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<h2 id="sec:consulting"><a id="sec:4.3"><span class="sec-nr">4.3</span> <span class="sec-title">Loading 
Prolog source files</span></a></h2>

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

<p>This section deals with loading Prolog source files. A Prolog source 
file is a plain text file containing a Prolog program or part thereof. 
Prolog source files come in three flavours:

<dl class="latex">
<dt><b> A traditional</b></dt>
<dd>
Prolog source file contains Prolog clauses and directives, but no <em>module 
declaration</em> (see
<a id="idx:module1:325"></a><a class="pred" href="mtoplevel.html#module/1">module/1</a>). 
They are normally loaded using <a id="idx:consult1:326"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
or <a id="idx:ensureloaded1:327"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a>. 
Currently, a non-module file can only be loaded into a single module.<sup class="fn">35<span class="fn-text">This 
limitation may be lifted in the future. Existing limitations in 
SWI-Prolog's source code administration make this non-trivial.</span></sup></dd>
<dt><b> A module</b></dt>
<dd>
Prolog source file starts with a module declaration. The subsequent 
Prolog code is loaded into the specified module, and only the <em>exported</em> 
predicates are made available to the context loading the module. Module 
files are normally loaded with <a id="idx:usemodule12:328"></a><span class="pred-ext">use_module/[1,2]</span>. 
See <a class="sec" href="modules.html">chapter 5</a> for details.</dd>
<dt><b> An include</b></dt>
<dd>
Prolog source file is loaded using the <a id="idx:include1:329"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
directive, textually including Prolog text into another Prolog source. A 
file may be included into multiple source files and is typically used to 
share <em>declarations</em> such as multifile or dynamic between source 
files.
</dd>
</dl>

<p>Prolog source files are located using <a id="idx:absolutefilename3:330"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a> 
with the following options:

<pre class="code">
locate_prolog_file(Spec, Path) :-
        absolute_file_name(Spec,
                           [ file_type(prolog),
                             access(read)
                           ],
                           Path).
</pre>

<p>The <code>file_type(prolog)</code> option is used to determine the 
extension of the file using <a id="idx:prologfiletype2:331"></a><a class="pred" href="consulting.html#prolog_file_type/2">prolog_file_type/2</a>. 
The default extension is
<code>.pl</code>. <var>Spec</var> allows for the <em>path alias</em> 
construct defined by <a id="idx:absolutefilename3:332"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a>. 
The most commonly used path alias is <code>library(LibraryFile)</code>. 
The example below loads the library file <code>ordsets.pl</code> 
(containing predicates for manipulating ordered sets).

<pre class="code">
:- use_module(library(ordsets)).
</pre>

<p>SWI-Prolog recognises grammar rules (<a id="idx:DCG:333">DCG</a>) as 
defined in
<cite><a class="cite" href="Bibliography.html#Clocksin:87">Clocksin &amp; 
Melish, 1987</a></cite>. The user may define additional compilation of 
the source file by defining the dynamic multifile predicates
<a id="idx:termexpansion2:334"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>, <a id="idx:termexpansion4:335"></a><a class="pred" href="consulting.html#term_expansion/4">term_expansion/4</a>, <a id="idx:goalexpansion2:336"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
and
<a id="idx:goalexpansion4:337"></a><a class="pred" href="consulting.html#goal_expansion/4">goal_expansion/4</a>. 
It is not allowed to use <a id="idx:assert1:338"></a><a class="pred" href="db.html#assert/1">assert/1</a>, <a id="idx:retract1:339"></a><a class="pred" href="db.html#retract/1">retract/1</a> 
or any other database predicate in <a id="idx:termexpansion2:340"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
other than for local computational purposes.<sup class="fn">36<span class="fn-text">It 
does work for normal loading, but not for <a id="idx:qcompile1:341"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>.</span></sup> 
Code that needs to create additional clauses must use <a id="idx:compileauxclauses1:342"></a><a class="pred" href="consulting.html#compile_aux_clauses/1">compile_aux_clauses/1</a>. 
See <code>library(library(apply_macros))</code> for an example.

<p>A <em>directive</em> is an instruction to the compiler. Directives 
are used to set (predicate) properties (see <a class="sec" href="dynamic.html">section 
4.14</a>), set flags (see <a id="idx:setprologflag2:343"></a><a class="pred" href="flags.html#set_prolog_flag/2">set_prolog_flag/2</a>) 
and load files (this section). Directives are terms of the form <code><code>:-</code></code> &lt;<var>term</var>&gt;. 
. Here are some examples:

<pre class="code">
:- use_module(library(lists)).
:- dynamic
        store/2.                % Name, Value
</pre>

<p>The directive <a id="idx:initialization1:344"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a> 
can be used to run arbitrary Prolog goals. The specified goal is started <em>after</em> 
loading the file in which it appears has completed.

<p>SWI-Prolog compiles code as it is read from the file, and directives 
are executed as <em>goals</em>. This implies that directives may call 
any predicate that has been defined before the point where the directive 
appears. It also accepts <code><code>?-</code></code> &lt;<var>term</var>&gt;. 
as a synonym.

<p><a id="idx:reconsult:345"></a>SWI-Prolog does not have a separate 
reconsult/1 predicate. Reconsulting is implied automatically by the fact 
that a file is consulted which is already loaded.

<p>Advanced topics are handled in subsequent sections: mutually 
dependent files (<a class="sec" href="consulting.html">section 4.3.2.1</a>), 
multithreaded loading (<a class="sec" href="consulting.html">section 
4.3.2.2</a>) and reloading running code (<a class="sec" href="consulting.html">section 
4.3.2.3</a>).

<p>The core of the family of loading predicates is <a id="idx:loadfiles2:346"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>. 
The predicates
<a id="idx:consult1:347"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>, <a id="idx:ensureloaded1:348"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a>, <a id="idx:usemodule1:349"></a><a class="pred" href="import.html#use_module/1">use_module/1</a>, <a id="idx:usemodule2:350"></a><a class="pred" href="import.html#use_module/2">use_module/2</a> 
and <a id="idx:reexport1:351"></a><a class="pred" href="reexport.html#reexport/1">reexport/1</a> 
pass the file argument directly to <a id="idx:loadfiles2:352"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
and pass additional options as expressed in the <a class="tab" href="consulting.html#tab:loadpreds">table 
4</a>:

<p><table class="latex frame-hsides center">
<tr><td><b>Predicate</b></td><td align=center><b>if</b></td><td align=center><b>must_be_module</b></td><td align=center><b>import </b></td></tr>
<tr class="hline"><td><a id="idx:consult1:353"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> </td><td align=center><code>true</code> </td><td align=center><code>false</code> </td><td align=center>all </td></tr>
<tr><td><a id="idx:ensureloaded1:354"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a> </td><td align=center><code>not_loaded</code> </td><td align=center><code>false</code> </td><td align=center>all </td></tr>
<tr><td><a id="idx:usemodule1:355"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> </td><td align=center><code>not_loaded</code> </td><td align=center><code>true</code> </td><td align=center>all</td></tr>
<tr><td><a id="idx:usemodule2:356"></a><a class="pred" href="import.html#use_module/2">use_module/2</a> </td><td align=center><code>not_loaded</code> </td><td align=center><code>true</code> </td><td align=center>specified </td></tr>
<tr><td><a id="idx:reexport1:357"></a><a class="pred" href="reexport.html#reexport/1">reexport/1</a> </td><td align=center><code>not_loaded</code> </td><td align=center><code>true</code> </td><td align=center>all </td></tr>
<tr><td><a id="idx:reexport2:358"></a><a class="pred" href="reexport.html#reexport/2">reexport/2</a> </td><td align=center><code>not_loaded</code> </td><td align=center><code>true</code> </td><td align=center>specified </td></tr>
</table>
<div class="caption"><b>Table 4 : </b>Properties of the file-loading 
predicates. The <i>import</i> column specifies what is imported if the 
loaded file is a module file.</div>
<a id="tab:loadpreds"></a>

