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<h2 id="sec:modulecompat"><a id="sec:5.15"><span class="sec-nr">5.15</span> <span class="sec-title">Compatibility 
of the Module System</span></a></h2>

<a id="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 id="idx:moduletransparent1:1482"></a><a class="pred" href="ctxmodule.html#module_transparent/1">module_transparent/1</a> 
directive and <a id="idx:stripmodule3:1483"></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 id="idx:metapredicate1:1484"></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 id="idx:metapredicate1:1485"></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 id="idx:module2:1486"></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 id="idx:addimportmodule3:1487"></a><a class="pred" href="importmodule.html#add_import_module/3">add_import_module/3</a> 
and <a id="idx:deleteimportmodule2:1488"></a><a class="pred" href="importmodule.html#delete_import_module/2">delete_import_module/2</a>.

<p>This mechanism can be used to realise a simple object-oriented system 
or a 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. I.e., operators are inherited from the
<em>import modules</em> (see above).

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

<p>
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swi-prolog-nox 6.6.6-5 / usr / lib / swi-prolog / doc / Manual / modulecompat.html
