yap-6.3/docs/yap.md

YAP 6-3.4 Manual

This file documents the YAP Prolog System version 6.3.4, a high-performance Prolog compiler developed at LIACC, Universidade do Porto. YAP is based on David H. D. Warren's WAM (Warren Abstract Machine), with several optimizations for better performance. YAP follows the Edinburgh tradition, and is largely compatible with DEC-10 Prolog, Quintus Prolog, and especially with C-Prolog.

• @ref download

• @ref install

• @ref run

• @ref consult

• @ref builtins

• @ref extensions

• @ref library

• @ref packages

• @ref swi

• @ref YAPProgramming

• @ref fli

\author Vitor Santos Costa, \author Luís Damas, \author Rogério Reis \author Rúben Azevedo

© 1989-2014 L. Damas, V. Santos Costa and Universidade do Porto. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.


This file contains extracts of the SWI-Prolog manual, as written by Jan Wielemaker. Our thanks to the author for his kind permission in allowing us to include his text in this document.

Introduction

This document provides User information on version 6.3.4 of YAP (Yet Another Prolog). The YAP Prolog System is a high-performance Prolog compiler developed at LIACC, Universidade do Porto. YAP provides several important features:

• Speed: YAP is widely considered one of the fastest available Prolog systems.

• Functionality: it supports stream Input/Output, sockets, modules, exceptions, Prolog debugger, C-interface, dynamic code, internal database, DCGs, saved states, co-routining, arrays, threads.

• We explicitly allow both commercial and non-commercial use of YAP.

YAP is based on the David H. D. Warren's WAM (Warren Abstract Machine), with several optimizations for better performance. YAP follows the Edinburgh tradition, and was originally designed to be largely compatible with DEC-10 Prolog, Quintus Prolog, and especially with C-Prolog.

YAP implements most of the ISO-Prolog standard. We are striving at full compatibility, and the manual describes what is still missing. The manual also includes a (largely incomplete) comparison with SICStus Prolog.

The document is intended neither as an introduction to Prolog nor to the implementation aspects of the compiler. A good introduction to programming in Prolog is the book @cite TheArtOfProlog , by L. Sterling and E. Shapiro, published by "The MIT Press, Cambridge MA". Other references should include the classical @cite ProgrammingInProlog , by W.F. Clocksin and C.S. Mellish, published by Springer-Verlag.

YAP 4.3 is known to build with many versions of gcc (<= gcc-2.7.2, >= gcc-2.8.1, >= egcs-1.0.1, gcc-2.95.*) and on a variety of Unixen: SunOS 4.1, Solaris 2.*, Irix 5.2, HP-UX 10, Dec Alpha Unix, Linux 1.2 and Linux 2.* (RedHat 4.0 thru 5.2, Debian 2.*) in both the x86 and alpha platforms. It has been built on Windows NT 4.0 using Cygwin from Cygnus Solutions (see README.nt) and using Visual C++ 6.0.

The overall copyright and permission notice for YAP4.3 can be found in the Artistic file in this directory. YAP follows the Perl Artistic license, and it is thus non-copylefted freeware.

If you have a question about this software, desire to add code, found a bug, want to request a feature, or wonder how to get further assistance, please send e-mail to . To subscribe to the mailing list, visit the page https://lists.sourceforge.net/lists/listinfo/yap-users.

On-line documentation is available for YAP at:

http://www.ncc.up.pt/~vsc/YAP/

Recent versions of YAP, including both source and selected binaries, can be found from this same URL.

This manual was written by Vítor Santos Costa, Luís Damas, Rogério Reis, and Rúben Azevedo. The manual is largely based on the DECsystem-10 Prolog User's Manual by D.L. Bowen, L. Byrd, F. C. N. Pereira, L. M. Pereira, and D. H. D. Warren. We have used comments from the Edinburgh Prolog library written by R. O'Keefe. Documentation from many built-ins is originally from the SWI-Prolog manual, with the gracious uathorization from Jan Wielemaker. We would also like to gratefully acknowledge the contributions from Ashwin Srinivasian.

