/usr/lib/swi-prolog/doc/Manual/modulecompat.html

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<H2><A NAME="sec:5.15"><SPAN class="sec-nr">5.15</SPAN> <SPAN class="sec-title">Compatibility 
of the Module System</SPAN></A></H2>

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

<P>The SWI-Prolog module system is largely derived from the Quintus 
Prolog module system, which is also adopted by SICStus, Ciao and YAP. 
Originally, the mechanism for defining meta-predicates in SWI-Prolog was 
based on the <A NAME="idx:moduletransparent1:1287"></A><A class="pred" href="ctxmodule.html#module_transparent/1">module_transparent/1</A> 
directive and <A NAME="idx:stripmodule3:1288"></A><A class="pred" href="ctxmodule.html#strip_module/3">strip_module/3</A>. 
Since 5.7.4 it supports the de-facto standard <A NAME="idx:metapredicate1:1289"></A><A class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</A> 
directive for implementing meta-predicates, providing much better 
compatibility.

<P>The support for the <A NAME="idx:metapredicate1:1290"></A><A class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</A> 
mechanism however is considerably different. On most systems, the <EM>caller</EM> 
of a meta-predicate is compiled differently to provide the required &lt;<VAR>module</VAR>&gt;:&lt;<VAR>term</VAR>&gt; 
qualification. This implies that the meta-declaration must be available 
to the compiler when compiling code that calls a meta-predicate. In 
practice, this implies that other systems pose the following 
restrictions on meta-predicates:

<P>
<UL class="latex">
<LI>Modules that provide meta-predicates for a module to-be-compiled 
must be loaded explicitly by that module.
<LI>The meta_predicate directives of exported predicates must follow the <A NAME="idx:module2:1291"></A><A class="pred" href="defmodule.html#module/2">module/2</A> 
directive immediately.
<LI>After changing a meta-declaration, all modules that <EM>call</EM> 
the modified predicates need to be recompiled.
</UL>

<P>In SWI-Prolog, meta-predicates are also <EM>module-transparent</EM> 
and qualifying the module sensitive arguments is done inside the 
meta-predicate. As a result, the caller need not be aware that it is 
calling a meta-predicate and none of the above restrictions hold for 
SWI-Prolog. However, code that aims at portability must obey the above 
rules.

<P>Other differences are listed below.

<P>
<UL class="latex">
<LI>If a module does not define a predicate, it is searched for in the
<EM>import modules</EM>. By default, the import module of any 
user-defined module is the <CODE>user</CODE> module. In turn, the
<CODE>user</CODE> module imports from the module <CODE>system</CODE> 
that provides all built-in predicates. The auto-import hierarchy can be 
changed using <A NAME="idx:addimportmodule3:1292"></A><A class="pred" href="importmodule.html#add_import_module/3">add_import_module/3</A> 
and <A NAME="idx:deleteimportmodule2:1293"></A><A class="pred" href="importmodule.html#delete_import_module/2">delete_import_module/2</A>.

<P>This mechanisms can be used to realise a simple object oriented 
system or hierarchical module system.

<P>
<LI>Operator declarations are local to a module and may be exported. In 
Quintus and SICStus all operators are global. YAP and Ciao also use 
local operators. SWI-Prolog provides global operator declarations from 
within a module by explicitly qualifying the operator name with the <CODE>user</CODE> 
module.

<PRE class="code">
:- op(precedence, type, user:(operatorname)).
</PRE>

<P>
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swi-prolog-nox 5.10.4-3ubuntu1 / usr / lib / swi-prolog / doc / Manual / modulecompat.html