<dl class="latex">
<dt class="pubdef"><a id="load_files/2"><strong>load_files</strong>(<var>:Files, 
+Options</var>)</a></dt>
<dd class="defbody">
The predicate <a id="idx:loadfiles2:359"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
is the parent of all the other loading predicates except for <a id="idx:include1:360"></a><a class="pred" href="consulting.html#include/1">include/1</a>. 
It currently supports a subset of the options of Quintus <a id="idx:loadfiles2:361"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>. <var>Files</var> 
is either a single source file or a list of source files. The 
specification for a source file is handed to <a id="idx:absolutefilename2:362"></a><a class="pred" href="files.html#absolute_file_name/2">absolute_file_name/2</a>. 
See this predicate for the supported expansions. <var>Options</var> is a 
list of options using the format
<var>OptionName</var>(<var>OptionValue</var>).

<p>The following options are currently supported:

<dl class="latex">
<dt><strong>autoload</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), indicate that this 
load is a
<em>demand</em> load. This implies that, depending on the setting of the 
Prolog flag <a class="flag" href="flags.html#flag:verbose_autoload">verbose_autoload</a>, 
the load action is printed at level <code>informational</code> or <code>silent</code>. 
See also <a id="idx:printmessage2:363"></a><a class="pred" href="exception.html#print_message/2">print_message/2</a> 
and <a id="idx:currentprologflag2:364"></a><a class="pred" href="flags.html#current_prolog_flag/2">current_prolog_flag/2</a>.</dd>
<dt><strong>derived_from</strong>(<var>File</var>)</dt>
<dd class="defbody">
Indicate that the loaded file is derived from <var>File</var>. Used by
<a id="idx:make0:365"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
to time-check and load the original file rather than the derived file.</dd>
<dt><strong>dialect</strong>(<var>+Dialect</var>)</dt>
<dd class="defbody">
Load <var>Files</var> with enhanced compatibility with the target Prolog 
system identified by <var>Dialect</var>. See <a id="idx:expectsdialect1:366"></a><a class="pred" href="dialect.html#expects_dialect/1">expects_dialect/1</a> 
and
<a class="sec" href="dialect.html">section C</a> for details.</dd>
<dt><strong>encoding</strong>(<var>Encoding</var>)</dt>
<dd class="defbody">
Specify the way characters are encoded in the file. Default is taken 
from the Prolog flag <a class="flag" href="flags.html#flag:encoding">encoding</a>. 
See <a class="sec" href="widechars.html">section 2.18.1</a> for details.</dd>
<dt><strong>expand</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code>, run the filenames through <a id="idx:expandfilename2:367"></a><a class="pred" href="files.html#expand_file_name/2">expand_file_name/2</a> 
and load the returned files. Default is <code>false</code>, except for <a id="idx:consult1:368"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
which is intended for interactive use. Flexible location of files is 
defined by <a id="idx:filesearchpath2:369"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.</dd>
<dt><strong>format</strong>(<var>+Format</var>)</dt>
<dd class="defbody">
Used to specify the file format if data is loaded from a stream using 
the <code>stream(Stream)</code> option. Default is <code>source</code>, 
loading Prolog source text. If <code>qlf</code>, load QLF data (see <a id="idx:qcompile1:370"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>).</dd>
<dt><strong>if</strong>(<var>Condition</var>)</dt>
<dd class="defbody">
Load the file only if the specified condition is satisfied. The value
<code>true</code> loads the file unconditionally, <code>changed</code> 
loads the file if it was not loaded before or has been modified since it 
was loaded the last time, and <code>not_loaded</code> loads the file if 
it was not loaded before.</dd>
<dt><strong>imports</strong>(<var>Import</var>)</dt>
<dd class="defbody">
Specify what to import from the loaded module. The default for
<a id="idx:usemodule1:371"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
is <code>all</code>. <var>Import</var> is passed from the second 
argument of <a id="idx:usemodule2:372"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>. 
Traditionally it is a list of predicate indicators to import. As part of 
the SWI-Prolog/YAP integration, we also support <var>Pred</var> as <var>Name</var> 
to import a predicate under another name. Finally, <var>Import</var> can 
be the term <code>except(Exceptions)</code>, where <var>Exceptions</var> 
is a list of predicate indicators that specify predicates that are <em>not</em> 
imported or <var>Pred</var> as <var>Name</var> terms to denote renamed 
predicates. See also <a id="idx:reexport2:373"></a><a class="pred" href="reexport.html#reexport/2">reexport/2</a> 
and
<a id="idx:usemodule2:374"></a><a class="pred" href="import.html#use_module/2">use_module/2</a>.<sup class="fn">bug<span class="fn-text"><var>Name</var>/<var>Arity</var> 
as <var>NewName</var> is currently implemented using a <em>link clause</em>. 
This harms efficiency and does not allow for querying the relation 
through <a id="idx:predicateproperty2:375"></a><a class="pred" href="examineprog.html#predicate_property/2">predicate_property/2</a>.</span></sup>

<p>If <var>Import</var> equals <code>all</code>, all operators are 
imported as well. Otherwise, operators are <em>not</em> imported. 
Operators can be imported selectively by adding terms <code>op(Pri,Assoc,Name)</code> 
to the
<var>Import</var> list. If such a term is encountered, all exported 
operators that unify with this term are imported. Typically, this 
construct will be used with all arguments unbound to import all 
operators or with only <var>Name</var> bound to import a particular 
operator.</dd>
<dt><strong>modified</strong>(<var>TimeStamp</var>)</dt>
<dd class="defbody">
Claim that the source was loaded at <var>TimeStamp</var> without 
checking the source. This option is intended to be used together with 
the <code>stream(Input)</code> option, for example after extracting the 
time from an HTTP server or database.</dd>
<dt><strong>must_be_module</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code>, raise an error if the file is not a module file. 
Used by
<a id="idx:usemodule12:376"></a><span class="pred-ext">use_module/[1,2]</span>.</dd>
<dt><strong>qcompile</strong>(<var>Atom</var>)</dt>
<dd class="defbody">
How to deal with quick-load-file compilation by <a id="idx:qcompile1:377"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>. 
Values are:

<dl class="latex">
<dt><strong>never</strong></dt>
<dd class="defbody">
Default. Do not use qcompile unless called explicitly.
</dd>
<dt><strong>auto</strong></dt>
<dd class="defbody">
Use qcompile for all writeable files. See comment below.
</dd>
<dt><strong>large</strong></dt>
<dd class="defbody">
Use qcompile if the file is `large'. Currently, files larger than 100&nbsp;Kbytes 
are considered large.
</dd>
<dt><strong>part</strong></dt>
<dd class="defbody">
If this <a id="idx:loadfile2:378"></a><span class="pred-ext">load_file/2</span> 
appears in a directive of a file that is compiled into Quick Load Format 
using <a id="idx:qcompile1:379"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>, 
the contents of the argument files are included in the <code>.qlf</code> 
file instead of the loading directive.
</dd>
</dl>