We are happy to include in YAP several excellent packages developed under separate licenses. Our thanks to the authors for their kind authorization to include these packages.

The packages are, in alphabetical order:

• The CHR package developed by Tom Schrijvers, Christian Holzbaur, and Jan Wielemaker.

• The CLP(BN) package and Horus toolkit developed by Tiago Gomes, and Vítor Santos Costa.

• The CLP(R) package developed by Leslie De Koninck, Bart Demoen, Tom Schrijvers, and Jan Wielemaker, based on the CLP(Q,R) implementation by Christian Holzbaur.

• The CPLint package developed by Fabrizio Riguzzi's research laboratory at the University of Ferrara. Please see

http://www.ing.unife.it/Docenti/FabrizioRiguzzi/

• The CUDA interface package developed by Carlos Martínez, Jorge Buenabad, Inês Dutra and Vítor Santos Costa.

• The GECODE interface package developed by Denys Duchier and Vítor Santos Costa.

• The JPL (Java-Prolog Library) package developed by .

• The Logtalk Object-Oriented system is developed at the University of Beira Interior, Portugal, by Paulo Moura:

  http://logtalk.org/

  Logtalk is no longer distributed with YAP. Please use the Logtalk standalone installer for a smooth integration with YAP.

• The minisat SAT solver interface developed by Michael Codish, Vitaly Lagoon, and Peter J. Stuckey.

• The MYDDAS relational data-base interface developed at the Universidade do Porto by Tiago Soares, Michel Ferreira, and Ricardo Rocha.

• The PRISM logic-based programming system for statistical modeling developed at the Sato Research Laboratory, TITECH, Japan.

• The ProbLog 1 system developed by the ProbLog team in the DTAI group of KULeuven. For general information on ProbLog 1 and 2, please see

  http://dtai.cs.kuleuven.be/problog

• The real R interface package developed by Nicos Angelopoulos, Vítor Santos Costa, João Azevedo, Jan Wielemaker, and Rui Camacho.

• YAP includes the yap2swi library that ports to YAP code from of SWI's PL interface. This includes the Input/Output Layer, the SWI Foreign Language Interface, and the RDF, archive, clib, http, odbc, plunit, semweb, sgml, and zlib packages written by Jan Wielemaker.

Downloading YAP

The latest development version of Yap-6 is yap-6.3.4 and can be obtained from the repositories

http://sourceforge.net/p/yap/yap-6.3

and

https://github.com/vscosta/yap-6.3

Several packages are shared with SWI-Prolog and need to be obtained from separate repositories. Proceed as follows:

cd yap-6.3
git submodule init
git submodule update

Most of these repositories are basically copies of the original repositories at the SWI-Prolog site. YAP-6 will work either with or without these packages.

Installing YAP

YAP is a configure based system. We discuss how to use configure to install YAP, and what are the major options.

Compiling YAP

To compile YAP it should be sufficient to:

1 autoconf. Recent versions of YAP try to follow GNU conventions on where to place software.

• The main executable is placed at $BINDIR. This executa§ble is actually a script that calls the Prolog engine, stored at $LIBDIR.

• $LIBDIR is the directory where libraries are stored. YAPLIBDIR is a subdirectory that contains the Prolog engine and a Prolog library.

• $INCLUDEDIR is used if you want to use YAP as a library.

• $INFODIR is where to store info files. Usually /usr/local/info, /usr/info, or /usr/share/info.

2 make.

3 If the compilation succeeds, try ./yap.

4 If you feel satisfied with the result, do make install.

5 In most systems you will need to be superuser in order to do make install and make info on the standard directories.

Tuning the Functionality of YAP

Compiling YAP with the standard options give you a plain vanilla Prolog. You can tune YAP to include extra functionality by calling configure with the appropriate options:

• --enable-rational-trees=yes gives you support for infinite rational trees.

