t fid = PL_open_foreign_frame();
      term_t g = PL_new_term_ref();
      BOOL rval;

      if ( PL_string_to_term(goal, g) )
        rval = PL_call(goal, NULL);
      else
        rval = FALSE;

      PL_discard_foreign_frame(fid);
      return rval;
    }

      ...
      call_chars("consult(load)");
      ...


char * PL_quote(int chr, const char *string)
    Return a quoted version of  string.  If chr is '\'', the result is a
    quoted  atom.  If chr  is '"', the result is  a string.  The  result
    string  is stored in the same ring of buffers as described  with the
    BUF_RING argument of PL_get_chars();

    In  the  current implementation,  the string  is  surrounded by  chr
    and  any  occurence  of   chr  is  doubled.     In  the  future  the
    behaviour  will depend  on the  character_escape prolog-flag.    See
    current_prolog_flag/2.


5.6.6 Calling Prolog from C

The  Prolog engine  can  be called  from  C.  There are  two  interfaces
for  this.    For the  first,  a  term is  created  that could  be  used
as an  argument to  call/1 and next  PL_call() is used  to call  Prolog.
This  system is  simple, but  does not  allow to  inspect the  different
answers  to a  non-deterministic  goal and  is  relatively slow  as  the
runtime  system needs  to  find the  predicate.    The  other  interface
is  based on  PL_open_query(),  PL_next_solution() and  PL_cut_query() or
PL_close_query().     This mechanism  is  more powerful,  but  also  more
complicated to use.


5.6.6.1 Predicate references

This  section  discusses   the  functions  used  to  communicate   about
predicates.   Though a Prolog predicate  may defined or not,  redefined,
etc., a Prolog predicate has a handle that is  not destroyed, nor moved.
This handle is known by the type predicate_t.


predicate_t PL_pred(functor_t f, module_t m)
    Return  a handle to a predicate for the specified name/arity  in the
    given module.   This function always succeeds, creating a handle for
    an undefined predicate if no handle was available.


predicate_t PL_predicate(const char *name, int arity, const char* module)

    Same   a  PL_pred(),   but   provides  a   more  convenient   inter-
    face to the C-programmer.


void PL_predicate_info(predicate_t p, atom_t *n, int *a, module_t *m)
    Return  information on the  predicate p.   The  name is stored  over
    n,  the  arity  over a,  while  m  receives the  definition  module.
    Note  that  the  latter need  not  be  the same  as  specified  with
    PL_predicate().    If the  predicate was  imported  into the  module
    given to  PL_predicate(), this function will  return the module where
    the predicate was defined.


5.6.6.2 Initiating a query from C

This  section discusses  the  functions  for creating  and  manipulating
queries from C. Note that a foreign context can have  at most one active
query.    This  implies it  is allowed  to  make strictly  nested  calls
between  C and  Prolog (Prolog  calls C,  calls Prolog,  calls C,  etc.,
but it  is not  allowed to  open multiple queries  and start  generating
solutions for  each of them by  calling PL_next_solution().   Be sure  to
call PL_cut_query() or PL_close_query() on any  query you opened  before
opening the next or returning control back to Prolog.


qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t +t0)

    Opens  a query  and returns  an identifier for  it.   This  function
    always  succeeds,  regardless whether  the predicate  is defined  or
    not.    ctx is  the context  module of  the goal.    When NULL,  the
    context  module of  the calling  context will  be used,  or user  if
    there  is no calling  context (as may  happen in embedded  systems).
    Note  that the  context module  only matters  for module_transparent
    predicates.   See context_module/1 and module_transparent/1.   The p
    argument  specifies the  predicate, and  should be the  result of  a
    call  to PL_pred() or PL_predicate().   Note that  it is allowed  to
    store  this handle as global data  and reuse it for future  queries.
    The  term-reference t0 is the  first of a vector of  term-references
    as returned by PL_new_term_refs(n).

    The  flags  arguments provides  some additional  options  concerning
    debugging  and  exception handling.    It  is a  bitwise or  of  the
    following values:

    PL_Q_NORMAL
         Normal operation.    The  debugger inherits  its settings  from
         the environment.   If an exception  occurs that is not  handled
         in Prolog, a  message is printed and  the tracer is started  to
         debug the error.

    PL_Q_NODEBUG
         Switch off the debugger while executing the goal.   This option
         is used by many  calls to hook-predicates to avoid  tracing the
         hooks.   An example is  print/1 calling portray/1 from  foreign
         code.

    PL_Q_CATCH_EXCEPTION
         If an  exception is  raised while  executing the  goal, do  not
         report it, but make it available for PL_exception().

    PL_Q_PASS_EXCEPTION
         As PL_Q_CATCH_EXCEPTION,  but do  not invalidate the  exception-
         term  while   calling  PL_close_query().      This  option   is
         experimental.

    The  example below opens a query to the predicate is_a/2  to find the
    ancestor of for some name.

    char *
    ancestor(const char *me)
    { term_t a0 = PL_new_term_refs(2);
      static predicate_t p;

      if ( !p )
        p = PL_predicate("is_a", 2, "database");

      PL_put_atom_chars(a0, me);
      PL_open_query(NULL, PL_Q_NORMAL, p, a0);
      ...
    }


int PL_next_solution(qid_t qid)
    Generate the first (next)  solution for the given query.  The return
    value  is TRUE if  a solution  was found, or  FALSE to indicate  the
    query  could not be proven.  This function may be  called repeatedly
    until it fails to generate all solutions to the query.


void PL_cut_query(qid)
    Discards  the query, but does not delete any of the data  created by
    the  query.   It just  invalidate qid,  allowing for a  new call  to
    PL_open_query() in this context.


void PL_close_query(qid)
    As  PL_cut_query(), but  all data and  bindings created by the  query
    are destroyed.


int PL_call_predicate(module_t m, int flags, predicate_t pred, term_t +t0)

    Shorthand  for  PL_open_query(),  PL_next_solution(),  PL_cut_query(),
    generating  a single solution.   The arguments  are the same as  for
    PL_open_query(), the return value is the same as PL_next_solution().


int PL_call(term_t, module_t)
    Call term just like  the Prolog predicate once/1.  Term is called in
    the  specified module, or in the context  module if module_t  = NULL.
    Returns  TRUE if  the call succeeds,  FALSE otherwise.   Figure  5.4
    shows an example to  obtain the number of defined atoms.  All checks
    are omitted to improve readability.


5.6.7 Discarding Data

The Prolog  data created and  term-references needed  to setup the  call
and/or analyse  the result can  in most cases  be discarded right  after
the  call.   PL_close_query() allows  for  destructing the  data,  while
leaving the  term-references.   The calls below may  be used to  destroy
term-references and data.  See figure 5.4 for an example.


fid_t PL_open_foreign_frame()
    Created  a foreign  frame,  holding a  mark that  allows the  system
    to  undo  bindings and  destroy data  created after  it  as well  as
    providing  the  environment  for  creating term-references.     This
    function   is  called  by  the  kernel  before  calling   a  foreign
    predicate.


void PL_close_foreign_frame(fid_t id)
    Discard  all term-references  created  after the  frame was  opened.
    All  other Prolog data is retained.  This function is called  by the
    kernel whenever a foreign function returns control back to Prolog.


void PL_discard_foreign_frame(fid_t id)
    Same  as PL_close_foreign_frame(),  but also  undo all bindings  made
    since the open and destroy all Prolog data.


void PL_rewind_foreign_frame(fid_t id)
    Undo all bindings  and discard all term-references created since the
    frame was created, but  does not pop the frame.  I.e. the same frame
    can  be rewinded multiple  times, and must  eventually be closed  or
    discarded.

It is obligatory to call either of the two  closing functions to discard
a foreign frame.  Foreign frames may be nested.

int
count_atoms()
{ fid_t fid = PL_open_foreign_frame();
  term_t goal  = PL_new_term_ref();
  term_t a1    = PL_new_term_ref();
  term_t a2    = PL_new_term_ref();
  functor_t s2 = PL_new_functor(PL_new_atom("statistics"), 2);

  int atoms;

  PL_put_atom_chars(a1, "atoms");
  PL_cons_functor(goal, s2, a1, a2);
  PL_call(goal, NULL);         /* call it in current module */

  PL_get_integer(a2, &atoms);
  PL_discard_foreign_frame(fid);

  return atoms;
}

                      Figure 5.4:  Calling Prolog


5.6.8 Foreign Code and Modules

Modules are  identified via a  unique handle.   The following  functions
are available to query and manipulate modules.


module_t PL_context()
    Return the module  identifier of the context module of the currently
    active foreign predicate.


int PL_strip_module(term_t +raw, module_t *m, term_t -plain)
    Utility  function.  If  raw is a  term, possibly holding the  module
    construct  <module>:<rest>this function will make plain a  reference
    to  <rest>  and fill  module *  with <module>.    For further  nested
    module  constructs  the inner  most module  is  returned via  module
    *.   If  raw is  not a module  construct arg will  simply be put  in
    plain.   If module * is NULL  it will be set to the  context module.
    Otherwise  it will be left untouched.   The following example  shows
    how  to obtain the  plain term and module  if the default module  is
    the user module:

    { module m = PL_new_module(PL_new_atom("user"));
      term_t plain = PL_new_term_ref();

      PL_strip_module(term, &m, plain);
      ...


atom_t PL_module_name(module_t)
    Return the name of module as an atom.


module_t PL_new_module(atom_t name)
    Find  an existing or create a new module with name specified  by the
    atom name.


5.6.9 Prolog exceptions in foreign code

This     section    discusses     PL_exception(),     PL_throw()     and
PL_raise_exception(),  the  interface functions  to  detect and  generate
Prolog  exceptions from  C-code.    PL_throw()  and PL_raise_exception()
from  the  C-interface   to  raise  an  exception  from  foreign   code.
PL_throw() exploits the  C-function longjmp()  to return immediately  to
the  innermost PL_next_solution().   PL_raise_exception() registers  the
exception term and returns FALSE. If a foreign  predicate returns FALSE,
while  and exception-term  is  registered  a Prolog  exception  will  be
raised by the virtual machine.

Calling these functions  outside the context of a function  implementing
a foreign predicate results in undefined behaviour.

PL_exception() may  be used  after a  call to  PL_next_solution() fails,
and returns a  term reference to an  exception term if an exception  was
raised, and 0 otherwise.

If  a C-function,  implementing  a predicate  calls Prolog  and  detects
an  exception using  PL_exception(),  it can  handle  this exception,  or
return  with the  exception.    Some caution  is required  though.    It
is  not allowed  to call  PL_close_query() or  PL_discard_foreign_frame()
afterwards,  as  this  will  invalidate  the  exception  term.     Below
is  the  code that  calls  a  Prolog defined  arithmetic  function  (see
arithmethic_function/1).

If PL_next_solution() succeeds,  the result  is analysed and  translated
to  a number,  after  which the  query  is closed  and all  Prolog  data
created  after  PL_open_foreign_frame() is  destroyed.    On  the  other
hand,  if  PL_next_solution() fails  and  if an  exception  was  raised,
just  pass it.     Otherwise generate  an  exception  (PL_error() is  an
internal  call  for  building  the  standard  error  terms  and  calling
PL_raise_exception()).    After this,  the Prolog  environment should  be
discarded  using  PL_cut_query() and  PL_close_foreign_frame() to  avoid
invalidating the exception term.

static int
prologFunction(ArithFunction f, term_t av, Number r)
{ int arity = f->proc->definition->functor->arity;
  fid_t fid = PL_open_foreign_frame();
  qid_t qid;
  int rval;

  qid = PL_open_query(NULL, PL_Q_NORMAL, f->proc, av);

  if ( PL_next_solution(qid) )
  { rval = valueExpression(av+arity-1, r);
    PL_close_query(qid);
    PL_discard_foreign_frame(fid);
  } else
  { term_t except;

    if ( (except = PL_exception(qid)) )
    { rval = PL_throw(except);          /* pass exception */
    } else
    { char *name = stringAtom(f->proc->definition->functor->name);

                                        /* generate exception */
      rval = PL_error(name, arity-1, NULL, ERR_FAILED, f->proc);
    }

    PL_cut_query(qid);                  /* donot destroy data */
    PL_close_foreign_frame(fid);        /* same */
  }

  return rval;
}


int PL_raise_exception(term_t exception)
    Generate  an exception (as  throw/1) and return  FALSE. Below is  an
    example returning an exception from foreign predicate:

    foreign_t
    pl_hello(term_t to)
    { char *s;

      if ( PL_get_atom_chars(to, &s) )
      { Sprintf("Hello \"%s\"\n", s);

        PL_succeed;
      } else
      { term_t except = PL_new_term_ref();

        PL_unify_term(except,
                      PL_FUNCTOR, PL_new_functor(PL_new_atom("type_error"), 2),
                        PL_CHARS, "atom",
                        PL_TERM,  to);

        return PL_raise_exception(except);
      }
    }


int PL_throw(term_t exception)
    Similar  to PL_raise_exception(),  but returns using the C  longjmp()
    function to the innermost PL_next_solution().


term_t PL_exception(qid_t qid)
    If  PL_next_solution() fails,  this  can be  due to  normal  failure
    of  the  Prolog  call, or  because  an  exception was  raised  using
    throw/1.   This function returns  a handle to the exception term  if
    an exception was raised, or 0 if the Prolog goal simply failed..


5.6.10 Foreign code and Prolog threads

If  SWI-Prolog   has  been   build  to   support  multi-threading   (see
section 3.39), all  foreign-code linked to Prolog should  be thread-safe
(reentrant) or  guarded in Prolog  using with_mutex/2 from  simultaneous
access from multiple  Prolog threads.   On Unix systems, this  generally
implies the  code should be compiled  with the -D_REENTRANT flag  passed
to  the compiler.    Please  note  that on  many  Unix systems  not  all
systemcalls and library-functions are thread-safe.   Consult your manual
for details.

If you are  using SWI-Prolog as an  embedded engine in a  multi-threaded
application you  can access the Prolog  engine from multiple threads  by
creating an engine in  each thread from which you call Prolog.   Without
creating an engine, a thread can only use functions that  do not use the
term_t type (for example PL_new_atom()).

Please note  that the  interface below  will only work  if threading  in
your application is based on the same thread-library as  used to compile
SWI-Prolog.


int PL_thread_self()
    Returns  the integer Prolog  identifier of the  engine or -1 if  the
    calling  thread  has  no Prolog  engine.    This  function  is  also
    provided  in the  single-threaded version  of  SWI-Prolog, where  it
    returns -2.


int PL_thread_attach_engine(PL_thread_attr_t *attr)
    Creates  a new Prolog engine in the calling thread.  If  the calling
    thread  already has an engine the  reference count of the engine  is
    incremented.  The  attr argument can be NULL to create a thread with
    default  attributes.  Otherwise it is a pointer to a  structure with
    the definition below.   For any field with value `0', the default is
    used.

    typedef struct
    { unsigned long     local_size;    /* Stack sizes (K-bytes) */
      unsigned long     global_size;
      unsigned long     trail_size;
      unsigned long     argument_size;
      char *            alias;         /* alias name */
    } PL_thread_attr_t;

    The  structure may be  destroyed after PL_thread_attach_engine() has
    returned.   If an error occurs,  -1 is returned.  If this  Prolog is
    not compiled for multi-threading, -2 is returned.


int PL_thread_destroy_engine()
    Destroy  the  Prolog engine  in  the calling  thread.    Only  takes
    effect  if  PL_thread_destroy_engine() is called  as many  times  as
    PL_thread_attach_engine() in this thread.   Returns TRUE on  success
    and  FALSE if the calling thread  has no engine or this Prolog  does
    not support threads.

    Please  note  that  construction  and  destruction  of  engines  are
    relatively  expensive  operations.     Only  destroy  an  engine  if
    performance  is  not critical  and memory  is  a critical  resource.
    The  engine  is  automatically  destroyed if  the  thread  finishes,
    regardless how many times PL_thread_attach_engine() has been called.


5.6.11 Miscellaneous


5.6.11.1 Term Comparison


int PL_compare(term_t t1, term_t t2)
    Compares  two terms using  the standard order  of terms and  returns
    -1, 0 or 1.  See also compare/3.


int PL_same_compound(term_t t1, term_t t2)
    Yields TRUE if t1  and t2 refer to physically the same compound term
    and FALSE otherwise.


5.6.11.2 Recorded database

In some  applications it is  useful to store  and retreive Prolog  terms
from C-code.  For example, the XPCE graphical  environment does this for
storing arbitrary Prolog data as slot-data of XPCE objects.

Please note  that the  returned handles  have no meaning  at the  Prolog
level  and  the recorded  terms  are  not  visible from  Prolog.     The
functions PL_recorded() and PL_erase() are  the only functions that  can
operate on the stored term.

Two groups  of functions are  provided.The first group  (PL_record() and
friends) store Prolog terms on the Prolog heap for  retrieval during the
same session.   These functions are also used by recorda/3  and friends.
The recorded  database may be used  to communicate Prolog terms  between
threads.


record_t PL_record(term_t +t)
    Record  the term t into the Prolog database as recorda/3  and return
    an  opaque handle to  the term.   The returned handle remains  valid
    until  PL_erase() is called on  it.   PL_recorded() is used to  copy
    recorded terms back to the Prolog stack.


void PL_recorded(record_t record, term_t -t)
    Copy a recorded term  back to the Prolog stack.  The same record may
    be used to copy  multiple instances at any time to the Prolog stack.
    See also PL_record() and PL_erase().


void PL_erase(record_t record)
    Remove  the recorded term from  the Prolog database, reclaiming  all
    associated memory resources.

