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<title>SWI-Prolog 7.3.6 Reference Manual: Section 6.3</title><link rel="home" href="index.html">
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<h2 id="sec:import"><a id="sec:6.3"><span class="sec-nr">6.3</span> <span class="sec-title">Importing
Predicates into a Module</span></a></h2>
<a id="sec:import"></a>
<p>Predicates can be added to a module by <em>importing</em> them from
another module. Importing adds predicates to the namespace of a module.
An imported predicate can be called exactly the same as a locally
defined predicate, although its implementation remains part of the
module in which it has been defined.
<p>Importing the predicates from another module is achieved using the
directives <a id="idx:usemodule1:1525"></a><a class="pred" href="import.html#use_module/1">use_module/1</a>
or <a id="idx:usemodule2:1526"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>.
Note that both directives take
<var>filename(s)</var> as arguments. That is, modules are imported based
on their filename rather than their module name.
<dl class="latex">
<dt class="pubdef"><a id="use_module/1"><strong>use_module</strong>(<var>+Files</var>)</a></dt>
<dd class="defbody">
Load the file(s) specified with <var>Files</var> just like <a id="idx:ensureloaded1:1527"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a>.
The files must all be module files. All exported predicates from the
loaded files are imported into the module from which this predicate is
called. This predicate is equivalent to <a id="idx:ensureloaded1:1528"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a>,
except that it raises an error if <var>Files</var> are not module files.
<p>The imported predicates act as <em>weak symbols</em> in the module
into which they are imported. This implies that a local definition of a
predicate overrides (clobbers) the imported definition. If the flag
<a class="flag" href="flags.html#flag:warn_override_implicit_import">warn_override_implicit_import</a>
is <code>true</code> (default), a warning is printed. Below is an
example of a module that uses library(lists), but redefines <a id="idx:flatten2:1529"></a><a class="pred" href="lists.html#flatten/2">flatten/2</a>,
giving it a totally different meaning:
<pre class="code">
:- module(shapes, []).
:- use_module(library(lists)).
flatten(cube, square).
flatten(ball, circle).
</pre>
<p>Loading the above file prints the following message:
<pre class="code">
Warning: /home/janw/Bugs/Import/t.pl:5:
Local definition of shapes:flatten/2
overrides weak import from lists
</pre>
<p>This warning can be avoided by (1) using <a id="idx:usemodule2:1530"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>
to only import the predicates from the <code>lists</code> library that
are actually used in the `shapes' module, (2) using the <code>except([flatten/2])</code>
option of <a id="idx:usemodule2:1531"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>,
(3) use
<code>:- abolish(<a id="idx:flatten2:1532"></a><a class="pred" href="lists.html#flatten/2">flatten/2</a>).</code>
before the local definition or (4) setting
<a class="flag" href="flags.html#flag:warn_override_implicit_import">warn_override_implicit_import</a>
to <code>false</code>. Globally disabling this warning is only
recommended if overriding imported predicates is common as a result of
design choices or the program is ported from a system that silently
overrides imported predicates.
<p>Note that it is always an error to import two modules with <a id="idx:usemodule1:1533"></a><a class="pred" href="import.html#use_module/1">use_module/1</a>
that export the same predicate. Such conflicts must be resolved with
<a id="idx:usemodule2:1534"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>
as described above.</dd>
<dt class="pubdef"><a id="use_module/2"><strong>use_module</strong>(<var>+File,
+ImportList</var>)</a></dt>
<dd class="defbody">
Load <var>File</var>, which must be a module file, and import the
predicates as specified by <var>ImportList</var>. <var>ImportList</var>
is a list of predicate indicators specifying the predicates that will be
imported from the loaded module. <var>ImportList</var> also allows for
renaming or import-everything-except. See also the <code>import</code>
option of
<a id="idx:loadfiles2:1535"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.
The first example below loads <a id="idx:member2:1536"></a><a class="pred" href="lists.html#member/2">member/2</a>
from the <code>lists</code> library and <a id="idx:append2:1537"></a><a class="pred" href="lists.html#append/2">append/2</a>
under the name <code>list_concat</code>, which is how this predicate is
named in YAP. The second example loads all exports from library <code>option</code>
except for <a id="idx:metaoptions3:1538"></a><a class="pred" href="option.html#meta_options/3">meta_options/3</a>.
These renaming facilities are generally used to deal with portability
issues with as few changes as possible to the actual code. See also <a class="sec" href="dialect.html">section
C</a> and
<a class="sec" href="reexport.html">section 6.7</a>.
<pre class="code">
:- use_module(library(lists), [ member/2,
append/2 as list_concat
]).
:- use_module(library(option), except([meta_options/3])).
</pre>
<p></dd>
</dl>
<p>The <a id="idx:module2:1539"></a><a class="pred" href="defmodule.html#module/2">module/2</a>, <a id="idx:usemodule1:1540"></a><a class="pred" href="import.html#use_module/1">use_module/1</a>
and <a id="idx:usemodule2:1541"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>
directives are sufficient to partition a simple Prolog program into
modules. The SWI-Prolog graphical cross-referencing tool <a id="idx:gxref0:1542"></a><a class="pred" href="xref.html#gxref/0">gxref/0</a>
can be used to analyse the dependencies between non-module files and
propose module declarations for each file.
<p></body></html>
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