• --enable-coroutining=yes gives you support for coroutining, including freezing of goals, attributed variables, and constraints. This will also enable support for infinite rational trees.

• --enable-depth-limit=yes allows depth limited evaluation, say for implementing iterative deepening.

• --enable-low-level-tracer=yes allows support for tracing all calls, retries, and backtracks in the system. This can help in debugging your application, but results in performance loss.

• --enable-wam-profile=yes allows profiling of abstract machine instructions. This is useful when developing YAP, should not be so useful for normal users.

• --enable-condor=yes allows using the Condor system that support High Throughput Computing (HTC) on large collections of distributively owned computing resources.

• --enable-tabling=yes allows tabling support. This option is still experimental.

• --enable-parallelism={env-copy,sba,a-cow} allows or-parallelism supported by one of these three forms. This option is still highly experimental.

• --with-max-workers allows definition of the maximum number of parallel processes (its value can be consulted at runtime using the flag max_workers).

• --with-gmp[=DIR] give a path to where one can find the GMP library if not installed in the default path.

• --enable-threads allows using of the multi-threading predicates provided by YAP. Depending on the operating system, the option --enable-pthread-locking may also need to be used.

• --with-max-threads allows definition of the maximum number of threads (the default value is 1024; its value can be consulted at runtime using the flag [max_threads](@ref max_threads)).

Next section discusses machine dependent details.

Tuning YAP for a Particular Machine and Compiler

The default options should give you best performance under GCC. Although the system is tuned for this compiler we have been able to compile versions of YAP under lcc in Linux, Sun's cc compiler, IBM's xlc, SGI's cc, and Microsoft's Visual C++ 6.0.

Tuning YAP for GCC.

YAP has been developed to take advantage of GCC (but not to depend on it). The major advantage of GCC is threaded code and explicit register reservation.

YAP is set by default to compile with the best compilation flags we know. Even so, a few specific options reduce portability. The option

• --enable-max-performance=yes will try to support the best available flags for a specific architectural model. Currently, the option assumes a recent version of GCC.
• --enable-debug-yap compiles YAP so that it can be debugged by tools such as dbx or gdb.

Here follow a few hints:

On x86 machines the flags:

YAP_EXTRAS= ... -DBP_FREE=1

tells us to use the %bp register (frame-pointer) as the emulator's program counter. This seems to be stable and is now default.

On Sparc/Solaris2 use:

YAP_EXTRAS= ...   -mno-app-regs -DOPTIMISE_ALL_REGS_FOR_SPARC=1

and YAP will get two extra registers! This trick does not work on SunOS 4 machines.

Note that versions of GCC can be tweaked to recognize different processors within the same instruction set, e.g. 486, Pentium, and PentiumPro for the x86; or Ultrasparc, and Supersparc for Sparc. Unfortunately, some of these tweaks do may make YAP run slower or not at all in other machines with the same instruction set, so they cannot be made default.

Last, the best options also depends on the version of GCC you are using, and it is a good idea to consult the GCC manual under the menus "Invoking GCC"/"Submodel Options". Specifically, you should check -march=XXX for recent versions of GCC/EGCS. In the case of GCC2.7 and other recent versions of GCC you can check:

• 486: In order to take advantage of 486 specific optimizations in GCC 2.7.*:

YAP_EXTRAS= ... -m486 -DBP_FREE=1

• Pentium:

YAP_EXTRAS= ... -m486 -malign-loops=2 -malign-jumps=2 \
                      -malign-functions=2

• PentiumPro and other recent Intel and AMD machines: PentiumPros are known not to require alignment. Check your version of GCC for the best -march option.

• Super and UltraSparcs:

YAP_EXTRAS= ... -msupersparc

• MIPS: if have a recent machine and you need a 64 bit wide address space you can use the abi 64 bits or eabi option, as in:

CC="gcc -mabi=64" ./configure --...