The  second group  (headed  by PL_record_external()) provides  the  same
functionality, but the returned data has properties  that enable storing
the  data on  an external  device.   It  has been  designed  to make  it
possible to store Prolog terms fast an compact in  an external database.
Here are the main features:

  o Independent of session
    Records  can  be communicated  to another  Prolog  session and  made
    visible using PL_recorded_external().

  o Binary
    The representation is  binary for maximum performance.  The returned
    data may contain 0-bytes.

  o Byte-order independent
    The   representation  can  be  transferred  between   machines  with
    different byte-order.

  o No alignment restrictions
    There are no  memory alignment restrictions and copies of the record
    can  thus be  moved freely.    For example,  it is  possible to  use
    this  representation to exchange  terms using shared memory  between
    different Prolog processes.

  o Compact
    It  is  assumed  that a  smaller  memory footprint  will  eventually
    outperform slightly faster representations.

  o Stable
    The  format is  designed  for future  enhancements without  breaking
    compatibility with older records.


char * PL_record_external(term_t +t, unsigned int *len)
    Record  the term t into the Prolog database as recorda/3  and return
    an  opaque handle to  the term.   The returned handle remains  valid
    until PL_erase() is called on it.

    It  is allowed  to copy the  data and use  PL_recorded_external() on
    the  copy.   The user  is responsible for  the memory management  of
    the  copy.    After copying,  the original  may  be discarded  using
    PL_erase_external().

    PL_recorded_external() is used to  copy such recorded terms back  to
    the Prolog stack.


int PL_recorded_external(const char *record, term_t -t)
    Copy a recorded term  back to the Prolog stack.  The same record may
    be used to copy  multiple instances at any time to the Prolog stack.
    See also PL_record_external() and PL_erase_external().


int PL_erase_external(char *record)
    Remove  the recorded term from  the Prolog database, reclaiming  all
    associated memory resources.


5.6.12 Catching Signals (Software Interrupts)

SWI-Prolog offers both  a C and Prolog  interface to deal with  software
interrupts (signals).   The Prolog mapping  is defined in section  3.10.
This subsection deals with handling signals from C.

If a signal is not  used by Prolog and the handler does not  call Prolog
in any way, the native signal interface routines may be used.

Some versions of SWI-Prolog, notably running on  popular Unix platforms,
handle SIG_SEGV  for guarding  the Prolog stacks.    If the  application
whishes to handle this signal too,  it should use PL_signal() to install
its handler after initialisating Prolog.   SWI-Prolog will pass SIG_SEGV
to the user  code if it detected the  signal is not related to a  Prolog
stack overflow.

Any handler that  wishes to call one  of the Prolog interface  functions
should call PL_signal() for its installation.


void (*)() PL_signal(sig, func)
    This  function  is equivalent  to  the BSD-Unix  signal()  function,
    regardless  of the  platform used.   The  signal handler is  blocked
    while  the signal routine  is active, and automatically  reactivated
    after the handler returns.

    After  a  signal handler  is  registered using  this  function,  the
    native  signal interface  redirects the signal  to a generic  signal
    handler  inside  SWI-Prolog.   This  generic  handler validates  the
    environment,   creates  a  suitable  environment  for   calling  the
    interface functions described  in this chapter and finally calls the
    registered user-handler.


5.6.13 Errors and warnings

PL_warning() prints a  standard Prolog warning  message to the  standard
error (user_error) stream.   Please note  that new code should  consider
using  PL_raise_exception() to  raise a  Prolog  exception.    See  also
section 3.9.


int PL_warning(format, a1, ...)
    Print  an  error message  starting  with `[WARNING: ',  followed  by
    the  output from  format, followed  by a `]'  and a newline.    Then
    start  the tracer.   format and  the arguments are  the same as  for
    printf(2).  Always returns FALSE.


5.6.14 Environment Control from Foreign Code


int PL_action(int, ...)
    Perform  some  action on  the  Prolog system.    int  describes  the
    action,  Remaining arguments depend  on the requested  action.   The
    actions are listed in table 5.1.
    ___________________________________________________________________
    | PL_ACTION_TRACE        |Start Prolog tracer (trace/0).   Requires|
    |                        |no arguments.                            |
    | PL_ACTION_DEBUG        |Switch  on Prolog  debug mode  (debug/0).|
    |                        |Requires no arguments.                   |
    | PL_ACTION_BACKTRACE    |Print   backtrace   on   current   output|
    |                        |stream.   The  argument (an  int) is  the|
    |                        |number of frames printed.                |

    | PL_ACTION_HALT         |Halt  Prolog  execution.     This  action|
    |                        |should be called rather  than Unix exit()|
    |                        |to give  Prolog the opportunity  to clean|
    |                        |up.   This  call does  not return.    The|
    |                        |argument (an int) is the  exit code.  See|
    |                        |halt/1.                                  |
    | PL_ACTION_ABORT        |Generate a Prolog abort  (abort/0).  This|

    |                        |call  does  not  return.     Requires  no|
    |                        |arguments.                               |
    | PL_ACTION_BREAK        |Create  a  standard  Prolog  break  envi-|
    |                        |ronment  (break/0).    Returns after  the|
    |                        |user  types  the  end-of-file  character.|
    ||                       |Requires|no arguments.                   ||

    | PL_ACTION_GUIAPP       |Win32:  Used to  indicate the kernel that|
    |                        |the application  is a GUI  application if|
    |                        |the  argument  is  not 0  and  a  console|
    |                        |application if  the argument  is 0.    If|
    |                        |a  fatal error  occurs,  the system  uses|
    |                        |a windows  messagebox to  report this  on|
    |                        |a GUI  application and simply  prints the|
    |                        |error and exits otherwise.               |
    | PL_ACTION_WRITE        |Write  the  argument,  a  char *  to  the|

    |                        |current output stream.                   |
    | PL_ACTION_FLUSH        |Flush  the current  output stream.    Re-|
    |________________________|quires_no_arguments._____________________|

                    Table 5.1:  PL_action() options


5.6.15 Querying Prolog


C_type PL_query(int)
    Obtain  status  information  on  the  Prolog system.     The  actual
    argument  type depends on the  information required.  int  describes
    what information is wanted.  The options are given in table 5.2.
    ___________________________________________________________________
    | PL_QUERY_ARGC          |Return an  integer holding the  number of|
    |                        |arguments given to Prolog from Unix.     |
    | PL_QUERY_ARGV          |Return  a char  **  holding the  argument|
    |                        |vector given to Prolog from Unix.        |
    | PL_QUERY_SYMBOLFILE    |Return  a  char  *  holding  the  current|

    |                        |symbol file of the running process.      |
    | PL_MAX_INTEGER         |Return a  long, representing  the maximal|
    |                        |integer  value  represented by  a  Prolog|
    |                        |integer.                                 |
    | PL_MIN_INTEGER         |Return a  long, representing  the minimal|
    |                        |integer value.                           |
    | PL_QUERY_VERSION       |Return a  long, representing  the version|

    |                        |as 10; 000M* +100m* +p, where  M is  the |
    |                        |major,  m the  minor version  number and |
    |                        |p the  patch-level.   For  example, 20717|
    |________________________|means_2.7.17.____________________________|

                     Table 5.2:  PL_query() options


5.6.16 Registering Foreign Predicates


int PL_register_foreign(const char *name, int arity, foreign_t (*function)(), int flags)

    Register   a   C-function   to   implement   a   Prolog   predicate.
    After  this call  returns successfully  a predicate  with name  name
    (a  char *) and  arity arity  (a C int)  is created.   As a  special
    case,  name may consist of a sequence of  alpha-numerical characters
    followed  by the colon (:).   In this case the name uptil  the colon
    is  taken to be the destination module and the rest of the  name the
    predicate name.

    When  called in  Prolog, Prolog  will call  function.   flags  forms
    bitwise or'ed list of options for the installation.  These are:
    ___________________________________________________________________
    | PL_FA_NOTRACE          |Predicate cannot be seen in the tracer   |
    | PL_FA_TRANSPARENT      |Predicate is module transparent          |
    | PL_FA_NONDETERMINISTIC |Predicate  is  non-deterministic.     See|
    |                        |also PL_retry().                         |

    |_PL_FA_VARARGS__________|Use_alternative_calling_convention.______|


void PL_load_extensions(PL_extension *e)
    Register  foreign  predicates from  a  table of  structures.    This
    is  an alternative  to multiple  calls to  PL_register_foreign() and
    simplifies  code that wishes  to use  PL_register_extensions()as  an
    alternative.  The type PL_extension is defined as:

    typedef struct _PL_extension
    { char          *predicate_name; /* Name of the predicate */
      short         arity;           /* Arity of the predicate */
      pl_function_t function;        /* Implementing functions */
      short         flags;           /* Or of PL_FA_... */
    } PL_extension;


void PL_register_extensions(PL_extension *e)
    The      function      PL_register_extensions()      behaves      as
    PL_load_extensions(),  but  is  the  only  PL_*  function  that  may
    be  called before  PL_initialise().    The predicates are  registered
    into  the module user after  registration of the SWI-Prolog  builtin
    foreign  predicates  and before  loading  the initial  saved  state.
    This implies that initialization/1 directives can refer to them.

    Here is an example of its usage:

    static PL_extension predicates[] = {
    { "foo",        1,      pl_foo, 0 },
    { "bar",        2,      pl_bar, PL_FA_NONDETERMINISTIC },
    { NULL,         0,      NULL,   0 }
    };

    main(int argc, char **argv)
    { PL_register_extensions(predicates);

      if ( !PL_initialise(argc, argv) )
        PL_halt(1);

      ...
    }


5.6.17 Foreign Code Hooks

For various specific applications some hooks re provided.


PL_dispatch_hook_t PL_dispatch_hook(PL_dispatch_hook_t)
    If  this hook  is not  NULL, this  function is  called when  reading
    from  the  terminal.     It is  supposed  to  dispatch  events  when
    SWI-Prolog  is connected  to a window  environment.   It can  return
    two  values:  PL_DISPATCH_INPUT indicates Prolog input is  available
    on  file descriptor 0 or PL_DISPATCH_TIMEOUT to indicate a  timeout.
    The  old hook is  returned.  The  type PL_dispatch_hook_t is  defined
    as:

    typedef int  (*PL_dispatch_hook_t)(void);


void PL_abort_hook(PL_abort_hook_t)
    Install  a hook  when abort/0 is  executed.   SWI-Prolog abort/0  is
    implemented  using C  setjmp()/longjmp() construct.   The hooks  are
    executed  in  the  reverse order  of  their registration  after  the
    longjmp()  took place and before  the Prolog toplevel is  reinvoked.
    The type PL_abort_hook_t is defined as:

    typedef void (*PL_abort_hook_t)(void);


int PL_abort_unhook(PL_abort_hook_t)
    Remove  a hook installed with PL_abort_hook().  Returns FALSE  if no
    such hook is found, TRUE otherwise.


void PL_on_halt(void (*f)(int, void *), void *closure)
    Register  the function f to be called if SWI-Prolog is halted.   The
    function  is  called with  two  arguments:   the  exit code  of  the
    process  (0 if this cannot  be determined on your operating  system)
    and  the closure argument passed to the PL_on_halt()call.   See also
    at_halt/1.


PL_agc_hook_t PL_agc_hook(PL_agc_hook_t new)
    Register    a   hook   with   the   atom-garbage    collector   (see
    garbage_collect_atoms/0  that  is   called  on  any  atom  that   is
    reclaimed.    The old hook  is returned.   If  no hook is  currently
    defined,  NULL is returned.  The argument of the called hook  is the
    atom  that is to be garbage collected.  The return value  is an int.
    If  the return value is zero, the  atom is not reclaimed.   The hook
    may invoke any Prolog predicate.

    The  example  below  defines  a foreign  library  for  printing  the
    garbage collected atoms for debugging purposes.

    #include <SWI-Stream.h>
    #include <SWI-Prolog.h>

    static int
    atom_hook(atom_t a)
    { Sdprintf("AGC: deleting %s\n", PL_atom_chars(a));

      return TRUE;
    }

    static PL_agc_hook_t old;

    install_t
    install()
    { old = PL_agc_hook(atom_hook);
    }

    install_t
    uninstall()
    { PL_agc_hook(old);
    }


5.6.18 Storing foreign data

This section  provides some hints for  handling foreign data in  Prolog.
With foreign  data, we refer  to data that is  used by foreign  language
predicates  and  needs  to  be passed  around  in  Prolog.     Excluding
combinations, there are three principal options for storing such data

  o Natural Prolog data
    E.i.  using the  representation  one would  choose if  there was  no
    foreign interface required.

  o Opaque packed Prolog data
    Data can also  be represetented in a foreign structure and stored on
    the  Prolog stacks using  PL_put_string_nchars()and retrieved  using
    PL_get_string_chars().   It is generally good  practice to wrap  the
    string  in a compound term with arity 1, so Prolog can  identify the
    type.    portray/1 rules  may be  used to  streamline printing  such
    terms during development.

  o Natural foreign data, passing a pointer
    An  alternative is to pass  a pointer to the  foreign data.   Again,
    this functor may be wrapped in a compound term.

The choice may be guided using the following distinctions

  o Is the data opaque to Prolog
    With  `opaque' data, we refer to data handled in  foreign functions,
    passed  around in  Prolog, but  of which Prolog  never examines  the
    contents  of the  data itself.   If  the data is  opaque to  Prolog,
    the  choosen representation  does not depend  on simple analysis  by
    Prolog,  and the selection  will be driven  solely by simplicity  of
    the interface and performance (both in time and space).

  o How big is the data
    Is  effient encoding  required?   For  examine, a  boolean aray  may
    be  expressed as  a compound  term, holding integers  each of  which
    contains a number of bits, or as a list of true and false.

  o What is the nature of the data
    For  examples in C,  constants are often  expressed using `enum'  or
    #define'd  integer values.   If  prolog needs  to handle this  data,
    atoms  are a more logical  choice.  Whether  or not this mapping  is
    used  depends on  whether Prolog  needs to interpret  the data,  how
    important debugging is and how important performance is.

  o What is the lifetime of the data
    We can distinguish three cases.

     1.  The lifetime  is dictated by  the accesibility  of the data  on
         the Prolog stacks.   Their is no way by which the  foreign code
         when the  data becomes `garbage',  and the  data thus needs  to
         be represented  on the Prolog  stacks using Prolog  data-types.
         (2),

     2.  The  data lives  on  the  `heap' and  is  explicitly  allocated
         and deallocated.   In  this case,  representing the data  using
         native foreign representation and passing a pointer to  it is a
         sensible choice.

     3.  The data lives as  during the lifetime of a  foreign predicate.
         If the predicate is deterministic, foreign  automatic variables
         are suitable.  if the predicate is  non-deterministic, the data
         may be allocated  using malloc() and  a pointer may be  passed.
         See section 5.6.1.1.


5.6.18.1 Examples for storing foreign data

In this section, we wull outline some examples,  covering typical cases.
In  the  first  example,  we will  deal  with  extending  Prolog's  data
representation with  integer-sets, represented as  bit-vectors.  In  the
second example, we  look at handling a  `netmask'.  Finally, we  discuss
the outline of the DDE interface.

Integer  sets with  not-to-far-apart  upper-  and  lower-bounds  can  be
represented using  bit-vectors.  Common  set operations, such as  union,
intersection,  etc.    are  reduced to  simple  and'ing and  or'ing  the
bitvectors.    This  can  be  done  in Prolog,  using  a  compound  term
holding  integer  arguments.    Especially  if  the  integers  are  kept
below  the maximum  tagged  integer  value (see  current_prolog_flag/2),
this representation  is fairly space-efficient (wasting  1 word for  the
functor and and  7 bits per integer for  the tags).  Arithmetic can  all
be performed in Prolog too.

For really demanding  applications, foreign representation will  perform
better,  especially  time-wise.    Bit-vectors are  natrually  expressed
using  string  objects.    If  the  string is  wrapped  in  bitvector/1,
lower-bound of the  vector is 0, and  the upperbound is not defined,  an
implementation for getting  and putting the setes  as well as the  union
predicate for it is below.

#include <SWI-Prolog.h>

#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))

static functor_t FUNCTOR_bitvector1;

static int
get_bitvector(term_t in, int *len, unsigned char **data)
{ if ( PL_is_functor(in, FUNCTOR_bitvector1) )
  { term_t a = PL_new_term_ref();

    PL_get_arg(1, in, a);
    return PL_get_string(a, (char **)data, len);
  }

  PL_fail;
}

static int
unify_bitvector(term_t out, int len, const unsigned char *data)
{ if ( PL_unify_functor(out, FUNCTOR_bitvector1) )
  { term_t a = PL_new_term_ref();

    PL_get_arg(1, out, a);

    return PL_unify_string_nchars(a, len, (const char *)data);
  }

  PL_fail;
}

static foreign_t
pl_bitvector_union(term_t t1, term_t t2, term_t u)
{ unsigned char *s1, *s2;
  int l1, l2;

  if ( get_bitvector(t1, &l1, &s1) &&
       get_bitvector(t2, &l2, &s2) )
  { int l = max(l1, l2);
    unsigned char *s3 = alloca(l);

    if ( s3 )
    { int n;
      int ml = min(l1, l2);

      for(n=0; n<ml; n++)
        s3[n] = s1[n] | s2[n];
      for( ; n < l1; n++)
        s3[n] = s1[n];
      for( ; n < l2; n++)
        s3[n] = s2[n];

      return unify_bitvector(u, l, s3);
    }

    return PL_warning("Not enough memory");
  }

  PL_fail;
}

install_t
install()
{ PL_register_foreign("bitvector_union", 3, pl_bitvector_union, 0);

  FUNCTOR_bitvector1 = PL_new_functor(PL_new_atom("bitvector"), 1);
}

Netmask's are  used with  TCP/IP  configuration.    Suppose we  have  an
application dealing with reasoning about a network configuration.   Such
an  application  requires  communicating  netmask  structures  from  the
operating system,  reasoning about  them and  possibly communicate  them
to the  user.   A  netmask consists  of 4  bitmasks between  0 and  255.
C-application  normally see  them as  an 4-byte  wide unsigned  integer.
SWI-Prolog cannot do that, as integers are always signed.