<p>If this option is not present, it uses the value of the Prolog flag <a class="flag" href="flags.html#flag:qcompile">qcompile</a> 
as default.</dd>
<dt><strong>redefine_module</strong>(<var>+Action</var>)</dt>
<dd class="defbody">
Defines what to do if a file is loaded that provides a module that is 
already loaded from another file. <var>Action</var> is one of <code>false</code> 
(default), which prints an error and refuses to load the file, or
<code>true</code>, which uses <a id="idx:unloadfile1:380"></a><a class="pred" href="consulting.html#unload_file/1">unload_file/1</a> 
on the old file and then proceeds loading the new file. Finally, there 
is <code>ask</code>, which starts interaction with the user. <code>ask</code> 
is only provided if the stream <code>user_input</code> is associated 
with a terminal.</dd>
<dt><strong>reexport</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code> re-export the imported predicate. Used by <a id="idx:reexport1:381"></a><a class="pred" href="reexport.html#reexport/1">reexport/1</a> 
and <a id="idx:reexport2:382"></a><a class="pred" href="reexport.html#reexport/2">reexport/2</a>.</dd>
<dt><strong>register</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>false</code>, do not register the load location and options. 
This option is used by <a id="idx:make0:383"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
and <a id="idx:loadhotfixes1:384"></a><span class="pred-ext">load_hotfixes/1</span> 
to avoid polluting the load-context database. See <a id="idx:sourcefileproperty2:385"></a><a class="pred" href="consulting.html#source_file_property/2">source_file_property/2</a>.</dd>
<dt><strong>sandboxed</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
Load the file in <em>sandboxed</em> mode. This option controls the flag <a class="flag" href="flags.html#flag:sandboxed_load">sandboxed_load</a>. 
The only meaningful value for
<var>Bool</var> is <code>true</code>. Using <code>false</code> while the 
Prolog flag is set to <code>true</code> raises a permission error.</dd>
<dt><strong>scope_settings</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
Scope <a id="idx:stylecheck1:386"></a><a class="pred" href="debugger.html#style_check/1">style_check/1</a> 
and <a id="idx:expectsdialect1:387"></a><a class="pred" href="dialect.html#expects_dialect/1">expects_dialect/1</a> 
to the file and files loaded from the file after the directive. Default 
is <code>true</code>. The system and user initialization files (see <strong>-f</strong> 
and
<strong>-F</strong>) are loading with <code>scope_settings(false)</code>.</dd>
<dt><strong>silent</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code>, load the file without printing a message. The 
specified value is the default for all files loaded as a result of 
loading the specified files. This option writes the Prolog flag
<a class="flag" href="flags.html#flag:verbose_load">verbose_load</a> 
with the negation of <var>Bool</var>.</dd>
<dt><strong>stream</strong>(<var>Input</var>)</dt>
<dd class="defbody">
This SWI-Prolog extension compiles the data from the stream
<var>Input</var>. If this option is used, <var>Files</var> must be a 
single atom which is used to identify the source location of the loaded 
clauses as well as to remove all clauses if the data is reconsulted.

<p>This option is added to allow compiling from non-file locations such 
as databases, the web, the <em>user</em> (see <a id="idx:consult1:388"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>) 
or other servers. It can be combined with <code>format(qlf)</code> to 
load QLF data from a stream.
</dd>
</dl>

<p>The <a id="idx:loadfiles2:389"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
predicate can be hooked to load other data or data from objects other 
than files. See <a id="idx:prologloadfile2:390"></a><a class="pred" href="loadfilehook.html#prolog_load_file/2">prolog_load_file/2</a> 
for a description and
<code>library(http/http_load)</code> for an example. All hooks for <a id="idx:loadfiles2:391"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
are documented in <a class="sec" href="loadfilehook.html">section B.9</a>.</dd>
<dt class="pubdef"><a id="consult/1"><strong>consult</strong>(<var>:File</var>)</a></dt>
<dd class="defbody">
Read <var>File</var> as a Prolog source file. Calls to <a id="idx:consult1:392"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
may be abbreviated by just typing a number of filenames in a list. 
Examples:

<p><table class="latex frame-void center">
<tr><td><code>?- consult(load).</code> </td><td>% consult <code>load</code> 
or <code>load.pl</code> </td></tr>
<tr><td><code>?- [library(lists)].</code> </td><td>% load library lists </td></tr>
<tr><td><code>?- [user].</code> </td><td>% Type program on the terminal </td></tr>
</table>

<p>The predicate <a id="idx:consult1:393"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
is equivalent to <code>load_files(File, [])</code>, except for handling 
the special file <code>user</code>, which reads clauses from the 
terminal. See also the <code>stream(Input)</code> option of <a id="idx:loadfiles2:394"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.</dd>
<dt class="pubdef"><a id="ensure_loaded/1"><strong>ensure_loaded</strong>(<var>:File</var>)</a></dt>
<dd class="defbody">
If the file is not already loaded, this is equivalent to <a id="idx:consult1:395"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>. 
Otherwise, if the file defines a module, import all public predicates. 
Finally, if the file is already loaded, is not a module file, and the 
context module is not the global user module, <a id="idx:ensureloaded1:396"></a><a class="pred" href="consulting.html#ensure_loaded/1">ensure_loaded/1</a> 
will call <a id="idx:consult1:397"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>.

<p>With this semantics, we hope to get as close as possible to the clear 
semantics without the presence of a module system. Applications using 
modules should consider using <a id="idx:usemodule12:398"></a><span class="pred-ext">use_module/[1,2]</span>.

<p>Equivalent to <code>load_files(Files, [if(not_loaded)]).</code><sup class="fn">37<span class="fn-text">On 
older versions the condition used to be <code>if(changed)</code>. Poor 
time management on some machines or copying often caused problems. The <a id="idx:make0:399"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
predicate deals with updating the running system after changing the 
source code.</span></sup></dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="include/1"><strong>include</strong>(<var>+File</var>)</a></dt>
<dd class="defbody">
Textually include the content of <var>File</var> in the file in which 
the
<em>directive</em> <code>:- include(File).</code> appears. The include 
construct is only honoured if it appears as a directive in a source 
file. <em>Textual</em> include (similar to C/C++ #include) is obviously 
useful for sharing declarations such as <a id="idx:dynamic1:400"></a><a class="pred" href="dynamic.html#dynamic/1">dynamic/1</a> 
or
<a id="idx:multifile1:401"></a><a class="pred" href="dynamic.html#multifile/1">multifile/1</a> 
by including a file with directives from multiple files that use these 
predicates.

<p>Textual including files that contain clauses is less obvious. 
Normally, in SWI-Prolog, clauses are <em>owned</em> by the file in which 
they are defined. This information is used to <em>replace</em> the old 
definition after the file has beeen modified and is reloaded by, e.g., <a id="idx:make0:402"></a><a class="pred" href="consulting.html#make/0">make/0</a>. 
As we understand it, <a id="idx:include1:403"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
is intended to include the same file multiple times. Including a file 
holding clauses multiple times into the same module is rather 
meaningless as it just duplicates the same clauses. Including a file 
holding clauses in multiple modules does not suffer from this problem, 
but leads to multiple equivalent <em>copies</em> of predicates. Using <a id="idx:usemodule1:404"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
can achieve the same result while
<em>sharing</em> the predicates.