Be careful. At least for some versions of GCC, compiling with -g seems to result in broken code.

Compiling Under MINGW's GCC

AT the time of this writing (Nov 2014), YAP uses the mkwin script to compile in WIN32. The script requires either a WIN32 environment, or a cross-compiler/emulator package.

YAP has been known to compile under VISUAL C++, and should compile and work under cygwin, but the favorite approach is to use a native msys/mingw environment. This approach has two key advantages:

• it does not need an interface layer and a DLL, like cygwin.

• it enables cross-compilation.

YAP uses rge mkwin script to generate a new YAP installer. The script is controlled by a set of of variables that should be defined early on in the text. It executes by first calling configure, next running make, and last (if all went well) executing nsys.

In more detail, the following mingw based environments have been tested to develop YAP:

• MSYS 1 and mingw32/64: most WIN32 development did occur in this native environment. Best results were achieved with MSYS-1.0.* and TDM-GCC:

  mingw: http://www.mingw.org/ original msys: http://www.mingw.org/wiki/MSYS mingw64: http://mingw-w64.sourceforge.net/ TDM-GCC: http://tdm-gcc.tdragon.net/

• This distribution was compiled with the MSYS2 integrated development, that supports 32 and 64 bit compilation. Setting up MSYS2 should be done with care, but it is worth it as the distribution works nicely in MINGW32 and MINGW64 mode. A third compilation mode, MSYS mode, has problems with compiling sockets.

  msys2: http://sourceforge.net/projects/msys2/

• cygwin and cygwin64 now can generate native applications

  cygwin: https://www.cygwin.com/

• Linux has a nice cross-compilation environment, with some of the best work done for Fedora.

  fedora mingw cross-compiler: http://fedoraproject.org/wiki/MinGW/CrossCompilerFramework

  One problem is that this environment requires emulation of WIN32 executables to generate the initial saved state and to compile chr. wine sometimes does the task, but it sometimes fails.

• OSX has the mxe package, a port of mingw that is in active development.

  mxe: http://mxe.cc/

  Note that OSX has technical limitations that preclude porting wine64. wine32 is distributed with package managers such as ports and brew.

Setting up WIN32 compilation

Compiling WIN32 packages depends on a number of parameters: chosen compiler, packages to install, directory setup. You may have to change these ones that control the mkwin script:

• VER: major/minor number
• PATCHID: third digit
• SRC: directory containing yap sources, in the local environment notation.
• SRC_WIN: same, but in WIN32 standard notation.
• THREADS: yes or no? controllable from the command line.
• ABI: "32" or "64", controllable from the command line.
• NSIS: installer generator, usually "/c/Program Files (x86)/NSIS/makensis".
• DOCS_DIR: where you have the doxygen output.
• GCC_DIR: root of gcc seup.
• HOST: argument to --host configure command.
• BUILD: build directory
• GMP: multi-precision package; yes, no, or the installation directory; usually in the distribution.
• CUDD: BDD package, usually in the distribution.
• JAVA: Java sdk directory, usually in the distribution.
• PYTHON: Python package, usually in the distribution.
• R: R environment package, usually in the distribution.
• GECODE: constraint solver package, usually not in the WIN32 distribution.

Compiling Under Visual C++

YAP used to compile cleanly under Microsoft's Visual C++ release 6.0. We next give a step-by-step review on how the core YAP compiled manually using this environment. Newer versions of YAP will use cmake for this purpose.

First, it is a good idea to build YAP as a DLL:

• create a project named yapdll using File.New. The project will be a DLL project, initially empty.

Notice that either the project is named yapdll or you must replace the preprocessors variable $YAPDLL_EXPORTS to match your project names in the files YAPInterface.h and c_interface.c.

• add all .c files in the $YAPSRC/C directory and in the $YAPSRC\OPTYAP directory to the Project's Source Files (use FileView).