We could use the string approach outlined above, but  this makes it hard
to handle  these terms in Prolog.   A better  choice is a compound  term
netmask/4, holding the 4 submasks as integer arguments.

As the implementation is trivial, we will omit this here.

The DDE  interface (see section  3.46) represents  another common  usage
of  the foreign  interface:   providing communication  to new  operating
system features.  The DDE interface requires knowledge  about active DDE
server and  client channels.   These  channels contains various  foreign
data-types.  Such an interface is normally achieved  using an open/close
protocol that creates and destroys a handle.  The  handle is a reference
to a foreign data-structure containing the relevant information.

There are a  couple of possibilities for  representing the handle.   The
choice  depends on  responsibilities  and  debugging facilities.     The
simplest  aproach is  to using  PL_unify_pointer() and PL_get_pointer().
This  approach is  fast and  easy, but  has the  drawbacks of  (untyped)
pointers:   there  is no  reliable way  to detect  the  validity of  the
pointer, not  to verify  it is pointing  to a  structure of the  desired
type.   The pointer  may be wrapped  into a compound  term with arity  1
(i.e.  dde_channel(<Pointer>)), making the type-problem less serious.

Alternatively  (used in  the  DDE  interface),  the interface  code  can
maintain a  (preferably variable  length) array of  pointers and  return
the index in this  array.  This provides better protection.   Especially
for debugging  purposes, wrapping  the handle  in a compound  is a  good
suggestion.


5.6.19 Embedding SWI-Prolog in a C-program

As  of  version  2.1.0,  SWI-Prolog may  be  embedded  in  a  C-program.
To  reach  at  a compiled  C-program  with  SWI-Prolog  as  an  embedded
application is very  similar to creating a statically linked  SWI-Prolog
executable as described in section 5.4.1.

The  file  \ldots/pl/include/stub.c  defines  SWI-Prologs  default  main
program:

int
main(int argc, char **argv)
{ if ( !PL_initialise(argc, argv) )
    PL_halt(1);

  PL_install_readline();        /* delete if you don't want readline */

  PL_halt(PL_toplevel() ? 0 : 1);
}

This  may be  replaced with  your own  main C-program.    The  interface
function  PL_initialise()  must  be  called  before  any  of  the  other
SWI-Prolog  foreign  language  functions  described  in   this  chapter.
PL_initialise() interprets  all the command-line  arguments, except  for
the -t toplevel flag that is interpreted by PL_toplevel().


int PL_initialise(int argc, char **argv)
    Initialises  the SWI-Prolog heap and  stacks, restores the boot  QLF
    file,  loads the system and personal initialisation files,  runs the
    at_initialization/1 hooks and finally runs the -g goal hook.

    Special  consideration  is required  for argv[0].    On  Unix,  this
    argument  passes the part of the commandline that is used  to locate
    the  executable.   Prolog  uses this to  find the  file holding  the
    running executable.   The Windows version uses this to find a module
    of  the  running executable.    If the  specified  module cannot  be
    found, it tries  the module libpl.dll, containing the Prolog runtime
    kernel.   In  all these cases,  the resulting file  is used for  two
    purposes

      o  See whether a Prolog saved-state  is appended to the file.   If
         this is  the case,  this state  will be loaded  instead of  the
         default boot.prc file from the SWI-Prolog home directory.   See
         also qsave_program/[1,2] and section 5.7.

      o  Find the Prolog home  directory.  This process is  described in
         detail in section 5.8.

    PL_initialise()  returns 1  if all  initialisation  succeeded and  0
    otherwise.

    In  most cases, argc and argv will be passed from the  main program.
    It  is allowed to create your own argument vector,  provided argv[0]
    is constructed according to the rules above.  For example:

    int
    main(int argc, char **argv)
    { char *av[10];
      int ac = 0;

      av[ac++] = argv[0];
      av[ac++] = "-x";
      av[ac++] = "mystate";
      av[ac]   = NULL;

      if ( !PL_initialise(ac, av) )
        PL_halt(1);
      ...
    }

    Please  note that the  passed argument vector  may be referred  from
    Prolog  at any time  and should  therefore be valid  as long as  the
    Prolog engine is used.

    A  good setup  in Windows is  to add  SWI-Prolog's bin directory  to
    your  PATH  and  either pass  a  module holding  a  saved-state,  or
    "libpl.dll" as argv[0].


int PL_is_initialised(int *argc, char ***argv)
    Test  whether the  Prolog engine  is already initialised.    Returns
    FALSE  if Prolog  is not  initialised and TRUE  otherwise.   If  the
    engine is initialised  and argc is not NULL, the argument count used
    with  PL_initialise() is  stored in  argc.   Same  for the  argument
    vector argv.


void PL_install_readline()
    Installs  the GNU-readline line-editor.  Embedded  applications that
    do  not use the  Prolog toplevel should  normally delete this  line,
    shrinking the Prolog kernel significantly.


int PL_toplevel()
    Runs  the  goal of  the -t toplevel  switch  (default prolog/0)  and
    returns 1 if successful, 0 otherwise.


void PL_cleanup(int status)
    This  function performs  the reverse  of PL_initialise().   It  runs
    the  PL_on_halt()and at_halt/1 handlers, closes all  streams (except
    for the `standard  I/O' streams which are flushed only), deallocates
    all  memory and restores all signal  handlers.  The status  argument
    is  passed  to  the  various termination  hooks  and  indicates  the
    exit-status.

    This  function allows deleting and  restarting the Prolog system  in
    the same  process.  Use it with care, as PL_initialise() is a costly
    function.   Unix  users should consider  using exec() (available  as
    part of the clib package,).


void PL_halt(int status)
    Cleanup  the Prolog environment using PL_cleanup() and calls  exit()
    with the status argument.


5.7 Linking embedded applications using plld

The  utility program  plld (Win32:   plld.exe)  may be  used  to link  a
combination of C-files  and Prolog files into a stand-alone  executable.
plld automates most of what is described in the previous sections.

In the normal  usage, a copy is  made of the default embedding  template
\ldots/pl/include/stub.c.    The  main()  routine is  modified  to  suit
your  application.    PL_initialise() must  be passed  the  program-name
(argv[0])  (Win32:    the  executing  program  can   be  obtained  using
GetModuleFileName()).   The other  elements of  the command-line may  be
modified.  Next, plld is typically invoked as:

plld -o output stubfile.c [other-c-or-o-files] [plfiles]

plld  will first  split the  options into  various groups  for both  the
C-compiler and the Prolog  compiler.  Next, it will add  various default
options to the  C-compiler and call it  to create an executable  holding
the  user's C-code  and the  Prolog kernel.    Then,  it  will call  the
SWI-Prolog compiler  to create a  saved state  from the provided  Prolog
files  and finally,  it  will attach  this saved  state to  the  created
emulator to create the requested executable.

Below, it  is described how the options  are split and which  additional
options are passed.

-help
    Print brief synopsis.

-pl prolog
    Select  the prolog to use.   This prolog  is used for two  purposes:
    get  the home-directory as well  as the compiler/linker options  and
    create a saved state of the Prolog code.

-ld linker
    Linker  used to  link the raw  executable.   Default is  to use  the
    C-compiler (Win32:  link.exe).

-cc C-compiler
    Compiler  for  .c  files found  on  the  commandline.    Default  is
    the  compiler used  to build SWI-Prolog  (see current_prolog_flag/2)
    (Win32:  cl.exe).

-c++ C++-compiler
    Compiler  for C++ sources (extensions  .cpp, .cxx, .cc or .C)  files
    found  on the commandline.  Default is c++ or g++ if  the C-compiler
    is gcc) (Win32:  cl.exe).

-nostate
    Just  relink  the  kernel,  do  not  add  any  Prolog  code  to  the
    new  kernel.     This  is  used  to  create  a  new  kernel  holding
    additional  foreign predicates on machines  that do not support  the
    shared-library  (DLL) interface, or if building the state  cannot be
    handled  by the default procedure used by plld.  In the  latter case
    the  state is created  seperately and appended  to the kernel  using
    cat <kernel> <state> > <out>(Win32:  copy /b <kernel>+<state> <out>)

-pl-options ,...
    Additional  options passed to Prolog when creating the  saved state.
    The  first character  immediately  following pl-options  is used  as
    separator  and  translated to  spaces when  the  argument is  built.
    Example:  -pl-options,-F,xpce  passed -F xpce as additional flags to
    Prolog.

-ld-options ,...
    Passes options to the linker, similar to -pl-options.

-cc-options ,...
    Passes options to the C/C++ compiler, similar to -pl-options.

-v
    Select  verbose operation,  showing the  various programs and  their
    options.

-o outfile
    Reserved to specify the final output file.

-llibrary
    Specifies  a library for the C-compiler.   By default, -lpl  (Win32:
    libpl.lib) and the libraries needed by the Prolog kernel are given.

-Llibrary-directory
    Specifies  a  library directory  for  the C-compiler.    By  default
    the  directory  containing  the  Prolog C-library  for  the  current
    architecture is passed.

-g | -Iinclude-directory | -Ddefinition
    These  options  are passed  to  the C-compiler.    By  default,  the
    include directory containing SWI-Prolog.h  is passed.  plld adds two
    additional * -Ddef flags:

    -D__SWI_PROLOG__
         Indicates the code is to be connected to SWI-Prolog.

    -D__SWI_EMBEDDED__
         Indicates the creation of an embedded program.

 *.o | *.c | *.C | *.cxx | *.cpp
    Passed as input files to the C-compiler

 *.pl |*.qlf
    Passed  as  input  files  to  the  Prolog  compiler  to  create  the
    saved-state.

 *
    I.e.  all other options.  These are passed as linker options  to the
    C-compiler.


5.7.1 A simple example

The  following is  a very  simple example  going through  all the  steps
outlined above.   It  provides an arithmetic expression  evaluator.   We
will call  the application calc  and define it in  the files calc.c  and
calc.pl.  The Prolog file is simple:

calc(Atom) :-
        term_to_atom(Expr, Atom),
        A is Expr,
        write(A),
        nl.

The  C-part  of   the  application  parses  the  command-line   options,
initialises the  Prolog engine, locates  the calc/1 predicate and  calls
it.  The coder is in figure 5.5.

#include <stdio.h>
#include <SWI-Prolog.h>

#define MAXLINE 1024

int
main(int argc, char **argv)
{ char expression[MAXLINE];
  char *e = expression;
  char *program = argv[0];
  char *plav[2];
  int n;

  /* combine all the arguments in a single string */

  for(n=1; n<argc; n++)
  { if ( n != 1 )
      *e++ = ' ';
    strcpy(e, argv[n]);
    e += strlen(e);

  }

  /* make the argument vector for Prolog */

  plav[0] = program;
  plav[1] = NULL;

  /* initialise Prolog */

  if ( !PL_initialise(1, plav) )
    PL_halt(1);

  /* Lookup calc/1 and make the arguments and call */

  { predicate_t pred = PL_predicate("calc", 1, "user");

    term_t h0 = PL_new_term_refs(1);
    int rval;

    PL_put_atom_chars(h0, expression);
    rval = PL_call_predicate(NULL, PL_Q_NORMAL, pred, h0);

    PL_halt(rval ? 0 : 1);
  }

  return 0;
}

             Figure 5.5:  C-source for the calc application

The application is now created using the following command-line:

% plld -o calc calc.c calc.pl

The following indicates the usage of the application:

% calc pi/2
1.5708


5.8 The Prolog `home' directory

Executables  embedding SWI-Prolog  should  be able  to find  the  `home'
directory  of  the  development  environment  unless   a  self-contained
saved-state has  been added to  the executable  (see qsave_program/[1,2]
and section 5.7).

If Prolog starts up, it will try to  locate the development environment.
To do so, it will try the following steps until one succeeds.

 1. If  the environment variable  SWI_HOME_DIRis  defined and points  to
    an existing directory, use this.

 2. If  the  environment variable  SWIPL  is defined  and points  to  an
    existing directory, use this.

 3. Locate  the  primary   executable  or  (Windows  only)  a  component
    (module)  thereof  and check  whether the  parent  directory of  the
    directory  holding this file contains the  file swipl.  If so,  this
    file  contains the (relative) path to  the home directory.  If  this
    directory  exists, use this.   This is the normal mechanism  used by
    the binary distribution.

 4. If  the precompiled path exists, use it.  This is only  useful for a
    source installation.

If  all fails  and  there is  no state  attached  to the  executable  or
provided Windows module (see PL_initialise()), SWI-Prolog gives up.   If
a state is attached, the current working directory is used.

The file_search_path/2 alias swi is set  to point to the home  directory
located.


5.9 Example of Using the Foreign Interface

Below is an example  showing all stages of the declaration of  a foreign
predicate that transforms atoms possibly holding  uppercase letters into
an atom only holding lower case letters.  Figure  5.6 shows the C-source
file, figure 5.7 illustrates compiling and loading of foreign code.
/*  Include file depends on local installation */

#include <SWI-Prolog.h>
#include <stdlib.h>
#include <ctype.h>

foreign_t
pl_lowercase(term_t u, term_t l)
{ char *copy;
  char *s, *q;

  int rval;

  if ( !PL_get_atom_chars(u, &s) )
    return PL_warning("lowercase/2: instantiation fault");
  copy = malloc(strlen(s)+1);

  for( q=copy; *s; q++, s++)

    *q = (isupper(*s) ? tolower(*s) : *s);
  *q = '\0';

  rval = PL_unify_atom_chars(l, copy);
  free(copy);

  return rval;
}

install_t
install()
{ PL_register_foreign("lowercase", 2, pl_lowercase, 0);
}

                   Figure 5.6:  Lowercase source file

% gcc -I/usr/local/lib/pl-\plversion/include -fpic -c lowercase.c
% gcc -shared -o lowercase.so lowercase.o

% pl
Welcome to SWI-Prolog (Version \plversion)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- load_foreign_library(lowercase).

Yes
2 ?- lowercase('Hello World!', L).

L = 'hello world!'

Yes

    Figure 5.7:  Compiling the C-source and loading the object file


5.10 Notes on Using Foreign Code


5.10.1 Memory Allocation

SWI-Prolog's  memory  allocation  is  based  on  the  malloc(3)  library
routines.   Foreign  applications can safely  use malloc(3),  realloc(3)
and free(3).   Memory allocation using brk(2) or sbrk(2) is  not allowed
as these calls conflict with malloc(3).


5.10.2 Debugging Foreign Code

Statically  linked foreign  code  or embedded  systems can  be  debugged
normally.     Most  modern  environments  provide  debugging  tools  for
dynamically  loaded shared  objects  or dynamic  load  libraries.    The
following example traces  the code of lowercase  using gdb(1) in a  Unix
environment.

% gcc -I/usr/local/lib/pl-2.2.0/include -fpic -c -g lowercase.c
% gcc -shared -o lowercase.so lowercase.o
% gdb pl
(gdb) r
Welcome to SWI-Prolog (Version \plversion)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

?- load_foreign_library(lowercase).
<type Control-C>
(gdb) shared                    % loads symbols for shared objects
(gdb) break pl_lowercase
(gdb) continue
?- lowercase('HELLO', X).


5.10.3 Name Conflicts in C modules

In  the  current  version  of the  system  all  public  C  functions  of
SWI-Prolog are in the symbol table.  This can lead  to name clashes with
foreign code.    Someday I  should write a  program to  strip all  these
symbols from the symbol table  (why does Unix not have that?).   For now
I can only suggest to give your function another name.   You can do this
using the C preprocessor.  If---for example---your  foreign package uses
a function warning(), which happens to exist in SWI-Prolog  as well, the
following macro should fix the problem.

#define warning warning_

Note  that shared  libraries do  not  have this  problem as  the  shared
library loader  will only look  for symbols in  the main executable  for
symbols that are not defined in the library itself.


5.10.4 Compatibility of the Foreign Interface

The term-reference  mechanism was first used  by Quintus Prolog  version
3.     SICStus  Prolog version  3  is  strongly  based  on  the  Quintus
interface.  The  described SWI-Prolog interface is similar to  using the
Quintus or SICStus interfaces, defining all  foreign-predicate arguments
of  type  +term.    SWI-Prolog  explicitly uses  type  functor_t,  while
Quintus  and SICStus  uses <name>  and  <arity>.   As  the  names of  the
functions differ  from Prolog to  Prolog, a  simple macro layer  dealing
with the names can also deal with this detail.  For example:

#define QP_put_functor(t, n, a) PL_put_functor(t, PL_new_functor(n, a))

The  PL_unify_*()  functions are  lacking from  the  Quintus and  SICStus
interface.    They can  easily  be emulated  or the  put/unify  approach
should be used to write compatible code.

The PL_open_foreign_frame()/PL_close_foreign_frame() combination is lack-
ing from both other  Prologs.  SICStus has PL_new_term_refs(0),  followed
by PL_reset_term_refs()that allows for discarding term references.