<p>Despite these observations, various projects seem to be using <a id="idx:include1:405"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
to load files holding clauses, typically loading them only once. Such 
usage would allow replacement by, e.g., <a id="idx:consult1:406"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>. 
Unfortunately, the same project might use <a id="idx:include1:407"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
to share directives. Another example of a limitation of mapping to <a id="idx:consult1:408"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
is that if the clauses of a predicate are distributed over two included 
files, <a id="idx:discontiguous1:409"></a><a class="pred" href="dynamic.html#discontiguous/1">discontiguous/1</a> 
is appropriate, while if they are distributed over two consulted files, 
one must use <a id="idx:multifile1:410"></a><a class="pred" href="dynamic.html#multifile/1">multifile/1</a>.

<p>To accommodate included files holding clauses, SWI-Prolog 
distinguishes between the source location of a clause (in this case the 
included file) and the <em>owner</em> of a clause (the file that 
includes the file holding the clause). The source location is used by, 
e.g., <a id="idx:edit1:411"></a><a class="pred" href="edit.html#edit/1">edit/1</a>, 
the graphical tracer, etc., while the owner is used to determine which 
clauses are removed if the file is modified. Relevant information is 
found with the following predicates:

<p>
<ul class="latex">
<li><a id="idx:sourcefile2:412"></a><a class="pred" href="consulting.html#source_file/2">source_file/2</a> 
describes the owner relation.
<li><a id="idx:predicateproperty2:413"></a><a class="pred" href="examineprog.html#predicate_property/2">predicate_property/2</a> 
describes the source location (of the first clause).
<li><a id="idx:clauseproperty2:414"></a><a class="pred" href="examineprog.html#clause_property/2">clause_property/2</a> 
provides access to both source and ownership.
<li><a id="idx:sourcefileproperty2:415"></a><a class="pred" href="consulting.html#source_file_property/2">source_file_property/2</a> 
can be used to query include relationships between files.
</ul>
</dd>
<dt class="pubdef"><a id="require/1"><strong>require</strong>(<var>+ListOfNameAndArity</var>)</a></dt>
<dd class="defbody">
Declare that this file/module requires the specified predicates to be 
defined ``with their commonly accepted definition''. This predicate 
originates from the Prolog portability layer for XPCE. It is intended to 
provide a portable mechanism for specifying that this module requires 
the specified predicates.

<p>The implementation normally first verifies whether the predicate is 
already defined. If not, it will search the libraries and load the 
required library.

<p>SWI-Prolog, having autoloading, does <b>not</b> load the library. 
Instead it creates a procedure header for the predicate if it does not 
exist. This will flag the predicate as `undefined'. See also <a id="idx:check0:416"></a><a class="pred" href="check.html#check/0">check/0</a> 
and
<a id="idx:autoload0:417"></a><a class="pred" href="runtime.html#autoload/0">autoload/0</a>.</dd>
<dt class="pubdef"><a id="encoding/1"><strong>encoding</strong>(<var>+Encoding</var>)</a></dt>
<dd class="defbody">
This directive can appear anywhere in a source file to define how 
characters are encoded in the remainder of the file. It can be used in 
files that are encoded with a superset of US-ASCII, currently UTF-8 and 
ISO Latin-1. See also <a class="sec" href="widechars.html">section 
2.18.1</a>.</dd>
<dt class="pubdef"><a id="make/0"><strong>make</strong></a></dt>
<dd class="defbody">
Consult all source files that have been changed since they were 
consulted. It checks <var>all</var> loaded source files: files loaded 
into a compiled state using <code>pl -c ...</code> and files loaded 
using <a id="idx:consult1:418"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
or one of its derivatives. The predicate <a id="idx:make0:419"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
is called after
<a id="idx:edit1:420"></a><a class="pred" href="edit.html#edit/1">edit/1</a>, 
automatically reloading all modified files. If the user uses an external 
editor (in a separate window), <a id="idx:make0:421"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
is normally used to update the program after editing. In addition, <a id="idx:make0:422"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
updates the autoload indices (see <a class="sec" href="autoload.html">section 
2.13</a>) and runs <a id="idx:listundefined0:423"></a><a class="pred" href="check.html#list_undefined/0">list_undefined/0</a> 
from the <code>library(check)</code> library to report on undefined 
predicates.</dd>
<dt class="pubdef"><a id="library_directory/1"><strong>library_directory</strong>(<var>?Atom</var>)</a></dt>
<dd class="defbody">
Dynamic predicate used to specify library directories. Default
<code>./lib</code>, <code> /lib/prolog</code> and the system's library 
(in this order) are defined. The user may add library directories using
<a id="idx:assertz1:424"></a><a class="pred" href="db.html#assertz/1">assertz/1</a>, <a id="idx:asserta1:425"></a><a class="pred" href="db.html#asserta/1">asserta/1</a> 
or remove system defaults using <a id="idx:retract1:426"></a><a class="pred" href="db.html#retract/1">retract/1</a>. 
Deprecated. New code should use <a id="idx:filesearchpath2:427"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.</dd>
<dt class="pubdef"><a id="file_search_path/2"><strong>file_search_path</strong>(<var>+Alias, 
?Path</var>)</a></dt>
<dd class="defbody">
Dynamic predicate used to specify `path aliases'. This feature is best 
described using an example. Given the definition:

<pre class="code">
file_search_path(demo, '/usr/lib/prolog/demo').
</pre>

<p>the file specification <code>demo(myfile)</code> will be expanded to
<code>/usr/lib/prolog/demo/myfile</code>. The second argument of
<a id="idx:filesearchpath2:428"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a> 
may be another alias.

<p>Below is the initial definition of the file search path. This path 
implies <code>swi(&lt;<var>Path</var>&gt;)</code> and refers to a file 
in the SWI-Prolog home directory. The alias <code>foreign(&lt;<var>Path</var>&gt;)</code> 
is intended for storing shared libraries (<code>.so</code> or <code>.DLL</code> 
files). See also
<a id="idx:loadforeignlibrary12:429"></a><span class="pred-ext">load_foreign_library/[1,2]</span>.

<pre class="code">
user:file_search_path(library, X) :-
        library_directory(X).
user:file_search_path(swi, Home) :-
        current_prolog_flag(home, Home).
user:file_search_path(foreign, swi(ArchLib)) :-
        current_prolog_flag(arch, Arch),
        atom_concat('lib/', Arch, ArchLib).
user:file_search_path(foreign, swi(lib)).
user:file_search_path(path, Dir) :-
        getenv('PATH', Path),
        (   current_prolog_flag(windows, true)
        -&gt;  atomic_list_concat(Dirs, (;), Path)
        ;   atomic_list_concat(Dirs, :, Path)
        ),
        member(Dir, Dirs).
</pre>

<p>The <a id="idx:filesearchpath2:430"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a> 
expansion is used by all loading predicates as well as by <a id="idx:absolutefilename23:431"></a><span class="pred-ext">absolute_file_name/[2,3]</span>.