• add all .h files in the $YAPSRC/H directory, $YAPSRC\include directory and in the $YAPSRC\OPTYAP subdirectory to the Project's Header Files.

• Ideally, you should now use m4 to generate extra .h from .m4 files and use configure to create a config.h. Or, you can be lazy, and fetch these files from $YAPSRC\VC\include.

• You may want to go to Build.Set Active Configuration and set Project Type to Release

• To use YAP's own include directories you have to set the Project option Project.Project Settings.C/C++.Preprocessor.Additional Include Directories to include the directories $YAPSRC\H, $YAPSRC\VC\include, $YAPSRC\OPTYAP and $YAPSRC\include. The syntax is:

$YAPSRC\H, $YAPSRC\VC\include, $YAPSRC\OPTYAP, $YAPSRC\include

• Build: the system should generate an yapdll.dll and an yapdll.lib.

• Copy the file yapdll.dll to your path. The file yapdll.lib should also be copied to a location where the linker can find it.

Now you are ready to create a console interface for YAP:

1. create a second project say `wyap` with `File.New`. The project will be a WIN32 console project, initially empty.

   • add $YAPSRC\console\yap.c to the Source Files.

   • add $YAPSRC\VC\include\config.h and the files in $YAPSRC\include to the Header Files.

   • You may want to go to Build.Set Active Configuration and set Project Type to Release.

   • you will eventually need to bootstrap the system by booting from boot.yap, so write:

           -b $YAPSRC\pl\boot.yap
   

   in Project.Project Settings.Debug.Program Arguments.

   • You need the sockets and yap libraries. Add

   ws2_32.lib yapdll.lib
   

   to Project.Project Settings.Link.Object/Library Modules

   You may also need to set the Link Path so that VC++ will find yapdll.lib.

   • set Project.Project Settings.C/C++.Preprocessor.Additional Include Directories to include the $YAPSRC/VC/include and $YAPSRC/include.

   The syntax is:

   $YAPSRC\VC\include, $YAPSRC\include
   

   • Build the system.

   • Use Build.Start Debug to boot the system, and then create the saved state with

   ['$YAPSRC\\pl\\init'].
   qsave_program('startup.yss').
   ^Z
   

   That's it, you've got YAP and the saved state!

Loading and Organising YAP Programs

@ingroup main

Next, we present the main predicates and directives available to load files and to control the Prolog environment.

• @ref YAPConsulting

• @ref YAPModules

+@ref YAPSaving

This chapter describes the predicates controlling the execution of Prolog programs.

In the description of the arguments of functors the following notation will be used:

• a preceding plus sign will denote an argument as an "input argument" - it cannot be a free variable at the time of the call;
• a preceding minus sign will denote an "output argument";
• an argument with no preceding symbol can be used in both ways.

Running YAP

We next describe how to invoke YAP in Unix systems.

Running YAP Interactively

Most often you will want to use YAP in interactive mode. Assuming that YAP is in the user's search path, the top-level can be invoked under Unix with the following command:

yap [-s n] [-h n] [-a n] [-c IP_HOST port ] [filename]

All the arguments and flags are optional and have the following meaning:

• -? print a short error message.
• -s Size allocate Size KBytes for local and global stacks. The user may specify M bytes.
• -h Size allocate Size KBytes for heap and auxiliary stacks
• -t Size allocate Size KBytes for the trail stack
• -L Size SWI-compatible option to allocate Size K bytes for local and global stacks, the local stack cannot be expanded. To avoid confusion with the load option, Size must immediately follow the letter L.
• -G Size SWI-compatible option to allocate Size K bytes for local and global stacks; the global stack cannot be expanded
• -T Size SWI-compatible option to allocate Size K bytes for the trail stack; the trail cannot be expanded.
• -l YAP_FILE compile the Prolog file YAP_FILE before entering the top-level.
• -L YAP_FILE compile the Prolog file YAP_FILE and then halt. This option is useful for implementing scripts.
• -g Goal run the goal Goal before top-level. The goal is converted from an atom to a Prolog term.
• -z Goal run the goal Goal as top-level. The goal is converted from an atom to a Prolog term.
• -b BOOT_FILE boot code is in Prolog file BOOT_FILE. The filename must define the predicate '$live'/0.
• -c IP_HOST port connect standard streams to host IP_HOST at port port
• filename restore state saved in the given file
• -f do not consult initial files
• -q do not print informational messages
• -- separator for arguments to Prolog code. These arguments are visible through the unix/1 built-in predicate.