The  Prolog interface  for  the graphical  user interface  package  XPCE
shares about  90% of the code  using a simple  macro layer to deal  with
different naming and calling conventions of the interfaces.


Chapter 6.  GENERATING RUNTIME APPLICATIONS

This  chapter  describes  the  features  of  SWI-Prolog  for  delivering
applications that can run without the development version  of the system
installed.

A SWI-Prolog runtime executable is a file consisting of two  parts.  The
first part  is the  emulator, which is  machine dependent.   The  second
part is the resource  archive, which contains the compiled program  in a
machine-independent format,  startup options  and possibly  user-defined
resources, see resource/3 and open_resource/3.

These two parts  can be connected in various  different ways.  The  most
common way  for distributed runtime applications  is to concatenate  the
two parts.   This can be achieved  using external commands (Unix:   cat,
Windows:   copy), or  using the  stand_alone option  to qsave_program/2.
The  second option  is  to  attach a  startup  script  in front  of  the
resource that  starts the  emulator with the  proper options.   This  is
the default  under Unix.   Finally,  an emulator  can be told  to use  a
specified resource file using the -x commandline switch.


qsave_program(+File, +ListOfOptions)
    Saves   the  current  state  of  the  program  to  the   file  File.
    The  result  is a  resource archive  containing  a saved-state  that
    expresses  all  Prolog   data  from  the  running  program  and  all
    user-defined  resources.   Depending on the stand_alone option,  the
    resource is headed by the emulator, a Unix shell-script or nothing.

    ListOfOptions  is a  list of  <Key>=<Value> or  <Key>(<Value>)  pairs.
    The available keys are described in table 6.1.
_________________________________________________________________________
|_Key________|Option_|____Type______|Description_________________________|
| local      | -L   |    K-bytes    |Size (Limit) of local stack         |
| global     | -G   |    K-bytes    |Size (Limit) of global stack        |
| trail      | -T   |    K-bytes    |Size (Limit) of trail stack         |

| argument   | -A   |    K-bytes    |Size (Limit) of argument stack      |
| goal       | -g   |     atom      |Initialisation goal                 |
| toplevel   | -t   |     atom      |Prolog toplevel goal                |
|_init_file___|-f___|_____atom______|Personal_initialisation_file________||||||
| class      |      |     atom      |If  runtime,  only  read  resources |
|            |      |               |from  the  state  (default).     If |

|            |      |               |kernel,  lock  all   predicates  as |
|            |      |               |system predicates  If  development, |
|            |      |               |save  the   predicates   in   their |
|            |      |               |current  state  and  keep   reading |
|            |      |               |resources  from  their  source  (if |
|            |      |               |present).  See also resource/3.     |
| autoload   |      |     bool      |If true, run autoload/0 first       |
| map        |      |     file      |File to write info on dump          |
| op         |      | save/standard |Save operator declarations?         |

| stand_alone |     |     bool      |Include the emulator in the state   |
| emulator   |      |     file      |Emulator attached  to  the  (stand- |
|            |      |               |alone) executable.  Default  is the |
|____________|______|_______________|running_emulator.___________________|

          Table 6.1:  <Key> = <Value> pairs for qsave_program/2

    Before   writing  the  data   to  file,  qsave_program/2  will   run
    autoload/0  to all  required  autoloading the  system can  discover.
    See autoload/0.

    Provided  the  application  does  not  require  any  of  the  Prolog
    libraries  to  be  loaded  at  runtime,   the  only  file  from  the
    SWI-Prolog development  environment required is the emulator itself.
    The  emulator may  be built in  two flavours.   The  default is  the
    development  emulator.  The  runtime emulator is similar, but  lacks
    the tracer.

    If the  option stand_alone(on)is present, the  emulator is the first
    part of the state.   If the emulator is started it will test whether
    a  boot-file (state)  is attached  to the emulator  itself and  load
    this  state.   Provided  the application has  all libraries  loaded,
    the  resulting executable is  completely independent of the  runtime
    environment or location where it was build.

    See also section 2.10.2.4.


qsave_program(+File)
    Equivalent to qsave_program(File, []).


autoload
    Check  the current Prolog program  for predicates that are  referred
    to,  are  undefined and  have a  definition in  the Prolog  library.
    Load the appropriate libraries.

    This  predicate is used  by qsave_program/[1,2] to ensure the  saved
    state  will not  depend  on one  of the  libraries.   The  predicate
    autoload/0  will find all direct references to predicates.   It does
    not  find predicates referenced via meta-predicates.   The predicate
    log/2  is  defined  in the  library(quintus)  to provide  a  quintus
    compatible means to compute  the natural logarithm of a number.  The
    following program will  behave correctly if its state is executed in
    an environment where the library(quintus) is not available:

    logtable(From, To) :-
            From > To, !.
    logtable(From, To) :-
            log(From, Value),
            format('~d~t~8|~2f~n', [From, Value]),
            F is From + 1,
            logtable(F, To).

    However,  the following implementation  refers to log/2 through  the
    meta-predicate  maplist/3.   Autoload will not  be able to find  the
    reference.   This problem may be fixed either by loading  the module
    libtary(quintus)  explicitly or  use  require/1 to  tell the  system
    that the predicate log/2 is required by this module.

    logtable(From, To) :-
            findall(X, between(From, To, X), Xlist),
            maplist(log, Xlist, SineList),
            write_table(Xlist, SineList).

    write_table([], []).
    write_table([I|IT], [V|VT]) :-
            format('~d~t~8|~2f~n', [I, V]),
            write_table(IT, VT).


volatile +Name/Arity, ...
    Declare  that  the clauses  of specified  predicates  should not  be
    saved  to the program.   The  volatile declaration is normally  used
    to avoid that  the clauses of dynamic predicates that represent data
    for the current session is saved in the state file.


6.1 Limitations of qsave_program

There  are  three  areas  that  require  special  attention  when  using
qsave_program/[1,2].

  o If  the  program  is an  embedded  Prolog  application or  uses  the
    foreign  language interface,  care has  to be taken  to restore  the
    appropriate foreign context.  See section 6.2 for details.

  o If  the program uses directives (:- goal. lines) that  perform other
    actions then  setting predicate attributes (dynamic, volatile, etc.)
    or  loading files  (consult,  etc.), the  directive may  need to  be
    prefixed with initialization/1.

  o Database  references as returned by clause/3, recorded/3, etc.   are
    not preserved and may thus not be part of the database when saved.


6.2 Runtimes and Foreign Code

Some  applications may  need  to  use the  foreign  language  interface.
Object code is  by definition machine-dependent and thus cannot  be part
of the saved program file.

To complicate the matter even further there are various  ways of loading
foreign code:

  o Using the library(shlib) predicates
    This  is the preferred way of dealing  with foreign code.   It loads
    quickly and ensures  an acceptable level of independence between the
    versions  of the emulator and the foreign code loaded.  It  works on
    Unix  machines supporting shared libraries and library  functions to
    load  them.  Most  modern Unixes, as  well as Win32 (Windows  95/NT)
    satisfy this constraint.

  o Static linking
    This  mechanism works on  all machines,  but generally requires  the
    same  C-compiler and linker to be  used for the external code as  is
    used to build SWI-Prolog itself.

To make  a runtime  executable that  can run on  multiple platforms  one
must make runtime  checks to find the correct  way of linking.   Suppose
we have  a source-file  myextension defining  the installation  function
install().

If this file  is compiled into a  shared library, load_foreign_library/1
will load this library and call the installation  function to initialise
the  foreign code.    If it  is  loaded as  a static  extension,  define
install() as the predicate install/0:

static foreign_t
pl_install()
{ install();

  PL_succeed;
}

PL_extension PL_extensions [] =
{
/*{ "name",     arity,  function,       PL_FA_<flags> },*/

  { "install",  0,      pl_install,     0 },
  { NULL,       0,      NULL,           0 }     /* terminating line */
};

Now, use the following Prolog code to load the foreign library:

load_foreign_extensions :-
        current_predicate(install, install), !, % static loaded
        install.
load_foreign_extensions :-                      % shared library
        load_foreign_library(foreign(myextension)).

:- initialization load_foreign_extensions.

The path  alias foreign  is defined by  file_search_path/2.   By  default
it  searches  the  directories <home>/lib/<arch> and  <home>/lib.     The
application can specify additional rules for file_search_path/2.


6.3 Using program resources

A  resource is  very  similar to  a  file.    Resources however  can  be
represented in two different formats:  on files, as well  as part of the
resource archive of a saved-state (see qsave_program/2).

A resource has a name  and a class.  The source data of the  resource is
a file.   Resources are declared by declaring the  predicate resource/3.
They are accessed using the predicate open_resource/3.

Before  going into  details,  let  us start  with  an  example.    Short
texts  can easily  be expressed  in Prolog  sourcecode,  but long  texts
are cumbersome.   Assume our application  defines a command `help'  that
prints a  helptext to the screen.   We put  the content of the  helptext
into a  file called help.txt.   The following  code implements our  help
command such that help.txt is incorperated into the runtime executable.

resource(help, text, 'help.txt').

help :-
        open_resource(help, text, In),
        copy_stream(In, user_output),
        close(In).

copy_stream(In, Out) :-
        get0(In, C),
        copy_stream(C, In, Out).

copy_stream(-1, _, _) :- !.
copy_stream(C, In, Out) :-
        put(Out, C),
        get0(In, C2),
        copy_stream(C2, In, Out).

The predicate  help/0 opens  the resource  as a Prolog  stream.   If  we
are executing this from the development environment,  this will actually
return  a stream  to  the  gelp.txt itself.     When executed  from  the
saved-state, the stream will actually be a stream  opened on the program
resource file, taking care of the offset and length of the resource.


6.3.1 Predicates Definitions


resource(+Name, +Class, +FileSpec)
    This  predicate is  defined  as a  dynamic predicate  in the  module
    user.    Clauses for  it may  be  defined in  any module,  including
    the  user module.    Name is  the name  of the  resource (an  atom).
    A  resource name  may contain  any character,  except for  $ and  :,
    which  are  reserved for  internal usage  by  the resource  library.
    Class  describes the what kind of object is stored in  the resource.
    In  the  current implementation,  it  is just  an  atom.    FileSpec
    is  a file  specification that  may exploit  file_search_path/2 (see
    absolute_file_name/2).

    Normally,  resources are  defined as unit  clauses (facts), but  the
    definition  of this  predicate also allows  for rules.   For  proper
    generation  of the  saved state,  it must be  possible to  enumerate
    the  available  resources by  calling this  predicate  with all  its
    arguments unbound.

    Dynamic  rules are useful to turn  all files in a certain  directory
    into  resources, without specifying a resources for each file.   For
    example,  assume the file_search_path/2icons refers to  the resource
    directory  containing icon-files.   The  following definition  makes
    all these images available as resources:

    resource(Name, image, icons(XpmName)) :-
            atom(Name), !,
            file_name_extension(Name, xpm, XpmName).
    resource(Name, image, XpmFile) :-
            var(Name),
            absolute_file_name(icons(.), [type(directory)], Dir)
            concat(Dir, '/*.xpm', Pattern),
            expand_file_name(Pattern, XpmFiles),
            member(XpmFile, XpmFiles).


open_resource(+Name, ?Class, -Stream)
    Opens  the resource specified by Name and  Class.  If the  latter is
    a  variable, it will be unified  to the class of the  first resource
    found that has the  specified Name.  If successful, Stream becomes a
    handle to a  binary input stream, providing access to the content of
    the resource.

    The  predicate  open_resource/3  first  checks  resource/3.     When
    succesful   it  will   open  the   returned  resource   source-file.
    Otherwise  it will  look in the  programs resource  database.   When
    creating  a  saved-state,  the system  normally saves  the  resource
    contents  into the resource archive, but does not save  the resource
    clauses.

    This   way,  the  development   environment  uses  the  files   (and
    modifications   to   the   resource/3  declarations   and/or   files
    containing  resource   info  thus  immediately  affect  the  running
    environment,  while the runtime  system quickly accesses the  system
    resources.


6.3.2 The plrc program

The  utility program  plrc can  be used  to examine  and manipulate  the
contents of  a SWI-Prolog resource  file.  The  options are inspired  by
the Unix ar program.  The basic command is:

% plrc option resource-file member ...

The options are described below.

l
    List contents of the archive.

x
    Extract  named (or  all)  members of  the archive  into the  current
    directory.

a
    Add  files  to the  archive.    If the  archive already  contains  a
    member  with the  same name,  the contents  is replaced.    Anywhere
    in   the  sequence  of  members,   the  options  --class=class   and
    --encoding=encoding may appear.   They affect the class and encoding
    of subsequent files.  The initial class is data and encoding none.

d
    Delete named members from the archive.

This command is also described in the pl(1) Unix manual page.


6.4 Finding Application files

If your application  uses files that are  not part of the saved  program
such as database  files, configuration files, etc., the  runtime version
has to be able to  locate these files.  The file_search_path/2 mechanism
in combination with  the -palias command-line argument is the  preferred
way to  locate runtime  files.   The first  step is to  define an  alias
for the  toplevel directory  of your  application.   We  will call  this
directory gnatdir in our examples.

A  good  place  for storing  data  associated  with  SWI-Prolog  runtime
systems is  below the emulator's  home-directory.   swi is a  predefined
alias for this directory.  The following is  a useful default definition
for the search path.

user:file_search_path(gnatdir, swi(gnat)).

The  application  should  locate   all  files  using  absolute_file_name.
Suppose   gnatdir   contains   a  file   config.pl   to   define   local
configuration.  Then use the code below to load this file:

configure_gnat :-
        (   absolute_file_name(gnatdir('config.pl'), ConfigFile)
            ->  consult(ConfigFile)
            ;   format(user_error, 'gnat: Cannot locate config.pl~n'),
            halt(1)
            ).


6.4.1 Passing a path to the application

Suppose   the  system   administrator  has   installed  the   SWI-Prolog
runtime  environment  in  /usr/local/lib/rt-pl-3.2.0.     A  user  wants
to  install  gnat,   but  gnat  will  look  for  its   configuration  in
/usr/local/lib/rt-pl-3.2.0/gnat where the user cannot write.

The user  decides to install the  gnat runtime files in  /users/bob/lib/
gnat.  For one-time  usage, the user may decide to start gnat  using the
command:

% gnat -p gnatdir=/users/bob/lib/gnat


6.5 The Runtime Environment


6.5.1 The Runtime Emulator

The sources  may be  used to built  two versions  of the emulator.    By
default,  the development emulator  is built.    This emulator  contains
all features  for interactive development  of Prolog  applications.   If
the system is  configured using --enable-runtime, make(1) will  create a
runtime version  of the emulator.   This emulator  is equivalent to  the
development version, except for the following features:

  o No input editing
    The  GNU library -lreadline  that provides EMACS compatible  editing
    of input lines will not be linked to the system.

  o No tracer
    The  tracer and  all its options  are removed,  making the system  a
    little faster too.

  o No profiler
    profile/3 and friends are  not supported.  This saves some space and
    provides better performance.

  o No interrupt
    Keyboard  interrupt (Control-C  normally)  is not  rebound and  will
    normally terminate the application.

  o current_prolog_flag(runtime, true) succeeds
    This  may  be used  to verify  your application  is  running in  the
    runtime environment rather than the development environment.

  o clause/[2,3] do not work on static predicates
    This  prolog-flag inhibits listing your program.  It is only  a very
    limited protection however.

The  following  fragment   is  an  example  for  building  the   runtime
environment in  \env{HOME}/lib/rt-pl-3.2.0.   If  possible, the  shared-
library interface should  be configured to ensure  it can serve a  large
number of applications.

% cd pl-3.2.0
% mkdir runtime
% cd runtime
% ../src/configure --enable-runtime --prefix=$HOME
% make
% make rt-install

The  runtime directory  contains  the components  listed  below.    This
directory may be tar'ed and shipped with your application.

           __________________________________________________
           |_README.RT____|Info_on_the_runtime_environment___|
           |_bin/<arch>/pl|The_emulator_itself_______________|

           |_man/pl.1_____|Manual_page_for_pl________________|
           |_swipl________|pointer_to_the_home_directory_(.)_|
           | lib/         |directory for shared libraries    |
           |_lib/<arch>/__|machine-specific_shared_libraries_|


Chapter 7.  HACKERS CORNER

This  appendix  describes  a  number  of  predicates  which  enable  the
Prolog  user  to  inspect the  Prolog  environment  and  manipulate  (or
even redefine)  the debugger.    They can be  used as  entry points  for
experiments with debugging  tools for Prolog.  The  predicates described
here should  be handled  with some  care as it  is easy  to corrupt  the
consistency of the Prolog system by misusing them.