<p>The Prolog flag <a class="flag" href="flags.html#flag:verbose_file_search">verbose_file_search</a> 
can be set to <code>true</code> to help debugging Prolog's search for 
files.</dd>
<dt class="pubdef"><a id="expand_file_search_path/2"><strong>expand_file_search_path</strong>(<var>+Spec, 
-Path</var>)</a></dt>
<dd class="defbody">
Unifies <var>Path</var> with all possible expansions of the filename 
specification <var>Spec</var>. See also <a id="idx:absolutefilename3:432"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a>.</dd>
<dt class="pubdef"><a id="prolog_file_type/2"><strong>prolog_file_type</strong>(<var>?Extension, 
?Type</var>)</a></dt>
<dd class="defbody">
This dynamic multifile predicate defined in module <code>user</code> 
determines the extensions considered by <a id="idx:filesearchpath2:433"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.
<var>Extension</var> is the filename extension without the leading dot, 
and
<var>Type</var> denotes the type as used by the <code>file_type(Type)</code> 
option of <a id="idx:filesearchpath2:434"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>. 
Here is the initial definition of
<a id="idx:prologfiletype2:435"></a><a class="pred" href="consulting.html#prolog_file_type/2">prolog_file_type/2</a>:

<pre class="code">
user:prolog_file_type(pl,       prolog).
user:prolog_file_type(Ext,      prolog) :-
        current_prolog_flag(associate, Ext),
        Ext \== pl.
user:prolog_file_type(qlf,      qlf).
user:prolog_file_type(Ext,      executable) :-
        current_prolog_flag(shared_object_extension, Ext).
</pre>

<p>Users can add extensions for Prolog source files to avoid conflicts 
(for example with <b>perl</b>) as well as to be compatible with another 
Prolog implementation. We suggest using <code>.pro</code> for avoiding 
conflicts with <b>perl</b>. Overriding the system definitions can stop 
the system from finding libraries.</dd>
<dt class="pubdef"><a id="source_file/1"><strong>source_file</strong>(<var>?File</var>)</a></dt>
<dd class="defbody">
True if <var>File</var> is a loaded Prolog source file. <var>File</var> 
is the absolute and canonical path to the source file.</dd>
<dt class="pubdef"><a id="source_file/2"><strong>source_file</strong>(<var>?Pred, 
?File</var>)</a></dt>
<dd class="defbody">
True if the predicate specified by <var>Pred</var> was loaded from file
<var>File</var>, where <var>File</var> is an absolute path name (see
<a id="idx:absolutefilename2:436"></a><a class="pred" href="files.html#absolute_file_name/2">absolute_file_name/2</a>). 
Can be used with any instantiation pattern, but the database only 
maintains the source file for each predicate. See also <a id="idx:clauseproperty2:437"></a><a class="pred" href="examineprog.html#clause_property/2">clause_property/2</a>. 
Note that the relation between files and predicates is more complicated 
if <a id="idx:include1:438"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
is used. The predicate describes the <em>owner</em> of the predicate. 
See <a id="idx:include1:439"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
for details.</dd>
<dt class="pubdef"><a id="source_file_property/2"><strong>source_file_property</strong>(<var>?File, 
?Property</var>)</a></dt>
<dd class="defbody">
True when <var>Property</var> is a property of the loaded file <var>File</var>. 
If <var>File</var> is non-var, it can be a file specification that is 
valid for <a id="idx:loadfiles2:440"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>. 
Defined properties are:

<dl class="latex">
<dt><strong>derived_from</strong>(<var>Original, OriginalModified</var>)</dt>
<dd class="defbody">
<var>File</var> was generated from the file <var>Original</var>, which 
was last modified at time <var>OriginalModified</var> at the time it was 
loaded. This property is available if <var>File</var> was loaded using 
the
<code>derived_from(Original)</code> option to <a id="idx:loadfiles2:441"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.</dd>
<dt><strong>includes</strong>(<var>IncludedFile, IncludedFileModified</var>)</dt>
<dd class="defbody">
<var>File</var> used <a id="idx:include1:442"></a><a class="pred" href="consulting.html#include/1">include/1</a> 
to include <var>IncludedFile</var>. The last modified time of <var>IncludedFile</var> 
was <var>IncludedFileModified</var> at the time it was included.</dd>
<dt><strong>included_in</strong>(<var>MasterFile, Line</var>)</dt>
<dd class="defbody">
<var>File</var> was included into <var>MasterFile</var> from line <var>Line</var>. 
This is the inverse of the <code>includes</code> property.</dd>
<dt><strong>load_context</strong>(<var>Module, Location, Options</var>)</dt>
<dd class="defbody">
<var>Module</var> is the module into which the file was loaded. If <var>File</var> 
is a module, this is the module into which the exports are imported. 
Otherwise it is the module into which the clauses of the non-module file 
are loaded. <var>Location</var> describes the file location from which 
the file was loaded. It is either a term &lt;<var>file</var>&gt;:&lt;<var>line</var>&gt; 
or the atom
<code>user</code> if the file was loaded from the terminal or another 
unknown source. <var>Options</var> are the options passed to <a id="idx:loadfiles2:443"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>. 
Note that all predicates to load files are mapped to <a id="idx:loadfiles2:444"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>, 
using the option argument to specify the exact behaviour.</dd>
<dt><strong>modified</strong>(<var>Stamp</var>)</dt>
<dd class="defbody">
File modification time when <var>File</var> was loaded. This is used by
<a id="idx:make0:445"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
to find files whose modification time is different from when it was 
loaded.</dd>
<dt><strong>module</strong>(<var>Module</var>)</dt>
<dd class="defbody">
<var>File</var> is a module file that declares the module <var>Module</var>.
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="unload_file/1"><strong>unload_file</strong>(<var>+File</var>)</a></dt>
<dd class="defbody">
Remove all clauses loaded from <var>File</var>. If <var>File</var> 
loaded a module, clear the module's export list and disassociate it from 
the file. <var>File</var> is a canonical filename or a file indicator 
that is valid for <a id="idx:loadfiles2:446"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.

<p>This predicate should be used with care. The multithreaded nature of 
SWI-Prolog makes removing static code unsafe. Attempts to do this should 
be reserved for development or situations where the application can 
guarantee that none of the clauses associated to <var>File</var> are 
active.</dd>
<dt class="pubdef"><a id="prolog_load_context/2"><strong>prolog_load_context</strong>(<var>?Key, 
?Value</var>)</a></dt>
<dd class="defbody">
Obtain context information during compilation. This predicate can be 
used from directives appearing in a source file to get information about 
the file being loaded. See also <a id="idx:sourcelocation2:447"></a><a class="pred" href="consulting.html#source_location/2">source_location/2</a> 
and <a id="idx:if1:448"></a><a class="pred" href="consulting.html#if/1">if/1</a>. 
The following keys are defined:

<p><table class="latex frame-box center">
<tr><td><b>Key</b></td><td><b>Description </b></td></tr>
<tr class="hline"><td><code>module</code> </td><td>Module into which 
file is loaded </td></tr>
<tr><td><code>source</code> </td><td>File being loaded. If the system is 
processing an included file, the value is the <em>main</em> file. 
Returns the original Prolog file when loading a
<code>.qlf</code> file. </td></tr>
<tr><td><code>file</code> </td><td>Similar to <code>source</code>, but 
returns the file being included when called while an include file is 
being processed </td></tr>
<tr><td><code>stream</code> </td><td>Stream identifier (see <a id="idx:currentinput1:449"></a><a class="pred" href="IO.html#current_input/1">current_input/1</a>) </td></tr>
<tr><td><code>directory</code> </td><td>Directory in which <code>source</code> 
lives </td></tr>
<tr><td><code>dialect</code> </td><td>Compatibility mode. See <a id="idx:expectsdialect1:450"></a><a class="pred" href="dialect.html#expects_dialect/1">expects_dialect/1</a>. </td></tr>
<tr><td><code>term_position</code> </td><td>Position of last term read. 
Term of the form
<code>'$stream_position'(0,&lt;<var>Line</var>&gt;,0,0,0)</code>. See 
also <a id="idx:streampositiondata3:451"></a><a class="pred" href="IO.html#stream_position_data/3">stream_position_data/3</a>. </td></tr>
<tr><td><code>variable_names</code></td><td>A list of `<var>Name</var> = <var>Var</var>' 
of the last term read. See <a id="idx:readterm2:452"></a><a class="pred" href="termrw.html#read_term/2">read_term/2</a> 
for details. </td></tr>
<tr><td><code>script</code> </td><td>Boolean that indicates whether the 
file is loaded as a script file (see
<strong>-s</strong>) </td></tr>
</table>

<p>The <code>directory</code> is commonly used to add rules to <a id="idx:filesearchpath2:453"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>, 
setting up a search path for finding files with <a id="idx:absolutefilename3:454"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a>. 
For example:

<pre class="code">
:- dynamic user:file_search_path/2.
:- multifile user:file_search_path/2.

:- prolog_load_context(directory, Dir),
   asserta(user:file_search_path(my_program_home, Dir)).

    ...
    absolute_file_name(my_program_home('README.TXT'), ReadMe,
                       [ access(read) ]),
    ...
</pre>

</dd>
<dt class="pubdef"><a id="source_location/2"><strong>source_location</strong>(<var>-File, 
-Line</var>)</a></dt>
<dd class="defbody">
If the last term has been read from a physical file (i.e., not from the 
file <code>user</code> or a string), unify <var>File</var> with an 
absolute path to the file and <var>Line</var> with the line number in 
the file. New code should use <a id="idx:prologloadcontext2:455"></a><a class="pred" href="consulting.html#prolog_load_context/2">prolog_load_context/2</a>.</dd>
<dt class="pubdef"><a id="at_halt/1"><strong>at_halt</strong>(<var>:Goal</var>)</a></dt>
<dd class="defbody">
Register <var>Goal</var> to be run from <a class="func" href="foreigninclude.html#PL_cleanup()">PL_cleanup()</a>, 
which is called when the system halts. The hooks are run in the reverse 
order they were registered (FIFO). Success or failure executing a hook 
is ignored. If the hook raises an exception this is printed using <a id="idx:printmessage2:456"></a><a class="pred" href="exception.html#print_message/2">print_message/2</a>. 
An attempt to call <a id="idx:halt01:457"></a><span class="pred-ext">halt/[0,1]</span> 
from a hook is ignored. Hooks may call
<a id="idx:cancelhalt1:458"></a><a class="pred" href="consulting.html#cancel_halt/1">cancel_halt/1</a>, 
causing <a id="idx:halt0:459"></a><a class="pred" href="toplevel.html#halt/0">halt/0</a> 
and <a class="func" href="foreigninclude.html#PL_halt()">PL_halt(0)</a> 
to print a message indicating that halting the system has been 
cancelled.</dd>
<dt class="pubdef"><a id="cancel_halt/1"><strong>cancel_halt</strong>(<var>+Reason</var>)</a></dt>
<dd class="defbody">
If this predicate is called from a hook registered with <a id="idx:athalt1:460"></a><a class="pred" href="consulting.html#at_halt/1">at_halt/1</a>, 
halting Prolog is cancelled and an informational message is printed that 
includes <var>Reason</var>. This is used by the development tools to 
cancel halting the system if the editor has unsafed data and the user 
decides to cancel.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="initialization/1">:- <strong>initialization</strong>(<var>:Goal</var>)</a></dt>
<dd class="defbody">
Call <var>Goal</var> <em>after</em> loading the source file in which 
this directive appears has been completed. In addition, <var>Goal</var> 
is executed if a saved state created using <a id="idx:qsaveprogram1:461"></a><a class="pred" href="runtime.html#qsave_program/1">qsave_program/1</a> 
is restored.

<p>The ISO standard only allows for using <code>:- Term</code> if <var>Term</var> 
is a
<em>directive</em>. This means that arbitrary goals can only be called 
from a directive by means of the <a id="idx:initialization1:462"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a> 
directive. SWI-Prolog does not enforce this rule.

<p>The <a id="idx:initialization1:463"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a> 
directive must be used to do program initialization in saved states (see <a id="idx:qsaveprogram1:464"></a><a class="pred" href="runtime.html#qsave_program/1">qsave_program/1</a>). 
A saved state contains the predicates, Prolog flags and operators 
present at the moment the state was created. Other resources (records, 
foreign resources, etc.) must be recreated using <a id="idx:initialization1:465"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a> 
directives or from the entry goal of the saved state.

<p>Up to SWI-Prolog 5.7.11, <var>Goal</var> was executed immediately 
rather than after loading the program text in which the directive 
appears as dictated by the ISO standard. In many cases the exact moment 
of execution is irrelevant, but there are exceptions. For example,
<a id="idx:loadforeignlibrary1:466"></a><a class="pred" href="foreignlink.html#load_foreign_library/1">load_foreign_library/1</a> 
must be executed immediately to make the loaded foreign predicates 
available for exporting. SWI-Prolog now provides the directive <a id="idx:useforeignlibrary1:467"></a><a class="pred" href="foreignlink.html#use_foreign_library/1">use_foreign_library/1</a> 
to ensure immediate loading as well as loading after restoring a saved 
state. If the system encounters a directive <code>:- 
initialization(load_foreign_library(...))</code>, it will load the 
foreign library immediately and issue a warning to update your code. 
This behaviour can be extended by providing clauses for the multifile 
hook predicate <code>prolog:initialize_now(Term, Advice)</code>, where <var>Advice</var> 
is an atom that gives advice on how to resolve the compatibility issue.</dd>
<dt class="pubdef"><a id="initialization/2"><strong>initialization</strong>(<var>:Goal, 
+When</var>)</a></dt>
<dd class="defbody">
Similar to <a id="idx:initialization1:468"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a>, 
but allows for specifying when <var>Goal</var> is executed while loading 
the program text:

<dl class="latex">
<dt><strong>now</strong></dt>
<dd class="defbody">
Execute <var>Goal</var> immediately.
</dd>
<dt><strong>after_load</strong></dt>
<dd class="defbody">
Execute <var>Goal</var> after loading program text. This is the same as <a id="idx:initialization1:469"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a>.
</dd>
<dt><strong>restore</strong></dt>
<dd class="defbody">
Do not execute <var>Goal</var> while loading the program, but <em>only</em> 
when restoring a state.
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="compiling/0"><strong>compiling</strong></a></dt>
<dd class="defbody">
True if the system is compiling source files with the <strong>-c</strong> 
option or <a id="idx:qcompile1:470"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a> 
into an intermediate code file. Can be used to perform conditional code 
optimisations in <a id="idx:termexpansion2:471"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
(see also the
<strong>-O</strong> option) or to omit execution of directives during 
compilation.
</dd>
</dl>

<p><h3 id="sec:progtransform"><a id="sec:4.3.1"><span class="sec-nr">4.3.1</span> <span class="sec-title">Conditional 
compilation and program transformation</span></a></h3>

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

<p><a id="idx:transformationofprogram:472"></a>ISO Prolog defines no way 
for program transformations such as macro expansion or conditional 
compilation. Expansion through <a id="idx:termexpansion2:473"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
and <a id="idx:expandterm2:474"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a> 
can be seen as part of the de-facto standard. This mechanism can do 
arbitrary translation between valid Prolog terms read from the source 
file to Prolog terms handed to the compiler. As
<a id="idx:termexpansion2:475"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
can return a list, the transformation does not need to be term-to-term.