Note that YAP will output an error message on the following conditions:

a file name was given but the file does not exist or is not a saved YAP state;

the necessary amount of memory could not be allocated;

the allocated memory is not enough to restore the state.

When restoring a saved state, YAP will allocate the

same amount of memory as that in use when the state was saved, unless a different amount is specified by flags in the command line. By default, YAP restores the file startup.yss from the current directory or from the YAP library.

YAP usually boots from a saved state. The saved state will use the default installation directory to search for the YAP binary unless you define the environment variable YAPBINDIR.

YAP always tries to find saved states from the current directory first. If it cannot it will use the environment variable YAPLIBDIR, if defined, or search the default library directory.

YAP will try to find library files from the YAPSHAREDIR/library directory.

Prolog Scripts

YAP can also be used to run Prolog files as scripts, at least in Unix-like environments. A simple example is shown next (do not forget that the shell comments are very important):

#!/usr/local/bin/yap -L --
#
# Hello World script file using YAP
#
# put a dot because of syntax errors .

:- write('Hello World'), nl.

The #! characters specify that the script should call the binary file YAP. Notice that many systems will require the complete path to the YAP binary. The -L flag indicates that YAP should consult the current file when booting and then halt. The remaining arguments are then passed to YAP. Note that YAP will skip the first lines if they start with # (the comment sign for Unix's shell). YAP will consult the file and execute any commands.

A slightly more sophisticated example is:

#!/usr/bin/yap -L --
#
# Hello World script file using YAP
# .

:- initialization(main).

main :- write('Hello World'), nl.

The initialization directive tells YAP to execute the goal main after consulting the file. Source code is thus compiled and main executed at the end. The . is useful while debugging the script as a Prolog program: it guarantees that the syntax error will not propagate to the Prolog code.

Notice that the -- is required so that the shell passes the extra arguments to YAP. As an example, consider the following script dump_args:

#!/usr/bin/yap -L --
#.

main( [] ).
main( [H|T] ) :-
        write( H ), nl,
        main( T ).

:- unix( argv(AllArgs) ), main( AllArgs ).

If you this run this script with the arguments:

./dump_args -s 10000

the script will start an YAP process with stack size 10MB, and the list of arguments to the process will be empty.

Often one wants to run the script as any other program, and for this it is convenient to ignore arguments to YAP. This is possible by using L -- as in the next version of dump_args:

#!/usr/bin/yap -L --

main( [] ).
main( [H|T] ) :-
        write( H ), nl,
        main( T ).

:- unix( argv(AllArgs) ), main( AllArgs ).

The -- indicates the next arguments are not for YAP. Instead, they must be sent directly to the argv built-in. Hence, running

./dump_args test

will write test on the standard output.

YAP Built-ins

• @ref YAPControl

• @ref arithmetic

• @ref YAPChars

• @ref YAP_Terms

• @ref InputOutput

• @ref YAPOS

• @ref Internal_Database

• @ref Sets

Extensions to core Prolog.

YAP includes a number of extensions over the original Prolog language. Next, we discuss how to use the most important ones.