7.1 Examining the Environment Stack


prolog_current_frame(-Frame)
    Unify   Frame  with  an  integer   providing  a  reference  to   the
    parent  of  the  current  local stack  frame.    A  pointer  to  the
    current  local frame  cannot be provided  as the predicate  succeeds
    deterministically  and therefore its frame is  destroyed immediately
    after succeeding.


prolog_frame_attribute(+Frame, +Key, -Value)
    Obtain  information  about  the local  stack  frame  Frame.    Frame
    is  a  frame reference  as obtained  through prolog_current_frame/1,
    prolog_trace_interception/4 or this predicate.   The key values  are
    described below.

    alternative
         Value is unified with  an integer reference to the  local stack
         frame in  which execution  is  resumed if  the goal  associated
         with Frame  fails.    Fails  if the  frame has  no  alternative
         frame.

    has_alternatives
         Value is unified  with true if Frame  still is a candidate  for
         backtracking.  false otherwise.

    goal
         Value is unified with the  goal associated with Frame.   If the
         definition module of the active predicate is not  user the goal
         is represented as <module>:<goal>.  Do not instantiate variables
         in this goal unless you know what you are doing!

    clause
         Value is  unified with  a  reference to  the currently  running
         clause.    Fails  if the  current  goal  is associated  with  a
         foreign  (C) defined  predicate.    See  also nth_clause/3  and
         clause_property/2.

    level
         Value is unified  with the recursion level  of Frame.  The  top
         level frame is at level `0'.

    parent
         Value is unified with an integer reference to  the parent local
         stack frame of Frame.  Fails if Frame is the top frame.

    context_module
         Value is  unified with the  name of the  context module of  the
         environment.

    top
         Value is  unified with  true if Frame  is the  top Prolog  goal
         from a recursive  call back from the  foreign language.   false
         otherwise.

    hidden
         Value is  unified with  true if the  frame is  hidden from  the
         user, either  because a  parent has  the hide-childs  attribute
         (all  system  predicates),  or  the  system   has  no  trace-me
         attribute.

    pc
         Value is unified with  the program-pointer saved on behalve  of
         the parent-goal if  the parent-goal is  not owned by a  foreign
         predicate.

    argument(N)
         Value is  unified with the N-th  slot of the  frame.   Argument
         1 is  the first  argument of  the goal.    Arguments above  the
         arity refer to local variables.  Fails silently if N  is out of
         range.


7.2 Intercepting the Tracer


prolog_trace_interception(+Port, +Frame, +PC, -Action)
    Dynamic  predicate, normally not defined.  This predicate  is called
    from  the SWI-Prolog debugger just before it would show a port.   If
    this  predicate succeeds the debugger  assumes the trace action  has
    been  taken care of and continues execution as described  by Action.
    Otherwise the normal Prolog debugger actions are performed.

    Port  is one  of call,  redo,  exit, fail  or unify.    Frame is  an
    integer  reference to  the current  local stack frame.    PC is  the
    current  value  of the  program-counter, relative  to  the start  of
    the  current clause,  or 0  if it  is invalid,  for example  because
    the  current frame runs a foreign  predicate, or no clause has  been
    selected  yet.    Action should  be unified  with one  of the  atoms
    continue  (just continue execution), retry (retry the  current goal)
    or fail (force the  current goal to fail).  Leaving it a variable is
    identical to continue.

    Together  with  the predicates  described in  section  3.42 and  the
    other  predicates of this chapter this predicate enables  the Prolog
    user  to define a complete new debugger in Prolog.  Besides  this it
    enables  the Prolog programmer monitor  the execution of a  program.
    The  example below records  all goals trapped  by the tracer in  the
    database.

    prolog_trace_interception(Port, Frame, _PC, continue) :-
            prolog_frame_attribute(Frame, goal, Goal),
            prolog_frame_attribute(Frame, level, Level),
            recordz(trace, trace(Port, Level, Goal)).

    To  trace the execution of `go' this way the following  query should
    be given:

    ?- trace, go, notrace.


prolog_skip_level(-Old, +New)
    Unify  Old with  the old  value of `skip  level' and  than set  this
    level  according to New.   New  is an integer,  or the special  atom
    very_deep  (meaning don't  skip).    The `skip  level' is  a  global
    variable  of the  Prolog system  that disables the  debugger on  all
    recursion  levels deeper than  the level of the  variable.  Used  to
    implement  the trace options  `skip' (sets skip  level to the  level
    of  the frame) and `up' (sets skip level to the level of  the parent
    frame (i.e. the level of this frame minus 1).


7.3 Hooks using the exception/3 predicate

This section describes  the predicate exception/3, which may  be defined
by the  user in the module  user as a multifile  predicate.  Unlike  the
name  suggests, this  is  actually a  hook predicate.    Exceptions  are
handled by  the ISO  predicates catch/3 and  throw/1.   They all  frames
created after the matching catch/3 to be discarded immediately.

The  predicate exception/3  is  called by  the  kernel  on a  couple  of
events,  allowing the user  to alter  the behaviour  on some  predefined
events.


exception(+Exception, +Context, -Action)
    Dynamic  predicate,  normally not  defined.   Called  by the  Prolog
    system  on run-time exceptions.  Currently exception/3 is  only used
    for  trapping undefined  predicates.   Future versions might  handle
    signal  handling, floating exceptions  and other runtime errors  via
    this mechanism.  The values for Exception are described below.

    undefined_predicate
         If Exception is undefined_predicate Context is  instantiated to
         a term  Name/Arity.    Name refers  to the  name  and Arity  to
         the  arity of  the  undefined predicate.    If  the  definition
         module of  the predicate is  not user, Context  will be of  the
         form <Module>:<Name>/<Arity>.   If  the  predicate fails  Prolog
         will generate an  esistence_error exception.  If the  predicate
         succeeds it should instantiate the last argument  either to the
         atom fail to tell Prolog to fail the predicate,  the atom retry
         to tell  Prolog to  retry the predicate  or error  to make  the
         system generate  an exception.    The action  retry only  makes
         sense if the exception handler has defined the predicate.


7.4 Hooks for integrating libraries

Some libraries  realise an entirely new  programming paradigm on top  of
Prolog.   An example is  XPCE which adds  an object-system to Prolog  as
well as an extensive  set of graphical primitives.   SWI-Prolog provides
several hooks to  improve the integration of  such libraries.  See  also
section 3.4  for editing hooks  and section 3.9.3  for hooking into  the
message system.


prolog_list_goal(:Goal)
    Hook, normally not defined.   This hook is called by the 'L' command
    of  the  tracer in  the module  user to  list  the currently  called
    predicate.   This hook may be defined to list only  relevant clauses
    of  the indicated  Goal  and/or show  the actual  source-code in  an
    editor.  See also portray/1 and multifile/1.


prolog:debug_control_hook(:Action)
    Hook for the  debugger-control predicates that allows the creator of
    more  high-level programming languages  to use the common  front-end
    predicates  to control de  debugger.  For  example, XPCE uses  these
    hooks  to allow for spying methods  rather then predicates.   Action
    is one of:

    spy(Spec)
         Hook in  spy/1.  If  the hook succeeds  spy/1 takes no  further
         action.

    nospy(Spec)
         Hook in nospy/1.   If the hook succeeds spy/1 takes  no further
         action.    If  spy/1  is  hooked,  it is  adviced  to  place  a
         complementary hook for nospy/1.

    nospyall
         Hook in nospyall/0.   Should remove all spy-points.   This hook
         is called in a failure-driven loop.

    debugging
         Hook in  debugging/0.   It can  be used in  two ways.   It  can
         report the status of the additional debug-points  controlled by
         the above hooks  and fail to let  the system report the  others
         or it succeed, overruling the entire behaviour of debugging/0.


prolog:help_hook(+Action)
    Hook  into help/0 and help/1.   If  the hook succeeds, the  built-in
    actions are not  executed.  For example, ?- help(picture). is caught
    by  the XPCE help-hook to give help  on the class picture.   Defined
    actions are:

    help
         User entered plain  help/0 to give default  help.  The  default
         performs help(help/1), giving help on help.

    help(What)
         Hook in help/1 on the topic What.

    apropos(What)
         Hook in apropos/1 on the topic What.


7.5 Readline Interaction

The       following       predicates       are        available       if
current_prolog_flag(readline, true) succeeds.      They  allow  for  di-
rect  interaction   with  the   GNU  readline   library.      See   also
readline(3)


rl_read_init_file(+File)
    Read  a readline  initialisation file.   Readline  by default  reads
    ~/.inputrc.     This  predicate  may be  used  to  read  alternative
    readline initialisation files.


rl_add_history(+Line)
    Add  a  line  to  the  Control-P/Control-N  history  system  of  the
    readline library.


Chapter 8.  GLOSSARY OF TERMS

anonymous [variable]
    The  variable  _  is  called  the  anonymous  variable.     Multiple
    occurrences of _ in a single term are not shared.

arguments
    Arguments  are terms that appear in a compound term.  A1  and a2 are
    the first and second argument of the term myterm(A1, a2).

arity
    Argument count (is number of arguments) of a compound term.

assert
    Add a clause to  a predicate.  Clauses can be added at either end of
    the clause-list of a predicate.  See assert/1 and assertz/1.

atom
    Textual  constant.   Used as name for  compound terms, to  represent
    constants or text.

backtracking
    Searching  process used by Prolog.   If a predicate offers  multiple
    clauses  to  solve  a goal,  they  are  tried one-by-one  until  one
    succeeds.    If  a subsequent  part of  the prove  is not  satisfied
    with  the resulting variable binding, it may ask for  an alternative
    solution (= binding  of the variables), causing Prolog to reject the
    previously chosen clause and try the next one.

binding [of a variable]
    Current value of the variable.  See also backtracking and query.

built-in [predicate]
    Predicate  that is part of the  Prolog system.  Built in  predicates
    cannot  be redefined  by the  user, unless this  is overruled  using
    redefine_system_predicate/1.

body
    Part of a clause behind the neck operator (:-).

clause
    `Sentence'  of a Prolog program.   A clause  consists of a head  and
    body  separated by  the neck operator  (:-) or it  is a fact.    For
    example:

    parent(X) :-
            father(X, _).

    Expressed  ``X is a parent if X is a father of someone''.   See also
    variable and predicate.

compile
    Process  where  a Prolog  program  is translated  to a  sequence  of
    instructions.   See  also interpreted.   SWI-Prolog always  compiles
    your program before executing it.

compound [term]
    Also  called  structure.    It  consists of  a  name followed  by  N
    arguments,  each of which are terms.   N is called the arity  of the
    term.

context module
    If  a term  is referring  to a predicate  in a  module, the  context
    module  is used to find the target module.  The context  module of a
    goal  is the module in which  the predicate is defined, unless  this
    predicate  is module transparent, in  which case the context  module
    is inherited from the parent goal.  See also module_transparent/1.

dynamic [predicate]
    A dynamic predicate  is a predicate to which clauses may be asserted
    and  from  which  clauses may  be  retracted  while the  program  is
    running.  See also update view.

exported [predicate]
    A  predicate is  said to  be exported from  a module  if it  appears
    in  the  public list.     This implies  that  the predicate  can  be
    imported  into another module  to make it visible  there.  See  also
    use_module/[1,2].

fact
    Clause  without a body.   This is called a fact because  interpreted
    as logic, there is  no condition to be satisfied.  The example below
    states john is a person.

    person(john).

fail
    A goal is said to haved failed if it could not be proven.

float
    Computers  cripled representation of a real number.   Represented as
    `IEEE double'.

foreign
    Computer code expressed  in other languages than Prolog.  SWI-Prolog
    can only cooperate directly with the C and C++ computer languages.

functor
    Combination  of name and arity of a compound term.  The  term foo(a,
    b,  c) is said to be a  term belonging to the functor foo/3.   foo/0
    is used to refer to the atom foo.

goal
    Question  stated to the Prolog engine.  A goal is either  an atom or
    a  compound term.  A goal  succeeds, in which case the  variables in
    the compound terms have  a binding or fails if Prolog fails to prove
    the goal.

hashing
    Indexing technique used for quick lookup.

head
    Part  of a clause before the neck  instruction.  This is an  atom or
    compound term.

imported [predicate]
    A  predicate is said to be imported  into a module if it  is defined
    in  another module  and made  available in this  module.   See  also
    chapter 4.

indexing
    Indexing  is a  technique used to  quickly select candidate  clauses
    of  a  predicate for  a specific  goal.    In most  Prolog  systems,
    including  SWI-Prolog, indexing  is done  on the  first argument  of
    the  head.  If  this argument is  instantiated to an atom,  integer,
    float or compound  term with functor, hashing is used quickly select
    all  clauses of which  the first argument  may unify with the  first
    argument of the goal.

integer
    Whole  number.   On most current  machines, SWI-Prolog integers  are
    represented  as `32-bit signed values', ranging from  -2147483648 to
    2147483647.  See also current_prolog_flag/2.

interpreted
    As  opposed  to  compiled,   interpreted  means  the  Prolog  system
    attempts  to prove  a goal  by directly reading  the clauses  rather
    than executing instructions  from an (abstract) instruction set that
    is not or only indirectly related to Prolog.

meta predicate
    A  predicate that reasons about other predicates, either  by calling
    them, (re)defining them or querying properties.

module
    Collection  of predicates.    Each module defines  a name-space  for
    predicates.   built-in predicates  are accessible from all  modules.
    Predicates  can be published (exported)  and imported to make  their
    definition available to other modules.

module transparent [predicate]
    A  predicate that  does not change  the context  module.   Sometimes
    also called a meta predicate.

multifile [predicate]
    Predicate  for which  the  definition is  distributed over  multiple
    source-files.  See multi_file/1.

neck
    Operator (:-) separating head from body in a clause.

operator
    Symbol (atom) that  may be placed before its operant (prefix), after
    its operant (postfix) or between its two operants (infix).

    In  Prolog, the expression a+b is exactly the same as  the canonical
    term +(a,b).

operant
    Argument of an operator.

precedence
    The  priority  of an  operator.    Operator  precedence is  used  to
    interpret a+b*c as +(a, *(b,c)).

predicate
    Collection  of clauses  with the same  functor (name/arity).   If  a
    goal  is proved,  the system  looks for  a predicate  with the  same
    functor,  then used  indexing to select  candidate clauses and  then
    tries these clauses one-by-one.  See also backtracking.

priority
    In the context of operators a synonym for precedence.

program
    Collection of predicates.

property
    Attribute  of  an object.    SWI-Prolog  defines  various *_property
    predicates to query the status of predicates, clauses.  etc.

prove
    Process  where Prolog attempts to prove a query using  the available
    predicates.

public list
    List of predicates exported from a module.

query
    See goal.

retract
    Remove  a clause from a  predicate.   See also dynamic, update  view
    and assert.

shared
    Two  variables  are called  shared after  they are  unified.    This
    implies  if either of them is bound, the other is bound to  the same
    value:

    ?- A = B, A = a.

    A = a,
    B = a

singleton [variable]
    Variable  appearing only one time in a clause.   SWI-Prolog normally
    warns  for  this to  avoid  you  making spelling  mistakes.    If  a
    variable  appears on purpose only  once in a  clause, write it as  _
    (see  anonymous) or make sure the first character is a _.   See also
    the style_check/1 option singletons.

solution
    Bindings resulting from a successfully proven goal.

structure
    Synonym for compound term.

string
    Used  for the  following representations  of text:   a packed  array
    (see  section 3.23, SWI-Prolog specific), a list of  character codes
    or a list of one-character atoms.

succeed
    A goal is said to have succeeded if it has been proven.

term
    Value in Prolog.   A term is either a variable, atom, integer, float
    or  compound term.   In addition,  SWI-Prolog also defines the  type
    string

transparent
    See module transparent.

unify
    Prolog  process to make  two terms equal  by assigning variables  in
    one term to  values at the corresponding location of the other term.
    For example:

    ?- foo(a, B) = foo(A, b).

    A = a,
    B = b

    Unlike  assignment (which does not exist in Prolog),  unification is
    not directed.

update view
    How  Prolog behaves when a dynamic predicate is changed while  it is
    running.   There are two models.   In most older Prolog  systems the
    change  becomes immediately visible to  the goal, in modern  systems
    including  SWI-Prolog, the running goal is  not affected.  Only  new
    goals `see' the new definition.

variable
    A  Prolog  variable is  a value  that `is  not yet  bound'.    After
    binding a variable, it  cannot be modified.  Backtracking to a point
    in  the execution before  the variable was  bound will turn it  back
    into a variable:

    ?- A = b, A = c.
    No
    ?- (A = b; true; A = c).
    A = b ;
    A = _G283 ;
    A = c ;
    No

    See also unify.


Chapter 9.  SUMMARY


9.1 Predicates

The  predicate summary  is used  by the  Prolog  predicate apropos/1  to
suggest predicates from a keyword.