<p>Various Prolog dialects provide the analogous <a id="idx:goalexpansion2:476"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
and
<a id="idx:expandgoal2:477"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a> 
that allow for translation of individual body terms, freeing the user of 
the task to disassemble each clause.

<dl class="latex">
<dt class="pubdef"><a id="term_expansion/2"><strong>term_expansion</strong>(<var>+Term1, 
-Term2</var>)</a></dt>
<dd class="defbody">
Dynamic and multifile predicate, normally not defined. When defined by 
the user all terms read during consulting are given to this predicate. 
If the predicate succeeds Prolog will assert <var>Term2</var> in the 
database rather than the read term (<var>Term1</var>). <var>Term2</var> 
may be a term of the form <code>?- Goal.</code> or <code>:- Goal</code>. <var>Goal</var> 
is then treated as a directive. If <var>Term2</var> is a list, all terms 
of the list are stored in the database or called (for directives). If
<var>Term2</var> is of the form below, the system will assert <var>Clause</var> 
and record the indicated source location with it:
<blockquote>
<code>'$source_location'(&lt;<var>File</var>&gt;, &lt;<var>Line</var>&gt;):&lt;<var>Clause</var>&gt;</code>
</blockquote>

<p>When compiling a module (see <a class="sec" href="modules.html">chapter 
5</a> and the directive <a id="idx:module2:478"></a><a class="pred" href="defmodule.html#module/2">module/2</a>),
<a id="idx:expandterm2:479"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a> 
will first try <a id="idx:termexpansion2:480"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
in the module being compiled to allow for term expansion rules that are 
local to a module. If there is no local definition, or the local 
definition fails to translate the term, <a id="idx:expandterm2:481"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a> 
will try <a id="idx:termexpansion2:482"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a> 
in module
<code>user</code>. For compatibility with SICStus and Quintus Prolog, 
this feature should not be used. See also <a id="idx:expandterm2:483"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a>, <a id="idx:goalexpansion2:484"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
and
<a id="idx:expandgoal2:485"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a>.</dd>
<dt class="pubdef"><a id="expand_term/2"><strong>expand_term</strong>(<var>+Term1, 
-Term2</var>)</a></dt>
<dd class="defbody">
This predicate is normally called by the compiler on terms read from the 
input to perform preprocessing. It consists of four steps, where each 
step processes the output of the previous step.

<p>
<ol class="latex">
<li>Test conditional compilation directives and translate all input to <code>[]</code> 
if we are in a `false branch' of the conditional compilation. See <a class="sec" href="consulting.html">section 
4.3.1.2</a>.

<p>
<li>Call <a id="idx:termexpansion2:486"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>. 
This predicate is first tried in the module that is being compiled and 
then in the module
<code>user</code>.

<p>
<li>Call DCG expansion (<a id="idx:dcgtranslaterule2:487"></a><a class="pred" href="consulting.html#dcg_translate_rule/2">dcg_translate_rule/2</a>).

<p>
<li>Call <a id="idx:expandgoal2:488"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a> 
on each body term that appears in the output of the previous steps.
</ol>
</dd>
<dt class="pubdef"><a id="goal_expansion/2"><strong>goal_expansion</strong>(<var>+Goal1, 
-Goal2</var>)</a></dt>
<dd class="defbody">
Like <a id="idx:termexpansion2:489"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>, <a id="idx:goalexpansion2:490"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
provides for macro expansion of Prolog source code. Between <a id="idx:expandterm2:491"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a> 
and the actual compilation, the body of clauses analysed and the goals 
are handed to <a id="idx:expandgoal2:492"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a>, 
which uses the <a id="idx:goalexpansion2:493"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
hook to do user-defined expansion.

<p>The predicate <a id="idx:goalexpansion2:494"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
is first called in the module that is being compiled, and then on the <code>user</code> 
module. If <var>Goal</var> is of the form <var>Module</var>:<var>Goal</var> 
where <var>Module</var> is instantiated,
<a id="idx:goalexpansion2:495"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
is called on <var>Goal</var> using rules from module
<var>Module</var> followed by <code>user</code>.

<p>Only goals appearing in the body of clauses when reading a source 
file are expanded using this mechanism, and only if they appear 
literally in the clause, or as an argument to a defined meta-predicate 
that is annotated using `0' (see <a id="idx:metapredicate1:496"></a><a class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</a>). 
Other cases need a real predicate definition.</dd>
<dt class="pubdef"><a id="expand_goal/2"><strong>expand_goal</strong>(<var>+Goal1, 
-Goal2</var>)</a></dt>
<dd class="defbody">
This predicate is normally called by the compiler to perform 
preprocessing using <a id="idx:goalexpansion2:497"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a>. 
The predicate computes a fixed-point by applying transformations until 
there are no more changes. If optimisation is enabled (see <strong>-O</strong> 
and
<a class="flag" href="flags.html#flag:optimise">optimise</a>), <a id="idx:expandgoal2:498"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a> 
simplifies the result by removing unneeded calls to <a id="idx:true0:499"></a><a class="pred" href="control.html#true/0">true/0</a> 
and <a id="idx:fail0:500"></a><a class="pred" href="control.html#fail/0">fail/0</a> 
as well as unreachable branches.</dd>
<dt class="pubdef"><a id="compile_aux_clauses/1"><strong>compile_aux_clauses</strong>(<var>+Clauses</var>)</a></dt>
<dd class="defbody">
Compile clauses on behalf of <a id="idx:goalexpansion2:501"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a>. 
This predicate compiles the argument clauses into static predicates, 
associating the predicates with the current file but avoids changing the 
notion of current predicate and therefore discontiguous warnings.</dd>
<dt class="pubdef"><a id="dcg_translate_rule/2"><strong>dcg_translate_rule</strong>(<var>+In, 
-Out</var>)</a></dt>
<dd class="defbody">
This predicate performs the translation of a term <code>Head--&gt;Body</code> 
into a normal Prolog clause. Normally this functionality should be 
accessed using <a id="idx:expandterm2:502"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a>.
</dd>
</dl>

<p><h4 id="sec:progtransform-layout"><a id="sec:4.3.1.1"><span class="sec-nr">4.3.1.1</span> <span class="sec-title">Program 
transformation with source layout info</span></a></h4>

<a id="sec:progtransform-layout"></a>

<p>This sections documents extended versions of the program 
transformation predicates that also transform the source layout 
information. Extended layout information is currently processed, but 
unused. Future versions will use for the following enhancements:

<p>
<ul class="latex">
<li>More precise locations of warnings and errors
<li>More reliable setting of breakpoints
<li>More reliable source layout information in the graphical debugger.
</ul>