• @ref Rational_Trees

• @ref AttributedVariables

• @ref DepthLimited

• @ref Tabling

• @ref Threads

• @ref Profiling

• @ref YAPArrays

• @ref Parallelism

The YAP Library

Library files reside in the library_directory path (set by the LIBDIR variable in the Makefile for YAP). Several files in the library are originally from the public-domain Edinburgh Prolog library.

• @ref apply
• @ref apply_macros
• @ref arg
• @ref Association_Lists
• @ref avl
• @ref bhash
• @ref block_diagram
• @ref c_alarms
• @ref charsio
• @ref clauses
• @ref cleanup
• @ref dbqueues
• @ref dbusage
• @ref dgraphs
• @ref exo_interval
• @ref flags
• @ref gensym
• @ref yap_hacks
• @ref heaps
• @ref lam_mpi
• @ref line_utils
• @ref swi_listing
• @ref lists
• @ref mapargs
• @ref maplist
• @ref matlab
• @ref matrix
• @ref nb
• @ref Ordered_Sets
• @ref parameters
• @ref queues
• @ref random
• @ref Pseudo_Random
• @ref rbtrees
• @ref regexp
• @ref rltrees
• @ref Splay_Trees
• @ref operating_system_support,
• @ref Terms
• @ref timeout
• @ref trees
• @ref tries
• @ref ugraphs
• @ref undgraphs
• @ref varnumbers
• @ref wdgraphs
• @ref wdgraphs
• @ref wdgraphs
• @ref wgraphs
• @ref wundgraphs
• @ref ypp

The YAP Packages

• @ref real

• @ref BDDs

• @ref Gecode

• @ref MYDDAS

• @ref PFL

• @ref ProbLog1

• @ref python

• @ref YAPRaptor

• @ref YAP-LBFGS

• @subpage yap-udi-indexers

Leuven packages ported from SWI-Prolog:

• @subpage chr

• @subpage clpqr

Compatibility

YAP has been designed to be as compatible as possible with other Prolog systems, originally with C-Prolog\cite x and SICStus Prolog~\cite x . More recent work on YAP has striven at making YAP compatible with the ISO-Prolog standard\cite x , and with Jan Wielemaker's SWI-Prolog\cite x .

SWI-Prolog and YAP have collaborated at improved compatibility \cite x . This resulted in Prolog extensions such as the dialect feature. YAP currently supports most of the SWI-Prolog foreign interface. The following SWI libraries have worked on YAP:

• @ref aggregate
• @ref base64
• @ref broadcast
• @ref ctypes
• @ref date
• @ref prolog_debug
• @ref prolog_edit
• @ref error
• @ref nb_set
• @ref prolog_operator
• @ref swi_option
• @ref pairs
• @ref pio
• @ref predicate_options,
• @ref predopts
• @ref prolog_clause
• @ref prolog_colour
• @ref prolog_source
• @ref prolog_xref
• @ref pure_input
• @ref quasi_quotations
• @ref read_util
• @ref record
• @ref settings
• @ref shlib
• @ref thread_pool
• @ref url
• @ref utf8
• @ref win_menu
• @ref www_browser

Note that in general SWI code may be from an earlier version than the one available with SWI-Prolog. SWI-Prolog are obviously not responsible for any incompatibilities and/or bugs in the YAP port.

Please do refer to the SWI-Prolog home page:

http://www.swi-prolog.org

for more information on SWI-Prolog and the SWI packages.

Compatibility with the C-Prolog interpreter

YAP was designed so that most C-Prolog programs should run under YAP without changes. The most important difference between YAP and C-Prolog is that, being YAP a compiler, some changes should be made if predicates such as assert/1, clause/1 and retract/1 are used. First predicates which will change during execution should be declared as dynamic by using commands like:

:- dynamic f/n.

where f is the predicate name and n is the arity of the predicate. Note that several such predicates can be declared in a single command:

 :- dynamic f/2, ..., g/1.