 !/0                           Cut (discard choicepoints)
 !/1                           Cut block.  See block/3
 ,/2                           Conjunction of goals
 ->/2                          If-then-else
 *->/2                         Soft-cut
 ./2                           Consult.  Also list constructor
 ;/2                           Disjunction of goals.  Same as |/2
 </2                           Arithmetic smaller

 =/2                           Unification
 =../2                         ``Univ.''  Term to list conversion
 =:=/2                         Arithmetic equal
 =</2                          Arithmetic smaller or equal
 ==/2                          Identical
 =@=/2                         Structural identical
 =\=/2                         Arithmetic not equal

 >/2                           Arithmetic larger
 >=/2                          Arithmetic larger or equal
 @</2                          Standard order smaller
 @=</2                         Standard order smaller or equal
 @>/2                          Standard order larger
 @>=/2                         Standard order larger or equal
 \+/1                          Negation by failure.  Same as not/1
 \=/2                          Not unifyable

 \==/2                         Not identical
 \=@=/2                        Not structural identical
 ^/2                           Existential quantification (bagof/3, setof/3)
 |/2                           Disjunction of goals.  Same as ;/2
 abolish/1                     Remove predicate definition from the database
 abolish/2                     Remove predicate definition from the database
 abort/0                       Abort execution, return to top level

 absolute_file_name/2          Get absolute path name
 absolute_file_name/3          Get absolute path name with options
 access_file/2                 Check access permissions of a file
 append/1                      Append to a file
 append/3                      Concatenate lists
 apply/2                       Call goal with additional arguments
 apropos/1                     library(online_help) Show related predicates and manual sections
 arg/3                         Access argument of a term

 arithmetic_function/1         Register an evaluable function
 assert/1                      Add a clause to the database
 assert/2                      Add a clause to the database, give reference
 asserta/1                     Add a clause to the database (first)
 asserta/2                     Add a clause to the database (first)
 assertz/1                     Add a clause to the database (last)
 assertz/2                     Add a clause to the database (last)

 attach_console/0              Attach I/O console to thread
 at_end_of_stream/0            Test for end of file on input
 at_end_of_stream/1            Test for end of file on stream
 at_halt/1                     Register goal to run at halt/1
 at_initialization/1           Register goal to run at start-up
 atom/1                        Type check for an atom
 atom_chars/2                  Convert between atom and list of characters
 atom_codes/2                  Convert between atom and list of ASCII values

 atom_length/2                 Determine length of an atom
 atom_prefix/2                 Test for start of atom
 atom_to_term/3                Convert between atom and term
 atomic/1                      Type check for primitive
 autoload/0                    Autoload all predicates now
 bagof/3                       Find all solutions to a goal
 between/3                     Integer range checking/generating

 block/3                       Start a block (`catch'/`throw')
 break/0                       Start interactive toplevel
 call/1                        Call a goal
 call/[2..]                    Call with additional arguments
 call_shared_object_function/2 UNIX: Call C-function in shared (.so) file
 call_with_depth_limit/3       Prove goal with bounded depth
 callable/1                    Test for atom or compound term
 catch/3                       Call goal, watching for exceptions

 char_code/2                   Convert between atom and ASCII value
 char_conversion/2             Provide mapping of input characters
 char_type/2                   Classify characters
 character_count/2             Get character index on a stream
 chdir/1                       Change working directory
 checklist/2                   Invoke goal on all members of a list
 clause/2                      Get clauses of a predicate

 clause/3                      Get clauses of a predicate
 clause_property/2             Get properties of a clause
 close/1                       Close stream
 close/2                       Close stream (forced)
 close_dde_conversation/1      Win32:  Close DDE channel
 close_shared_object/1         UNIX: Close shared library (.so file)
 compare/3                     Compare, using a predicate to determine the order
 compiling/0                   Is this a compilation run?

 compound/1                    Test for compound term
 atom_concat/3                 Append two atoms
 code_type/2                   Classify a character-code
 concat_atom/2                 Append a list of atoms
 concat_atom/3                 Append a list of atoms with separator
 consult/1                     Read (compile) a Prolog source file
 context_module/1              Get context module of current goal

 convert_time/8                Break time stamp into fields
 convert_time/2                Convert time stamp to string
 copy_stream_data/2            Copy all data from stream to stream
 copy_stream_data/3            Copy n bytes from stream to stream
 copy_term/2                   Make a copy of a term
 current_arithmetic_function/1 Examine evaluable functions
 current_atom/1                Examine existing atoms
 current_char_conversion/2     Query input character mapping

 current_flag/1                Examine existing flags
 current_foreign_library/2     library(shlib) Examine loaded shared libraries (.so files)
 current_format_predicate/2    Enumerate user-defined format codes
 current_functor/2             Examine existing name/arity pairs
 current_input/1               Get current input stream
 current_key/1                 Examine existing database keys
 current_module/1              Examine existing modules

 current_module/2              Examine existing modules
 current_mutex/3               Examine existing mutexes
 current_op/3                  Examine current operator declarations
 current_output/1              Get the current output stream
 current_predicate/2           Examine existing predicates
 current_signal/3              Current software signal mapping
 current_stream/3              Examine open streams
 current_thread/2              Examine Prolog threads

 dde_current_connection/2      Win32:  Examine open DDE connections
 dde_current_service/2         Win32:  Examine DDE services provided
 dde_execute/2                 Win32:  Execute command on DDE server
 dde_register_service/2        Win32:  Become a DDE server
 dde_request/3                 Win32:  Make a DDE request
 dde_poke/3                    Win32:  POKE operation on DDE server
 dde_unregister_service/1      Win32:  Terminate a DDE service

 debug/0                       Test for debugging mode
 debug_control_hook/1          (hook) Extend spy/1, etc.
 debugging/0                   Show debugger status
 default_module/2              Get the default modules of a module
 delete/3                      Delete all matching members from a list
 delete_directory/1            Remove a folder from the file system
 delete_file/1                 Remove a file from the file system
 discontiguous/1               Indicate distributed definition of a predicate

 dwim_match/2                  Atoms match in ``Do What I Mean'' sense
 dwim_match/3                  Atoms match in ``Do What I Mean'' sense
 dwim_predicate/2              Find predicate in ``Do What I Mean'' sense
 dynamic/1                     Indicate predicate definition may change
 edit/1                        Edit a file
 ensure_loaded/1               Consult a file if that has not yet been done
 erase/1                       Erase a database record or clause

 exception/3                   (hook) Handle runtime exceptions
 exists_directory/1            Check existence of directory
 exists_file/1                 Check existence of file
 exit/2                        Exit from named block.  See block/3
 expand_answer/2               Expand answer of query
 expand_file_name/2            Wildcard expansion of file names
 expand_file_search_path/2     Wildcard expansion of file paths
 expand_goal/2                 Compiler:  expand goal in clause-body

 expand_query/4                Expanded entered query
 expand_term/2                 Compiler:  expand read term into clause(s)
 explain/1                     library(explain) Explain argument
 explain/2                     library(explain) 2nd argument is explanation of first
 export/1                      Export a predicate from a module
 export_list/2                 List of public predicates of a module
 fail/0                        Always false

 fail/1                        Immediately fail named block.  See block/3
 current_prolog_flag/2         Get system configuration parameters
 file_base_name/2              Get file part of path
 file_directory_name/2         Get directory part of path
 file_name_extension/3         Add, remove or test file extensions
 file_search_path/2            Define path-aliases for locating files
 fileerrors/2                  Do/Don't warn on file errors
 findall/3                     Find all solutions to a goal

 flag/3                        Simple global variable system
 flatten/2                     Transform nested list into flat list
 float/1                       Type check for a floating point number
 flush_output/0                Output pending characters on current stream
 flush_output/1                Output pending characters on specified stream
 forall/2                      Prove goal for all solutions of another goal
 format/1                      Formatted output

 format/2                      Formatted output with arguments
 format/3                      Formatted output on a stream
 format_predicate/2            Program format/[1,2]
 free_variables/2              Find unbound variables in a term
 functor/3                     Get name and arity of a term or construct a term
 garbage_collect/0             Invoke the garbage collector
 garbage_collect_atoms/0       Invoke the atom garbage collector
 gensym/2                      Generate unique atoms from a base

 get/1                         Read first non-blank character
 get/2                         Read first non-blank character from a stream
 get0/1                        Read next character
 get0/2                        Read next character from a stream
 get_byte/1                    Read next byte (ISO)
 get_byte/2                    Read next byte from a stream (ISO)
 get_char/1                    Read next character as an atom (ISO)

 get_char/2                    Read next character from a stream (ISO)
 get_code/1                    Read next character (ISO)
 get_code/2                    Read next character from a stream (ISO)
 get_single_char/1             Read next character from the terminal
 get_time/1                    Get current time
 getenv/2                      Get shell environment variable
 goal_expansion/2              Hook for macro-expanding goals
 ground/1                      Verify term holds no unbound variables

 guitracer/0                   Install hooks for the graphical debugger
 halt/0                        Exit from Prolog
 halt/1                        Exit from Prolog with status
 hash_term/2                   Hash-value of ground term
 help/0                        Give help on help
 help/1                        Give help on predicates and show parts of manual
 help_hook/1                   (hook) User-hook in the help-system

 ignore/1                      Call the argument, but always succeed
 import/1                      Import a predicate from a module
 include/1                     Include a file with declarations
 index/1                       Change clause indexing
 initialization/1              Initialization directive
 int_to_atom/2                 Convert from integer to atom
 int_to_atom/3                 Convert from integer to atom (non-decimal)
 integer/1                     Type check for integer

 interactor/0                  Start new thread with console and toplevel
 intersection/3                Set intersection
 is/2                          Evaluate arithmetic expression
 is_absolute_file_name/1       True if arg defines an absolute path
 is_list/1                     Type check for a list
 is_set/1                      Type check for a set
 keysort/2                     Sort, using a key

 last/2                        Last element of a list
 leash/1                       Change ports visited by the tracer
 length/2                      Length of a list
 library_directory/1           (hook) Directories holding Prolog libraries
 limit_stack/2                 Limit stack expansion
 line_count/2                  Line number on stream
 line_position/2               Character position in line on stream
 list_to_set/2                 Remove duplicates

 listing/0                     List program in current module
 listing/1                     List predicate
 load_files/2                  Load source files with options
 load_foreign_library/1        library(shlib) Load shared library (.so file)
 load_foreign_library/2        library(shlib) Load shared library (.so file)
 make/0                        Reconsult all changed source files
 make_directory/1              Create a folder on the file system

 make_fat_filemap/1            Win32:  Create file containing non-FAT filenames
 make_library_index/1          Create autoload file INDEX.pl
 maplist/3                     Transform all elements of a list
 member/2                      Element is member of a list
 memberchk/2                   Deterministic member/2
 merge/3                       Merge two sorted lists
 merge_set/3                   Merge two sorted sets
 message_hook/3                Intercept print_message/2

 message_to_string/2           Translate message-term to string
 meta_predicate/1              Quintus compatibility
 module/1                      Query/set current type-in module
 module/2                      Declare a module
 module_transparent/1          Indicate module based meta predicate
 msort/2                       Sort, do not remove duplicates
 multifile/1                   Indicate distributed definition of predicate

 mutex_create/1                Create a thread-synchronisation device
 mutex_destroy/1               Destroy a mutex
 mutex_lock/1                  Become owner of a mutex
 mutex_trylock/1               Become owner of a mutex (non-blocking)
 mutex_unlock/1                Release ownership of mutex
 mutex_unlock_all/0            Release ownership of all mutexes
 name/2                        Convert between atom and list of ASCII characters
 nl/0                          Generate a newline

 nl/1                          Generate a newline on a stream
 nodebug/0                     Disable debugging
 noguitracer/0                 Disable the graphical debugger
 nonvar/1                      Type check for bound term
 noprotocol/0                  Disable logging of user interaction
 nospy/1                       Remove spy point
 nospyall/0                    Remove all spy points

 not/1                         Negation by failure (argument not provable).  Same as \+/1
 notrace/0                     Stop tracing
 notrace/1                     Do not debug argument goal
 nth0/3                        N-th element of a list (0-based)
 nth1/3                        N-th element of a list (1-based)
 nth_clause/3                  N-th clause of a predicate
 number/1                      Type check for integer or float
 number_chars/2                Convert between number and one-char atoms

 number_codes/2                Convert between number and ASCII values
 numbervars/4                  Enumerate unbound variables of a term using a given base
 on_signal/3                   Handle a software signal
 once/1                        Call a goal deterministically
 op/3                          Declare an operator
 open/3                        Open a file (creating a stream)
 open/4                        Open a file (creating a stream)

 open_dde_conversation/3       Win32:  Open DDE channel
 open_null_stream/1            Open a stream to discard output
 open_resource/3               Open a program resource as a stream
 open_shared_object/2          UNIX: Open shared library (.so file)
 open_shared_object/3          UNIX: Open shared library (.so file)
 peek_byte/1                   Read byte without removing
 peek_byte/2                   Read byte without removing
 peek_char/1                   Read character without removing

 peek_char/2                   Read character without removing
 peek_code/1                   Read character-code without removing
 peek_code/2                   Read character-code without removing
 phrase/2                      Activate grammar-rule set
 phrase/3                      Activate grammar-rule set (returning rest)
 please/3                      Query/change environment parameters
 plus/3                        Logical integer addition

 portray/1                     (hook) Modify behaviour of print/1
 portray_clause/1              Pretty print a clause
 predicate_property/2          Query predicate attributes
 predsort/3                    Sort, using a predicate to determine the order
 preprocessor/2                Install a preprocessor before the compiler
 print/1                       Print a term
 print/2                       Print a term on a stream
 print_message/2               Print message from (exception) term

 print_message_lines/3         Print message to stream
 profile/3                     Obtain execution statistics
 profile_count/3               Obtain profile results on a predicate
 profiler/2                    Obtain/change status of the profiler
 prolog/0                      Run interactive toplevel
 prolog_current_frame/1        Reference to goal's environment stack
 prolog_edit:locate/2          Locate targets for edit/1

 prolog_edit:locate/3          Locate targets for edit/1
 prolog_edit:edit_source/1     Call editor for edit/1
 prolog_edit:edit_command/2    Specify editor activation
 prolog_edit:load/0            Load edit/1 extensions
 prolog_file_type/2            Define meaning of file extension
 prolog_frame_attribute/3      Obtain information on a goal environment
 prolog_list_goal/1            Hook.  Intercept tracer 'L' command
 prolog_load_context/2         Context information for directives

 prolog_skip_level/2           Indicate deepest recursion to trace
 prolog_to_os_filename/2       Convert between Prolog and OS filenames
 prolog_trace_interception/4   library(user) Intercept the Prolog tracer
 prompt1/1                     Change prompt for 1 line
 prompt/2                      Change the prompt used by read/1
 proper_list/1                 Type check for list
 protocol/1                    Make a log of the user interaction

 protocola/1                   Append log of the user interaction to file
 protocolling/1                On what file is user interaction logged
 put/1                         Write a character
 put/2                         Write a character on a stream
 put_byte/1                    Write a byte
 put_byte/2                    Write a byte on a stream
 put_char/1                    Write a character
 put_char/2                    Write a character on a stream

 put_code/1                    Write a character-code
 put_code/2                    Write a character-code on a stream
 qcompile/1                    Compile source to Quick Load File
 qsave_program/1               Create runtime application
 qsave_program/2               Create runtime application
 read/1                        Read Prolog term
 read/2                        Read Prolog term from stream

 read_clause/1                 Read clause
 read_clause/2                 Read clause from stream
 read_history/6                Read using history substitution
 read_link/3                   Read a symbolic link
 read_term/2                   Read term with options
 read_term/3                   Read term with options from stream
 recorda/2                     Record term in the database (first)
 recorda/3                     Record term in the database (first)

 recorded/2                    Obtain term from the database
 recorded/3                    Obtain term from the database
 recordz/2                     Record term in the database (last)
 recordz/3                     Record term in the database (last)
 redefine_system_predicate/1   Abolish system definition
 rename_file/2                 Change name of file
 repeat/0                      Succeed, leaving infinite backtrack points

 require/1                     This file requires these predicates
 reset_profiler/0              Clear statistics obtained by the profiler
 resource/3                    Declare a program resource
 retract/1                     Remove clause from the database
 retractall/1                  Remove unifying clauses from the database
 reverse/2                     Inverse the order of the elements in a list
 same_file/2                   Succeeds if arguments refer to same file
 see/1                         Change the current input stream

 seeing/1                      Query the current input stream
 seek/4                        Modify the current position in a stream
 seen/0                        Close the current input stream
 select/3                      Select element of a list
 set_input/1                   Set current input stream from a stream
 set_output/1                  Set current output stream from a stream
 set_prolog_flag/2             Define a system feature

 set_stream/2                  Set stream attribute
 set_stream_position/2         Seek stream to position
 set_tty/2                     Set `tty' stream
 setarg/3                      Destructive assignment on term
 setenv/2                      Set shell environment variable
 setof/3                       Find all unique solutions to a goal
 sformat/2                     Format on a string
 sformat/3                     Format on a string

 shell/0                       Execute interactive subshell
 shell/1                       Execute OS command
 shell/2                       Execute OS command
 show_profile/1                Show results of the profiler
 size_file/2                   Get size of a file in characters
 skip/1                        Skip to character in current input
 skip/2                        Skip to character on stream

 rl_add_history/1              Add line to readline(3) history
 rl_read_init_file/1           Read readline(3) init file
 sleep/1                       Suspend execution for specified time
 sort/2                        Sort elements in a list
 source_file/1                 Examine currently loaded source files
 source_file/2                 Obtain source file of predicate
 source_location/2             Location of last read term
 spy/1                         Force tracer on specified predicate

 stack_parameter/4             Some systems:  Query/Set runtime stack parameter
 statistics/0                  Show execution statistics
 statistics/2                  Obtain collected statistics
 stream_property/2             Get stream properties
 string/1                      Type check for string
 string_concat/3               atom_concat/3for strings
 string_length/2               Determine length of a string

 string_to_atom/2              Conversion between string and atom
 string_to_list/2              Conversion between string and list of ASCII
 style_check/1                 Change level of warnings
 sub_atom/5                    Take a substring from an atom
 sublist/3                     Determine elements that meet condition
 subset/2                      Check subset relation for unordered sets
 sub_string/5                  Take a substring from a string
 subtract/3                    Delete elements that do not meet condition

 succ/2                        Logical integer successor relation
 swritef/2                     Formatted write on a string
 swritef/3                     Formatted write on a string
 tab/1                         Output number of spaces
 tab/2                         Output number of spaces on a stream
 tell/1                        Change current output stream
 telling/1                     Query current output stream

 term_expansion/2              (hook) Convert term before compilation
 term_to_atom/2                Convert between term and atom
 thread_at_exit/1              Register goal to be called at exit
 thread_create/3               Create a new Prolog task
 thread_exit/1                 Terminate Prolog task with value
 thread_get_message/1          Wait for message
 thread_join/2                 Wait for Prolog task-completion
 thread_peek_message/1         Test for message in queue

 thread_self/1                 Get identifier of current thread
 thread_send_message/2         Send message to another thread
 thread_signal/2               Execute goal in another thread
 threads/0                     List running threads
 throw/1                       Raise an exception (see catch/3)
 time/1                        Determine time needed to execute goal
 time_file/2                   Get last modification time of file

 tmp_file/2                    Create a temporary filename
 told/0                        Close current output
 trace/0                       Start the tracer
 trace/1                       Set trace-point on predicate
 trace/2                       Set/Clear trace-point on ports
 tracing/0                     Query status of the tracer
 trim_stacks/0                 Release unused memory resources
 true/0                        Succeed

 tty_get_capability/3          Get terminal parameter
 tty_goto/2                    Goto position on screen
 tty_put/2                     Write control string to terminal
 tty_size/2                    Get row/column size of the terminal
 ttyflush/0                    Flush output on terminal
 union/3                       Union of two sets
 unify_with_occurs_check/2     Logically sound unification

 unix/1                        OS interaction
 unknown/2                     Trap undefined predicates
 unload_foreign_library/1      library(shlib) Detach shared library (.so file)
 unsetenv/1                    Delete shell environment variable
 use_module/1                  Import a module
 use_module/2                  Import predicates from a module
 var/1                         Type check for unbound variable
 visible/1                     Ports that are visible in the tracer

 volatile/1                    Predicates that are not saved
 wait_for_input/3              Wait for input with optional timeout
 wildcard_match/2              Csh(1) style wildcard match
 win_exec/2                    Win32:  spawn Windows task
 win_shell/2                   Win32:  open document through Shell
 win_registry_get_value/3      Win32:  get registry value
 with_mutex/2                  Run goal while holding mutex

 write/1                       Write term
 write/2                       Write term to stream
 write_ln/1                    Write term, followed by a newline
 write_canonical/1             Write a term with quotes, ignore operators
 write_canonical/2             Write a term with quotes, ignore operators on a stream
 write_term/2                  Write term with options
 write_term/3                  Write term with options to stream
 writef/1                      Formatted write

 writef/2                      Formatted write on stream
 writeq/1                      Write term, insert quotes
 writeq/2                      Write term, insert quotes on stream


