/usr/share/doc/swi-prolog-doc/Manual/dialect.html

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<H1><A NAME="sec:C"><SPAN class="sec-nr">C</SPAN> <SPAN class="sec-title">Compatibility 
with other Prolog dialects</SPAN></A></H1>

<A NAME="sec:dialect"></A>

<P><A NAME="idx:YAPprolog:1588"></A><A NAME="idx:portableprologcode:1589"></A>This 
chapter explains issues for writing portable Prolog programs. It was 
started after discussion with Vitor Santos Costa, the leading developer 
of YAP Prolog<SUP class="fn">90<SPAN class="fn-text"><A class="url" href="http://yap.sourceforge.net/">http://yap.sourceforge.net/</A></SPAN></SUP> 
YAP and SWI-Prolog have expressed the ambition to enhance the 
portability beyond the trivial Prolog examples, including complex 
libraries involving foreign code.

<P>Although it is our aim to enhance compatibility, we are still faced 
with many incompatibilities between the dialects. As a first step both 
YAP and SWI will provide some instruments that help developing portable 
code. A first release of these tools appeared in SWI-Prolog 5.6.43. Some 
of the facilities are implemented in the base system. Others in the 
library <CODE>library(dialect.pl)</CODE>.

<P>
<UL>
<LI>The Prolog flag <A class="flag" href="flags.html#flag:dialect">dialect</A> 
is an unambiguous and fast way to find out which Prolog dialect executes 
your program. It has the value <CODE>swi</CODE> for SWI-Prolog and <CODE>yap</CODE> 
on YAP.

<P>
<LI>The Prolog flag <A class="flag" href="flags.html#flag:version_data">version_data</A> 
is bound to a term
<CODE>swi(Major, Minor, Patch, Extra)</CODE>

<P>
<LI>Conditional compilation using <CODE>:- if(Condition)</CODE> ...<CODE>:- 
endif</CODE> is supported. See <A class="sec" href="consulting.html">section 
4.3.1.1</A>.

<P>
<LI>The predicate <A NAME="idx:expectsdialect1:1590"></A><A class="pred" href="dialect.html#expects_dialect/1">expects_dialect/1</A> 
allows for specifying for which Prolog system the code was written.

<P>
<LI>The predicates <A NAME="idx:existssource1:1591"></A><A class="pred" href="dialect.html#exists_source/1">exists_source/1</A> 
and <A NAME="idx:sourceexports2:1592"></A><A class="pred" href="dialect.html#source_exports/2">source_exports/2</A> 
can be used to query the library content. The <A NAME="idx:require1:1593"></A><A class="pred" href="consulting.html#require/1">require/1</A> 
directive can be used to get access to predicates without knowing their 
location.

<P>
<LI>The module predicates <A NAME="idx:usemodule1:1594"></A><A class="pred" href="import.html#use_module/1">use_module/1</A>, <A NAME="idx:usemodule2:1595"></A><A class="pred" href="import.html#use_module/2">use_module/2</A> 
have been extended with a notion for `import-except' and `import-as'. 
This is particulary useful together with <A NAME="idx:reexport1:1596"></A><A class="pred" href="reexport.html#reexport/1">reexport/1</A> 
and <A NAME="idx:reexport2:1597"></A><A class="pred" href="reexport.html#reexport/2">reexport/2</A> 
to compose modules from other modules and mapping names.

<P>
<LI>Foreign code can expect <CODE>__SWI_PROLOG__</CODE> when compiled 
for SWI-Prolog and <CODE>__YAP_PROLOG__</CODE> when compiled on YAP.
</UL>

<DL>
<DT class="pubdef"><A NAME="expects_dialect/1">:- <STRONG>expects_dialect</STRONG>(<VAR>+Dialect</VAR>)</A></DT>
<DD class="defbody">
This directive states that the code following the directive is written 
for the given Prolog <VAR>Dialect</VAR>. See also <A class="flag" href="flags.html#flag:dialect">dialect</A>. 
The declaration holds until the end of the file in which it appears. The 
current dialect is available using <A NAME="idx:prologloadcontext2:1598"></A><A class="pred" href="consulting.html#prolog_load_context/2">prolog_load_context/2</A>.

<P>The exact behaviour of this predicate is still subject to discussion. 
Of course, if <VAR>Dialect</VAR> matches the running dialect the 
directive has no effect. Otherwise we check for the existence of
<CODE>library(dialect/Dialect)</CODE> and load it if the file is found. 
Currently, this file has this functionality:

<P>
<UL>
<LI>Define system predicates of the requested dialect we do not have.

<P>
<LI>Apply <A NAME="idx:goalexpansion2:1599"></A><A class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</A> 
rules that map conflicting predicates to versions emulating the 
requested dialect. These expansion rules reside in the dialect 
compatibility module, but are applied if prolog_load_context(dialect, 
Dialect) is active.

<P>
<LI>Modify the search path for library directories, putting libraries 
compatible to the target dialect before the native libraries.

<P>
<LI>Setup support for the default filename extension of the dialect.
</UL>
</DD>
<DT class="pubdef"><A NAME="exists_source/1"><STRONG>exists_source</STRONG>(<VAR>+Spec</VAR>)</A></DT>
<DD class="defbody">
Is true if <VAR>Spec</VAR> exists as a Prolog source. <VAR>Spec</VAR> 
uses the same conventions as <A NAME="idx:loadfiles2:1600"></A><A class="pred" href="consulting.html#load_files/2">load_files/2</A>. 
Fails without error if <VAR>Spec</VAR> cannot be found.</DD>
<DT class="pubdef"><A NAME="source_exports/2"><STRONG>source_exports</STRONG>(<VAR>+Spec, 
+Export</VAR>)</A></DT>
<DD class="defbody">
Is true if source <VAR>Spec</VAR> exports <VAR>Export</VAR>, a predicate 
indicator. Fails without error otherwise.
</DD>
</DL>

<P>
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<H2>Section Index</H2>

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<DIV class="toc">
<DIV class="toc-h2"><A class="sec" href="portabilitystrategies.html"><SPAN class="sec-nr">C.1</SPAN> <SPAN class="sec-title">Some 
considerations for writing portable code</SPAN></A></DIV>
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swi-prolog-doc 5.6.59-1 / usr / share / doc / swi-prolog-doc / Manual / dialect.html