Primitive predicates such as retract apply only to dynamic predicates. Finally note that not all the C-Prolog primitive predicates are implemented in YAP. They can easily be detected using the unknown system predicate provided by YAP.

Last, by default YAP enables character escapes in strings. You can disable the special interpretation for the escape character by using:

:- yap_flag(character_escapes,off).

or by using:

:- yap_flag(language,cprolog).

Compatibility with the Quintus and SICStus Prolog systems

The Quintus Prolog system was the first Prolog compiler to use Warren's Abstract Machine. This system was very influential in the Prolog community. Quintus Prolog implemented compilation into an abstract machine code, which was then emulated. Quintus Prolog also included several new built-ins, an extensive library, and in later releases a garbage collector. The SICStus Prolog system, developed at SICS (Swedish Institute of Computer Science), is an emulator based Prolog system largely compatible with Quintus Prolog. SICStus Prolog has evolved through several versions. The current version includes several extensions, such as an object implementation, co-routining, and constraints.

Both YAP and SICStus Prolog obey the Edinburgh Syntax and are based on the WAM. Even so, there are major important differences:

• Differently from SICStus Prolog, both consulted and dynamic code in YAP are compiled, not interpreted. All code in YAP is compiled.

• The following SICStus Prolog v3 built-ins are not (currently) implemented in YAP (note that this is only a partial list): stream_interrupt/3, reinitialize/0, help/0, help/1, trimcore/0, and require/1.

• The consult/1 predicate in YAP follows C-Prolog semantics. That is, it adds clauses to the data base, even for preexisting procedures. This is different from consult/1 in SICStus Prolog or SWI-Prolog.

• This list is incomplete.

Compatibility with the ISO Prolog standard

The Prolog standard was developed by ISO/IEC JTC1/SC22/WG17, the international standardization working group for the programming language Prolog. The book "Prolog: The Standard" by Deransart, Ed-Dbali and Cervoni gives a complete description of this standard. Development in YAP from YAP4.1.6 onwards have striven at making YAP compatible with ISO Prolog. As such:

• YAP now supports all of the built-ins required by the ISO-standard, and,
• Error-handling is as required by the standard.

YAP by default is not fully ISO standard compliant. You can set the language flag to iso to obtain better compatibility. Setting this flag changes the following:

• By default, YAP implements the atom_chars/2 (see Testing Terms), and number_chars/2, (see Testing Terms), built-ins as per the original Quintus Prolog definition, and not as per the ISO definition.

Calling set_prolog_flag(to_chars_mode,iso) will switch YAP to use the ISO definition for atom_chars/2 and number_chars/2.

• By default, YAP allows executable goals in directives. In ISO mode most directives can only be called from top level (the exceptions are set_prolog_flag/2 and op/3).

• Error checking for meta-calls under ISO Prolog mode is stricter than by default.

• The strict_iso flag automatically enables the ISO Prolog standard. This feature should disable all features not present in the standard.

The following incompatibilities between YAP and the ISO standard are known to still exist (please check Ulrich Neumerkel's page for more details):

• Currently, YAP does not handle overflow errors in integer operations, and handles floating-point errors only in some architectures. Otherwise, YAP follows IEEE arithmetic.

  Please inform the authors on other incompatibilities that may still exist.

  Foreign Language interface for YAP

  YAP provides the user with three facilities for writing predicates in a language other than Prolog. Under Unix systems, most language implementations were linkable to C, and the first interface exported the YAP machinery to the C language. YAP also implements most of the SWI-Prolog foreign language interface. This gives portability with a number of SWI-Prolog packages and avoids garnage collection by using @ref slotInterface. Last, a new C++ based interface is being designed to work with the swig (www.swig.orgv) interface compiler.

  • The @ref c-interface exports the YAP engine.

  • The @ref swi-c-interface emulates Jan Wielemaker's SWI foreign language interface.

  • The @ref yap-cplus-interface is desiged to interface with the SWI ackage \cite x Object-Oriented systems.