9.2 Arithmetic Functions

 */2                     Multiplication
 **/2                    Power function

 +/2                     Addition
 -/1                     Unary minus
 -/2                     Subtraction
 //2                     Division
 ///2                    Integer division
 /\/2                    Bitwise and
 <</2                    Bitwise left shift
 >>/2                    Bitwise right shift

 ./2                     List of one character:  character code
 \/1                     Bitwise negation
 \//2                    Bitwise or
 ^/2                     Power function
 abs/1                   Absolute value
 acos/1                  Inverse (arc) cosine
 asin/1                  Inverse (arc) sine

 atan/1                  Inverse (arc) tangent
 atan/2                  Rectangular to polar conversion
 ceil/1                  Smallest integer larger than arg
 ceiling/1               Smallest integer larger than arg
 cos/1                   Cosine
 cputime/0               Get CPU time
 e/0                     Mathematical constant
 exp/1                   Exponent (base e)

 float/1                 Explicitly convert to float
 float_fractional_part/1 Fractional part of a float
 float_integer_part/1    Integer part of a float
 floor/1                 Largest integer below argument
 integer/1               Round to nearest integer
 log/1                   Natural logarithm
 log10/1                 10 base logarithm

 max/2                   Maximum of two numbers
 min/2                   Minimum of two numbers
 mod/2                   Remainder of division
 random/1                Generate random number
 rem/2                   Remainder of division
 round/1                 Round to nearest integer
 truncate/1              Truncate float to integer
 pi/0                    Mathematical constant

 sign/1                  Extract sign of value
 sin/1                   Sine
 sqrt/1                  Square root
 tan/1                   Tangent
 xor/2                   Bitwise exclusive or


9.3 Operators

 $                     1    fx   Bind toplevel variable
 ^                   200   xfy   Predicate
 ^                   200   xfy   Arithmetic function
 mod                 300   xfx   Arithmetic function
 *                   400   yfx   Arithmetic function
 /                   400   yfx   Arithmetic function
 //                  400   yfx   Arithmetic function
 <<                  400   yfx   Arithmetic function

 >>                  400   yfx   Arithmetic function
 xor                 400   yfx   Arithmetic function
 +                   500    fx   Arithmetic function
 -                   500    fx   Arithmetic function
 ?                   500    fx   XPCE: obtainer
 \                   500    fx   Arithmetic function
 +                   500   yfx   Arithmetic function

 -                   500   yfx   Arithmetic function
 /\                  500   yfx   Arithmetic function
 \/                  500   yfx   Arithmetic function
 :                   600   xfy   module:term separator
 <                   700   xfx   Predicate
 =                   700   xfx   Predicate
 =..                 700   xfx   Predicate
 =:=                 700   xfx   Predicate

 <                   700   xfx   Predicate
 ==                  700   xfx   Predicate
 =@=                 700   xfx   Predicate
 =\=                 700   xfx   Predicate
 >                   700   xfx   Predicate
 >=                  700   xfx   Predicate
 @<                  700   xfx   Predicate

 @=<                 700   xfx   Predicate
 @>                  700   xfx   Predicate
 @>=                 700   xfx   Predicate
 is                  700   xfx   Predicate
 \=                  700   xfx   Predicate
 \==                 700   xfx   Predicate
 =@=                 700   xfx   Predicate
 not                 900    fy   Predicate

 \+                  900    fy   Predicate
 ,                  1000   xfy   Predicate
 ->                 1050   xfy   Predicate
 *->                1050   xfy   Predicate
 ;                  1100   xfy   Predicate
 |                  1100   xfy   Predicate
 discontiguous      1150    fx   Predicate

 dynamic            1150    fx   Predicate
 module_transparent 1150    fx   Predicate
 multifile          1150    fx   Predicate
 volatile           1150    fx   Predicate
 initialization     1150    fx   Predicate
 :-                 1200    fx   Introduces a directive
 ?-                 1200    fx   Introduces a directive
 -->                1200   xfx   DCGrammar:  rewrite

 :-                 1200   xfx   head :- body.  separator


Bibliography

[Anjewierden & Wielemaker, 1989]  A.  Anjewierden  and  J.   Wielemaker.
                                  Extensible  objects.  ESPRIT   Project
                                  1098 Technical Report  UvA-C1-TR-006a,
                                  University of Amsterdam, March 1989.

[BIM, 1989]                       BIM  Prolog  release  2.4.   Everberg,
                                  Belgium, 1989.

[Bowen & Byrd, 1983]              D. L. Bowen and L. M. Byrd. A portable
                                  Prolog compiler.  In  L.  M.  Pereira,
                                  editor,  Proceedings   of  the   Login
                                  Programming  Workshop  1983,  Lisabon,
                                  Portugal, 1983.  Universidade nova  de
                                  Lisboa.

[Bratko, 1986]                    I. Bratko. Prolog Programming for  Ar-
                                  tificial Intelligence. Addison-Wesley,
                                  Reading, Massachusetts, 1986.

[Clocksin & Melish, 1987]         W.  F.  Clocksin  and  C.  S.  Melish.
                                  Programming   in  Prolog.    Springer-
                                  Verlag, New York,  Third, Revised  and
                                  Extended edition, 1987.

[Deransart et al., 1996]          P.  Deransart,    A.  Ed-Dbali,    and
                                  L. Cervoni.  Prolog:    The  Standard.
                                  Springer-Verlag, New York, 1996.

[Kernighan & Ritchie, 1978]       B. W. Kernighan and D. M. Ritchie. The
                                  C Programming Language. Prentice-Hall,
                                  Englewood Cliffs, New Jersey, 1978.

[OKeefe, 1990]                    R. A.  OKeefe.  The Craft  of  Prolog.
                                  MIT Press, Massachussetts, 1990.

[Pereira, 1986]                   F. Pereira.  C-Prolog  User's  Manual,
                                  1986.

[Qui, 1997]                       Quintus   Prolog,   User   Guide   and
                                  Reference  Manual.  Berkhamsted,   UK,
                                  1997.

[Sterling & Shapiro, 1986]        L.  Sterling  and   E.  Shapiro.   The
                                  Art of Prolog.  MIT Press,  Cambridge,
                                  Massachusetts, 1986.

[Warren, 1983]                    D.   H.  D.   Warren.    The   runtime
                                  environment  for  a  prolog   compiler
                                  using  a  copy  algorithm.   Technical
                                  Report 83/052,  SUNY and Stone  Brook,
                                  New York, 1983.  Major revision  March
                                  1984.

                                  860


Index

'MANUAL' library, 35                   PL_new_term_ref(), 639
-lpl library, 814                      PL_new_term_refs(), 640
-lreadline library, 838                PL_next_solution(), 753
=:=/2, 354                             PL_on_halt(), 803
/\/2, 383                              PL_open_foreign_frame(), 759
=\=/2, 353                             PL_open_query(), 752
|/2, 140                               PL_pred(), 748
,/2, 138                               PL_predicate(), 749
!/0, 137                               PL_predicate_info(), 750
!/1, 170                               PL_put_atom(), 714
//2, 361                               PL_put_atom_chars(), 715
./2, 369                               PL_put_atom_nchars(), 696
=/2, 123                               PL_put_float(), 721
==/2, 121                              PL_put_functor(), 722
>=/2, 352                              PL_put_integer(), 719
>/2, 349                               PL_put_list(), 723
^/2, 398                               PL_put_list_chars(), 718
///2, 364                              PL_put_list_nchars(), 699
->/2, 141                              PL_put_list_ncodes(), 698
=</2, 351                              PL_put_nil(), 724
<</2, 381                              PL_put_pointer(), 720
</2, 350                               PL_put_string_chars(), 716
-/1, 357                               PL_put_string_nchars(), 697, 717
-/2, 359                               PL_put_term(), 725
\=/2, 125                              PL_put_variable(), 713
\/1, 385                               PL_query(), 794
\==/2, 122                             PL_quote(), 745
\+/1, 143                              PL_raise_exception(), 769
\//2, 382                              PL_record(), 781
+/2, 358                               PL_record_external(), 784
**/2, 397                              PL_recorded(), 782
>>/2, 380                              PL_recorded_external(), 785
;/2, 139                               PL_register_atom(), 662
*->/2, 142                             PL_register_extensions(), 798
=@=/2, 126                             PL_register_foreign(), 796
\=@=/2, 127                            PL_reset_term_refs(), 642
@>=/2, 131                             PL_retry(), 650
@>/2, 130                              PL_retry_address(), 651
*/2, 360                               PL_rewind_foreign_frame(), 762
@=</2, 129                             PL_same_compound(), 779
@</2, 128                              PL_signal(), 788
=../2, 309                             PL_strip_module(), 765
_PL_get_arg(), 691                     PL_succeed(), 647
call/1                                 PL_term_type(), 666
   cut in, 137                         PL_thread_attach_engine(), 774

abolish/1, 17, 175, 176                PL_thread_destroy_engine(),P775L_thread_self(), 773
abolish/2, 176, 634                    PL_throw(), 770
abolish/[1                             PL_toplevel(), 811
   2], 53                              PL_unify(), 730
abort/0,  33, 44,  53, 224,  519,      PL_unify_arg(), 742
      529, 792, 801                    PL_unify_atom(), 731
abs/1, 365                             PL_unify_atom_chars(), 732
absolute_file_name/2, 490              PL_unify_atom_nchars(), 700
absolute_file_name/3, 491              PL_unify_float(), 737
absolute_file_name/2, 24,  72, 83,     PL_unify_functor(), 739
      492, 832                         PL_unify_integer(), 736
absolute_file_name/3, 53,  55, 71,     PL_unify_list(), 740
      80, 490, 491, 632                PL_unify_list_chars(), 733
absolute_file_name/[2                  PL_unify_list_nchars(), 703
   3], 79                              PL_unify_list_ncodes(), 702
access_file/2, 480                     PL_unify_nil(), 741
access_file/2, 53, 491                 PL_unify_pointer(), 738
acos/1, 391                            PL_unify_string_chars(), 734
address/2, 513                         PL_unify_string_nchars(), 701, 735
Alpha                                  PL_unify_term(), 743
   DEC, 16                             PL_unregister_atom(), 663
append/1, 215, 217, 218                PL_warning(), 790
append/3, 324, 408                     plus/3, 146, 348
apply/2, 147                           portray/1, 44, 53, 55, 284,  285,
apropos/1, 37, 38, 55, 851, 857              293, 295, 752, 805, 849
arg/3, 307                             portray_clause/1, 106
arithmethic_function/1, 768            portray_clause/1, 104
arithmetic_function/1, 403             predicate_property/2, 208
arithmetic_function/1, 402             predicate_property/2, 616
asin/1, 390                            predsort/3, 432
assert/1, 71,  78, 174, 180--183,      preprocessor/2, 94
      194, 198, 604, 619, 855          print/1, 53, 285, 293, 295,  447,
assert/2, 183--185, 192, 211                 452, 752, 857
asserta/1, 33, 78, 181                 print/2, 294
asserta/2, 184, 192                    print/[1
assertz/1, 182, 855                       2], 284
assertz/2, 185, 192                    print_message/2, 158, 857
at_end_of_stream/0, 280                print_message_lines/3, 159
at_end_of_stream/1, 281                print_message/2,   20,   53,   55,
at_halt/1, 91                                157--160, 300
at_initialization/1, 90                print_message_lines/3,   20,   158,
at_end_of_stream/[0                          160
   1], 229                             profile file, 31
at_halt/1, 90, 507, 530, 803, 812      profile/3, 564, 838
at_initialization/1, 92, 808           profile_count/3, 568
atan/1, 392                            profiler/2, 564, 566
atan/2, 393                            prolog/0, 33, 53, 154, 528,  532,
atom/1, 113, 664                             533, 618, 811
atom_chars/2, 21, 315                  prolog:debug_control_hook/1,    55,
atom_codes/2, 314                            850
atom_concat/3, 324, 857                prolog:help_hook/1, 55, 851
atom_length/2, 327                     prolog_current_frame/1, 841
atom_prefix/2, 328                     prolog_edit:edit_command/2,     55,
atom_to_term/3, 323                          102
atom_chars/2,  53,  70, 265,  313,     prolog_edit:edit_source/1, 55, 101
      317, 332                         prolog_edit:load/0, 103
atom_codes/2,  21,  53,  70,  313,     prolog_edit:locate/2, 100
      315, 332                         prolog_edit:locate/3, 55, 99
atom_concat/3, 325, 337                prolog_file_type/2, 81
atom_length/2, 53, 336                 prolog_frame_attribute/3, 842
atom_to_term/3, 284                    prolog_list_goal/1, 55, 849
atomic/1, 115                          prolog_load_context/2, 84
attach_console/0, 525                  prolog_skip_level/2, 845
attach_console/0, 524                  prolog_to_os_filename/2, 495
autoload/0, 76, 824, 826               prolog_trace_interception/4,    55,

backcomp library, 17, 21               prolog844_current_frame/1, 842
bagof/3, 434--436, 857                 prolog_edit:edit_command/2, 101
between/3, 346                         prolog_edit:edit_source/1, 98
block/3, 166, 167, 857                 prolog_edit:locate/3,   97,    98,
break/0, 33, 44, 528, 792                    100, 101

