yap-6.3/docs/md/swi.md

Compatibility with other Prolog systems

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 been adapted to 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.