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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>
<HR>
<CENTER>
<H2>Section Index</H2>
</CENTER>
<HR>
<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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