call/1,  21, 88,  117, 137,  145,      prolog_file_type/2,p71,r81olog_frame_attribute/3, 213
      147, 153, 548, 746, 861          prolog_load_context/2, 85
call/2, 146                            prolog_to_os_filename/2, 492
call/[2-6], 146                        prolog_trace_interception/4,   544,
call_shared_object_function/2, 630           842
call_with_depth_limit/3, 151           prompt/2, 302--304
call_with_depth_limit/3, 151           prompt1/1, 304
callable/1, 117                        proper_list/1, 407
catch/3,  17, 19,  152--155, 166,      proper_list/1, 421
      300, 504, 846, 857               protocol/1, 536, 537, 539
ceil/1, 379                            protocola/1, 537, 539
ceiling/1, 378, 379                    protocolling/1, 539
char_code/2, 316                       put/1, 248, 250, 254
char_conversion/2, 343                 put/2, 249, 251, 253, 255
char_type/2, 331                       put_byte/1, 250
char_code/2, 70                        put_byte/2, 251
char_conversion/2, 53, 344             put_char/1, 252
char_type/2, 332                       put_code/1, 254
character_count/2, 241                 put_code/2, 255
chdir/1, 475, 500                      put_byte/[1
check/0, 76                               2], 70
check_old_select/0, 22                 put_char/1, 252
checklist/2, 438                       put_char/[1
checkselect library, 22                   2], 70
clause/2, 210, 211                     put_code/[1
clause/3, 183, 192, 211, 213, 828         2], 70
clause/[2
   3], 53, 838                         qcompile/1, 95, 96
clause_property/2, 213                 qsave_program/1, 825
clause_property/2, 83, 842             qsave_program/2, 824
clib                                   qsave_program/2, 14, 51,  53, 823,
   package, 812                              824, 830
close/1, 227                           qsave_program/[1
close/2, 228                              2], 13,  18, 33,  50, 53,  92,
close_dde_conversation/1, 578                625, 808, 816, 826, 828
close_shared_object/1, 629             quiet, 33, 158
code_type/2, 332                       quintus library, 21, 611, 620
code_type/2, 330
commandline                            random/1, 370
   arguments, 33                       read/1, 53,  66, 224,  258,  291,
compare/3, 132, 432, 778                     296,  298, 300,  303,  558,
compiling/0, 93                              857
compound/1, 116                        read/2, 297
concat_atom/2, 325                     read_clause/1, 298
concat_atom/3, 326                     read_clause/2, 299
concat_atom/2, 326                     read_history/6, 302
consult/1,  28, 31,  46, 71,  73,      read_link/3, 496
      74, 95, 96, 199, 298, 558        read_term/2, 300
context_module/1, 614                  read_term/3, 301
context_module/1, 614, 752             read_clause/1, 299, 558
convert_time/2, 478                    read_history/6, 302
convert_time/8, 477                    read_term/2,  42,  53,  298,  301,
convert_time/2, 477                          302, 323
convert_time/8, 478                    read_term/3, 300, 343, 533
convert_time/[2                        read_term/[2
   8], 489                                3], 300
copy_stream_data/2, 283                reconsult/1, 71
copy_stream_data/3, 282                recorda/2, 187
copy_term/2, 312                       recorda/3, 186,  188,  192,  206,
copy_term/2, 312                             780, 781, 784
cos/1, 388                             recorded/2, 191
cputime/0, 401                         recorded/3, 190, 192, 828
ctype library, 330                     recordz/2, 189
current_arithmetic_function/1, 404     recordz/3, 188
current_atom/1, 203                    redefine_system_predicate/1, 177
current_char_conversion/2, 344         redefine_system_predicate/1,    11,
current_flag/1, 205                          855
current_foreign_library/2, 635         registry, 64
current_format_predicate/2, 458        rem/2, 363
current_functor/2, 204                 rename_file/2, 487
current_input/1, 237                   repeat/0, 136, 151
current_key/1, 206                     require/1, 76, 826
current_module/1, 612                  reset_profiler/0, 567
current_module/2, 613                  reset_profiler/0, 564
current_mutex/3, 520                   resource/3,  18,  55,  823,  824,
current_op/3, 341                            830, 832, 833
current_output/1, 238                  retract/1, 71, 78, 174, 175, 178,
current_predicate/2, 207                     194, 198, 619
current_prolog_flag/2, 53              retractall/1, 174, 175, 179
current_signal/3, 164                  reverse/2, 416, 596
current_stream/3, 21, 230              rl_add_history/1, 854
current_thread/2, 504                  rl_read_init_file/1, 853
current_atom/1, 203                    round/1, 371, 372
current_char_conversion/2, 343
current_input/1, 84                    same_file/2, 484
current_predicate/2, 568               see/1, 20, 215, 216, 224
current_prolog_flag/2, 21, 33, 52,     seeing/1, 215, 219
      53,  56, 61, 70,  120, 155,      seek/4, 229, 231, 232
      158,  284,  289, 300,  532,      seen/0, 221
      558,  583,  624, 636,  745,      select/3, 22, 412
      806, 814, 855                    set_feature/2, 21
current_signal/3, 163                  set_input/1, 235
current_stream/3, 230                  set_output/1, 236
current_thread/2, 504, 505, 507        set_prolog_flag/2,s54et_stream/2, 233
DCG, 71, 171                           set_stream_position/2, 231
dde_current_connection/2, 586          set_tty/2, 463
dde_current_service/2, 585             set_feature/2, 21
dde_execute/2, 580                     set_input/1, 233
dde_poke/4, 581                        set_prolog_flag/2, 21, 41, 52, 345
dde_register_service/2, 583            set_stream/2, 224
dde_request/3, 579                     setarg/3, 308
dde_unregister_service/1, 584          setenv/2, 101, 473
debug/0, 44, 155,  547, 549, 550,      setof/3, 436, 857
      792                              sformat/2, 455
debugging                              sformat/3, 284, 454
   exceptions, 155                     shell/0, 468, 475
debugging/0, 55, 547, 551, 850         shell/1, 102, 467, 475
DEC                                    shell/2, 466
   Alpha, 16                           shell/[0-2], 473
default_module/2, 617                  shell/[1
delete/3, 411                             2], 466
delete_directory/1, 499                shlib library, 857
delete_file/1, 486                     show_profile/1, 565
discontiguous/1, 197, 200              show_profile/1, 564, 565
display/1, 447, 711                    sign/1, 366
display/[1                             silent, 158
   2], 17                              sin/1, 356, 387
displayq/1, 452                        size_file/2, 488
displayq/[1                            skip/1, 277, 278
   2], 17                              skip/2, 278
dup/2, 21                              sleep/1, 592
dup_stream/2, 21                       sort/2, 422, 429, 430, 432, 436
dup_stream/2, 21                       source_file/1, 82
dwim_match/2, 588                      source_file/2, 83
dwim_match/3, 589                      source_location/2, 85
dwim_predicate/2, 209                  source_file/2, 96, 208
dwim_match/2, 209, 589                 source_file/[1
dynamic/1,  53,  174,  197,  198,         2], 613
      208, 610                         source_location/2, 84

e/0, 400                               spy/1,s44,q53,r55,t552,/850,1857, 386
edit/1, 23, 55,  77, 97--99, 101,      stack
      857                                 memory management, 65
edit_source/1, 102                     stack_parameter/4, 574
Emacs, 34                              startup file, 31
emacs/swi_prolog library, 23           statistics/0, 561
ensure_loaded/1, 74                    statistics/2, 401, 560
ensure_loaded/1, 46, 71, 74, 598       stream_property/2, 229
erase/1, 183, 186, 192, 211            stream_position/3, 224, 232, 300
exception/3, 55, 846, 847              stream_property/2, 231
exceptions                             string/1, 114, 452
   debugging, 155                      string_concat/3, 337
exists_directory/1, 485                string_length/2, 336
exists_file/1, 481                     string_to_atom/2, 334
exists_file/1, 53                      string_to_list/2, 335
exit/2, 167, 168                       string_concat/3, 324
exp/1, 356, 396                        style_check/1, 558
expand_answer/2, 534                   style_check/1, 66, 67, 200, 855
expand_file_name/2, 494                sub_atom/5, 329
expand_file_search_path/2, 80          sub_string/5, 338
expand_goal/2, 89                      sub_atom/5, 338
expand_query/4, 533                    sublist/3, 440
expand_term/2, 87                      subset/2, 426
expand_answer/2, 533                   subtract/3, 424
expand_file_name/2, 53, 473, 490       succ/2, 347
expand_goal/2, 53, 86, 88              swi_edit library, 103
expand_term/2, 86, 88, 93, 171         swi_help library, 35
explain library, 857                   swritef/2, 449
explain/1, 39                          swritef/3, 448
explain/2, 40
export/1, 607, 615                     tab/1, 256
export_list/2, 616                     tab/2, 257

fail/0, 134                            tan/1,t389ell/1, 20, 215, 217, 218, 224
fail/1, 169                            telling/1, 215, 220
feature/2, 21                          term_expansion/2, 55, 86
file_base_name/2, 483                  term_to_atom/2, 322
file_directory_name/2, 482             term_expansion/2, 55, 71,  86--88,
file_name_extension/3, 493                   96, 533
file_search_path/2, 55, 79             term_position/3, 229
file_base_name/2, 482                  term_to_atom/2, 284, 744
file_search_path/2,  31,  33,  73,     thread_at_exit/1, 507
      79,   81,  632,  816,  829,      thread_create/3, 502
      832, 835                         thread_exit/1, 506
fileerrors/2, 53, 244                  thread_get_message/1, 510
findall/3, 434, 619                    thread_join/2, 505
flag/3, 53, 193, 205                   thread_peek_message/1, 511
flatten/2, 417                         thread_self/1, 503
float/1, 111, 355, 373                 thread_send_message/2, 509
float_fractional_part/1, 374           thread_signal/2, 512
float_integer_part/1, 375              thread_exit/1, 504, 505
float_integer_part/1, 376              thread_join/2, 502, 504, 505
floor/1, 377                           thread_peek_message/1, 510
flush_output/0, 258                    thread_self/1, 509
flush_output/1, 259                    thread_signal/2, 512, 519
flush_output/0, 258                    threads/0, 523
flush_output/1, 159                    throw/1, 17,  44, 152--154,  166,
flush_output/[0                              504,  512, 529,  769,  771,
   1], 224, 260                              846
foo/0, 855                             time/1, 401, 562, 564
foo/3, 855                             time_file/2, 489
forall/2, 88, 442                      time_file/2, 477
format/1, 159, 451                     tmp_file/2, 497
format/2, 452--454                     told/0, 222
format/3, 159, 453                     trace/0, 44, 512, 541, 547, 792
format/[1                              trace/1, 53, 546
   2], 53, 284, 443, 857               trace/2, 547
format/[2                              tracing/0, 542
   3], 61                              trim_stacks/0, 573
format_predicate/2, 457                trim_stacks/0, 53, 569, 570
free_variables/2, 311                  true/0, 53, 135, 151
free_variables/2, 300                  truncate/1, 376
functor/3, 6, 116, 117, 306            tty_get_capability/3, 460

garbage_collect/0, 570                 tty_goto/2,t461ty_put/2, 462
garbage_collect_atoms/0, 571           tty_size/2, 464
garbage_collect_atoms/0, 804           tty_get_capability/2, 464
gensym/2, 591                          tty_get_capability/3, 462
get/1, 269                             tty_goto/2, 463
get/2, 270                             tty_put/2, 463
get0/1, 224, 267, 277, 279             tty_size/2, 464
get0/2, 268                            ttyflush/0, 260, 447
get_byte/1, 261
get_byte/2, 262                        unify_with_occurs_check/2, 124
get_char/1, 265                        union/3, 425
get_char/2, 266                        unix, 53
get_code/1, 263                        unix/1, 21, 475
get_code/2, 264                        unknown/2, 56, 198, 557, 607
get_single_char/1, 279                 unload_foreign_library/1, 634
get_time/1, 476                        unsetenv/1, 474
get_byte/1, 267, 271                   update view, 194
get_byte/2, 266, 268, 272              URL, 470
get_byte/[1                            use_module/1, 598
   2], 70                              use_module/2, 599
get_char/1, 263, 275                   use_module/2, 56
get_char/2, 266, 276                   use_module/[1
get_char/[1                               2], 46, 71, 72, 74,  597, 600,
   2], 70                                    602, 615, 855
get_code/1, 273                        user library, 857
get_code/2, 266, 274                   user profile file, 31
get_code/[1                            utf-8, 70
   2], 70
get_single_char/1, 33, 53              var/1, 12, 108, 664
get_time/1, 477, 478, 489              verbose, 33
getenv/2, 472, 473                     visible/1, 556
GNU-Emacs, 34                          volatile/1, 827
go/0, 32
goal_expansion/2, 55, 88               wait_for_input/3, 240
goal_expansion/2, 86, 88, 89           wait_for_input/3, 240
Graphics, 8                            wildcard_match/2, 590
ground/1, 118, 196                     win_exec/2, 469
GUI, 8                                 win_registry_get_value/3, 471
guitracer/0, 23, 540, 544              win_shell/2,w470in_exec/2, 466

halt/0, 44, 530, 531                   win_shell/2, 466
halt/1, 531, 792, 857                  Window interface, 8
halt/[0                                windows, 53
   1], 91                              with_mutex/2, 521
hash_term/2, 196                       with_mutex/2, 516, 772
hash_term/2, 195                       write/1, 53, 289, 293, 322,  327,
help/0, 36, 55, 830, 851                     447, 452, 681, 711
help/1, 35--37, 55, 851                write/2, 290
helpidx library, 35                    write_canonical/1, 287
hooks, 55                              write_canonical/2, 288

ignore/1, 150                          write_ln/1,w445rite_term/2, 285
immediate                              write_term/3, 286
   update view, 194                    write_canonical/[1
import/1, 597, 600, 607                   2], 17
include/1, 71, 75                      write_term/2, 44,  53, 124,  285--
index/1, 195, 201, 208                       287, 447, 452
initialization/1,  92, 625,  798,      write_term/3, 55
      828                              write_term/[2
install/0, 829                            3], 17
int_to_atom/2, 321                     writef/1, 446
int_to_atom/3, 320                     writef/2, 24, 61, 284, 447, 448
integer/1, 110, 372                    writef/[1
interactor/0, 233, 524                    2], 443
intersection/3, 423                    writeq/1, 291, 447, 452
is/2, 53, 355, 373, 403                writeq/2, 292
is_absolute_file_name/1, 492
is_list/1, 406                         X11, 8
is_set/1, 421                          xor/2, 384
is_list/1, 407                         XPCE, 8

keysort/2, 431, 432
last/2, 415
leash/1, 44, 555, 556
length/2, 418
library_directory/1, 55, 78
library_directory/1, 56, 73
likes/2, 28
limit_stack/2, 572
limit_stack/2, 569
line_count/2, 242
line_position/2, 243
line_count/2, 461
line_position/2, 461
list_to_set/2, 422
listing/0, 105
listing/1, 44, 104, 105
load_files/2, 72
load_foreign_library/1, 633
load_foreign_library/2, 632
load_files/1, 55
load_files/2, 72
load_foreign_library/1, 829
load_foreign_library/2, 633
load_foreign_library/[1
   2], 79, 627
log/1, 394
log10/1, 395
logical
   update view, 194

main/0, 51
make/0, 7, 56, 77
make_directory/1, 498
make_library_index/1, 57
make_library_index/1, 56
maplist/3, 439, 605, 826
max/2, 367
member/2, 44, 212, 409, 410, 412,
      857
memberchk/2, 410
memory
   layout, 65
merge/3, 419
merge_set/3, 427
message_hook/3, 55, 160
message_to_string/2, 161
message_hook/3, 20, 157--159
message_to_string/2, 158, 160
meta_predicate/1, 611, 620
meta_predicate/1, 619, 620
min/2, 368
mod/2, 362
module/1, 618
module/2, 86, 596, 609, 615
module_transparent/1, 610
module_transparent/1,   208,  619,
      752, 855
msort/2, 430, 431
multi_file/1, 855
multifile/1,  97, 197,  199, 208,
      849
mutex_create/1, 514
mutex_destroy/1, 515
mutex_lock/1, 516
mutex_trylock/1, 517
mutex_unlock/1, 518
mutex_unlock_all/0, 519
mutex_create/1, 516
mutex_create/2, 521
mutex_lock/1, 517

name/2, 313, 319
netmask/4, 806
nl/0, 246
nl/1, 247
nl/[0
   1], 447
nodebug/0, 550
noguitracer/0, 545
nonvar/1, 109
noprotocol/0, 538
nospy/1, 44, 55, 553, 850
nospyall/0, 55, 554, 850
not/1, 88, 148, 857
notrace/0, 543
notrace/1, 548
nth0/3, 413
nth1/3, 414
nth_clause/3, 212
nth_clause/3, 213, 842
number/1, 112
number_chars/2, 21, 317
number_codes/2, 318
number_chars/2, 21, 70, 318
number_codes/2, 21, 70, 313
numbervars/4, 310

on_signal/3, 163
on_signal/3, 19, 163, 164
once/1, 149, 150,  521, 548, 562,
      757
online_help library, 857
op/3, 21, 69, 197, 285, 340, 341
open/3, 53, 223, 225
open/4,  13, 70,  224, 225,  229,
      281
open_dde_conversation/3, 577
open_null_stream/1, 226
open_resource/3, 833
open_shared_object/2, 627
open_shared_object/3, 628
open_resource/3,   18,  823,  830,
      833
open_shared_object/2, 53, 624, 628
operator
   and modules, 339

package
   clib, 812
peek_byte/1, 271
peek_byte/2, 272
peek_char/1, 275
peek_char/2, 276
peek_code/1, 273
peek_code/2, 274
peek_byte/[1
   2], 70
peek_char/[1
   2], 70
peek_code/[1
   2], 70
phrase/2, 171, 172
phrase/3, 171, 173
pi/0, 399
PL_abort_hook(), 801
PL_abort_unhook(), 802
PL_action(), 792
PL_agc_hook(), 804
PL_atom_chars(), 657
PL_atom_nchars(), 705
PL_call(), 757
PL_call_predicate(), 756
PL_chars_to_term(), 744
PL_cleanup(), 812
PL_close_foreign_frame(), 760
PL_close_query(), 755
PL_compare(), 778
PL_cons_functor(), 726
PL_cons_functor_v(), 727
PL_cons_list(), 728
PL_context(), 764
PL_copy_term_ref(), 641
PL_cut_query(), 754
PL_discard_foreign_frame(), 761
PL_dispatch_hook(), 800
PL_erase(), 783
PL_erase_external(), 786
PL_exception(), 771
PL_fail(), 648
PL_foreign_context(), 653
PL_foreign_context_address(), 654
PL_foreign_control(), 652
PL_functor_arity(), 660
PL_functor_name(), 659
PL_get_arg(), 690
PL_get_atom(), 678
PL_get_atom_chars(), 679
PL_get_atom_nchars(), 693
PL_get_chars(), 681
PL_get_float(), 686
PL_get_functor(), 687
PL_get_head(), 708
PL_get_integer(), 683
PL_get_list(), 707
PL_get_list_chars(), 682
PL_get_list_nchars(), 694
PL_get_long(), 684
PL_get_module(), 689
PL_get_name_arity(), 688
PL_get_nchars(), 695
PL_get_nil(), 710
PL_get_pointer(), 685
PL_get_string_chars(), 680
PL_get_tail(), 709
PL_halt(), 813
PL_initialise(), 808
PL_install_readline(), 810
PL_is_atom(), 668
PL_is_atomic(), 675
PL_is_compound(), 672
PL_is_float(), 671
PL_is_functor(), 673
PL_is_initialised(), 809
PL_is_integer(), 670
PL_is_list(), 674
PL_is_number(), 676
PL_is_string(), 669
PL_is_variable(), 667
PL_load_extensions(), 797
PL_module_name(), 766
PL_new_atom(), 656
PL_new_atom_nchars(), 704
PL_new_functor(), 658
PL_new_module(), 767

                                  861
