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

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<h2 id="sec:overrule"><a id="sec:5.5"><span class="sec-nr">5.5</span> <span class="sec-title">Overruling 
Module Boundaries</span></a></h2>

<a id="sec:overrule"></a>

<p>The module system described so far is sufficient to distribute 
programs over multiple modules. There are, however, cases in which we 
would like to be able to overrule this schema and explicitly call a 
predicate in some module or assert explicitly into some module. Calling 
in a particular module is useful for debugging from the user's top level 
or to access multiple implementations of the same interface that reside 
in multiple modules. Accessing the same interface from multiple modules 
cannot be achieved using importing because importing a predicate with 
the same name and arity from two modules results in a name conflict. 
Asserting in a different module can be used to create models dynamically 
in a new module. See <a class="sec" href="dynamic-modules.html">section 
5.12</a>.

<p>Direct addressing of modules is achieved using a <code><code>:</code>/2</code> 
explicitly in a program and relies on the module qualification mechanism 
described in <a class="sec" href="metapred.html">section 5.4</a>. Here 
are a few examples:

<pre class="code">
?- assert(world:done).   % asserts done/0 into module world
?- world:asserta(done).  % the same
?- world:done.           % calls done/0 in module world
</pre>

<p>Note that the second example is the same due to the Prolog flag
<a class="flag" href="flags.html#flag:colon_sets_calling_context">colon_sets_calling_context</a>. 
The system predicate <a id="idx:asserta1:1436"></a><a class="pred" href="db.html#asserta/1">asserta/1</a> 
is called in the module <code>world</code>, which is possible because 
system predicates are <em>visible</em> in all modules. At the same time, 
the
<em>calling context</em> is set to <code>world</code>. Because meta 
arguments are qualified with the calling context, the resulting call is 
the same as the first example.

<p><h3 id="sec:set-calling-context"><a id="sec:5.5.1"><span class="sec-nr">5.5.1</span> <span class="sec-title">Explicit 
manipulation of the calling context</span></a></h3>

<a id="sec:set-calling-context"></a>

<p>Quintus' derived module systems have no means to separate the lookup 
module (for finding predicates) from the calling context (for qualifying 
meta arguments). Some other Prolog implementations (e.g., ECLiPSe and 
IF/Proloog) distinguish these operations, using <b>@/2</b> for setting 
the calling context of a goal. This is provided by SWI-Prolog, currently 
mainly to support compatibility layers.

<dl class="latex">
<dt class="pubdef"><a id="@/2"><strong>@</strong>(<var>:Goal, +Module</var>)</a></dt>
<dd class="defbody">
Execute <var>Goal</var>, setting the calling context to <var>Module</var>. 
Setting the calling context affects meta-predicates, for which meta 
arguments are qualified with <var>Module</var> and transparent 
predicates (see
<a id="idx:moduletransparent1:1437"></a><a class="pred" href="ctxmodule.html#module_transparent/1">module_transparent/1</a>). 
It has no implications for other predicates.

<p>For example, the code <code>asserta(done)@world</code> is the same as
<code>asserta(world:done)</code>. Unlike in <code>world:asserta(done)</code>,
<a id="idx:asserta1:1438"></a><a class="pred" href="db.html#asserta/1">asserta/1</a> 
is resolved in the current module rather than the module
<code>world</code>. This makes no difference for system predicates, but 
usually does make a difference for user predicates.

<p>Not that SWI-Prolog does not define <code>@</code> as an operator. 
Some systems define this construct using <code>op(900, xfx, @)</code>.
</dd>
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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / overrule.html