<dl class="latex">
<dt class="pubdef"><a id="expand_goal/4"><strong>expand_goal</strong>(<var>+Goal1, 
?Layout1, -Goal2, -Layout2</var>)</a></dt>
<dt class="pubdef"><a id="goal_expansion/4"><strong>goal_expansion</strong>(<var>+Goal1, 
?Layout1, -Goal2, -Layout2</var>)</a></dt>
<dt class="pubdef"><a id="expand_term/4"><strong>expand_term</strong>(<var>+Term1, 
?Layout1, -Term2, -Layout2</var>)</a></dt>
<dt class="pubdef"><a id="term_expansion/4"><strong>term_expansion</strong>(<var>+Term1, 
?Layout1, -Term2, -Layout2</var>)</a></dt>
<dt class="pubdef"><a id="dcg_translate_rule/4"><strong>dcg_translate_rule</strong>(<var>+In, 
?LayoutIn, -Out, -LayoutOut</var>)</a></dt>
<dd class="defbody">
These versions are called <em>before</em> their 2-argument counterparts. 
The input layout term is either a variable (if no layout information is 
available) or a term carrying detailed layout information as returned by 
the <code>subterm_positions</code> of <a id="idx:readterm2:503"></a><a class="pred" href="termrw.html#read_term/2">read_term/2</a>.
</dd>
</dl>

<p><h4 id="sec:conditionalcompilation"><a id="sec:4.3.1.2"><span class="sec-nr">4.3.1.2</span> <span class="sec-title">Conditional 
compilation</span></a></h4>

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

<p><a id="idx:ifdirective:504"></a>Conditional compilation builds on the 
same principle as
<a id="idx:termexpansion2:505"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>, <a id="idx:goalexpansion2:506"></a><a class="pred" href="consulting.html#goal_expansion/2">goal_expansion/2</a> 
and the expansion of grammar rules to compile sections of the source 
code conditionally. One of the reasons for introducing conditional 
compilation is to simplify writing portable code. See <a class="sec" href="dialect.html">section 
C</a> for more information. Here is a simple example:

<pre class="code">
:- if(\+source_exports(library(lists), suffix/2)).

suffix(Suffix, List) :-
        append(_, Suffix, List).

:- endif.
</pre>

<p>Note that these directives can only appear as separate terms in the 
input. Typical usage scenarios include:

<p>
<ul class="compact">
<li>Load different libraries on different dialects.
<li>Define a predicate if it is missing as a system predicate.
<li>Realise totally different implementations for a particular part of 
the code due to different capabilities.
<li>Realise different configuration options for your software.
</ul>

<dl class="latex">
<dt class="pubdef"><a id="if/1">:- <strong>if</strong>(<var>:Goal</var>)</a></dt>
<dd class="defbody">
Compile subsequent code only if <var>Goal</var> succeeds. For enhanced 
portability, <var>Goal</var> is processed by <a id="idx:expandgoal2:507"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a> 
before execution. If an error occurs, the error is printed and 
processing proceeds as if
<var>Goal</var> has failed.</dd>
<dt class="pubdef"><a id="elif/1">:- <strong>elif</strong>(<var>:Goal</var>)</a></dt>
<dd class="defbody">
Equivalent to <code>:- else. :-if(Goal).</code> ... <code>:- endif.</code> 
In a sequence as below, the section below the first matching <code>elif</code> 
is processed. If no test succeeds, the else branch is processed.

<pre class="code">
:- if(test1).
section_1.
:- elif(test2).
section_2.
:- elif(test3).
section_3.
:- else.
section_else.
:- endif.
</pre>

</dd>
<dt class="pubdef"><a id="else/0">:- <strong>else</strong></a></dt>
<dd class="defbody">
Start `else' branch.</dd>
<dt class="pubdef"><a id="endif/0">:- <strong>endif</strong></a></dt>
<dd class="defbody">
End of conditional compilation.
</dd>
</dl>

<p><h3 id="sec:loadrunningcode"><a id="sec:4.3.2"><span class="sec-nr">4.3.2</span> <span class="sec-title">Loading 
files, active code and threads</span></a></h3>

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

<p>Traditionally, Prolog environments allow for reloading files holding 
currently active code. In particular, the following sequence is a valid 
use of the development environment:

<p>
<ul class="compact">
<li>Trace a goal
<li>Find unexpected behaviour of a predicate
<li>Enter a <em>break</em> using the <b>b</b> command
<li>Fix the sources and reload them using <a id="idx:make0:508"></a><a class="pred" href="consulting.html#make/0">make/0</a>
<li>Exit the break, <em>retry</em> using the <b>r</b> command
</ul>

<p>Goals running during the reload keep running on the old definition, 
while new goals use the reloaded definition, which is why the
<em>retry</em> must be used <em>after</em> the reload. This implies that 
clauses of predicates that are active during the reload cannot be 
reclaimed. Normally a small amount of dead clauses should not be an 
issue during development. Such clauses can be reclaimed with
<a id="idx:garbagecollectclauses0:509"></a><a class="pred" href="consulting.html#garbage_collect_clauses/0">garbage_collect_clauses/0</a>.

<dl class="latex">
<dt class="pubdef"><a id="garbage_collect_clauses/0"><strong>garbage_collect_clauses</strong></a></dt>
<dd class="defbody">
Clean up all <em>dirty</em> predicates, where dirty predicates are 
defined to be predicates that have both old and new definitions due to 
reloading a source file while the predicate was active. Of course, 
predicates that are active using <a id="idx:garbagecollectclauses0:510"></a><a class="pred" href="consulting.html#garbage_collect_clauses/0">garbage_collect_clauses/0</a> 
cannot be reclaimed and remain <em>dirty</em>. Predicates are, like 
atoms, shared resources and therefore all threads are suspended during 
the execution of this predicate.
</dd>
</dl>

<p><h4 id="sec:depload"><a id="sec:4.3.2.1"><span class="sec-nr">4.3.2.1</span> <span class="sec-title">Compilation 
of mutually dependent code</span></a></h4>

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

<p>Large programs are generally split into multiple files. If file <var>A</var> 
accesses predicates from file <var>B</var> which accesses predicates 
from file
<var>A</var>, we consider this a mutual or circular dependency. If 
traditional load predicates (e.g., <a id="idx:consult1:511"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>) 
are used to include file <var>B</var> from <var>A</var> and <var>A</var> 
from <var>B</var>, loading either file results in a loop. This is 
because
<a id="idx:consult1:512"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
is mapped to <a id="idx:loadfiles2:513"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
using the option <code>if(true)(if(true))</code> Such programs are 
typically loaded using a <em>load file</em> that consults all required 
(non-module) files. If modules are used, the dependencies are made 
explicit using <a id="idx:usemodule1:514"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
statements. The
<a id="idx:usemodule1:515"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
predicate, however, maps to <a id="idx:loadfiles2:516"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
with the option
<code>if(not_loaded)(if(not_loaded))</code> A <a id="idx:usemodule1:517"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
on an already loaded file merely makes the public predicates of the used 
module available.

<p>Summarizing, mutual dependency of source files is fully supported 
with no precautions when using modules. Modules can use each other in an 
arbitrary dependency graph. When using <a id="idx:consult1:518"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>, 
predicate dependencies between loaded files can still be arbitrary, but 
the consult relations between files must be a proper tree.

<p><h4 id="sec:mtload"><a id="sec:4.3.2.2"><span class="sec-nr">4.3.2.2</span> <span class="sec-title">Compilation 
with multiple threads</span></a></h4>

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

<p>This section discusses compiling files for the first time. For 
reloading, see <a class="sec" href="consulting.html">section 4.3.2.3</a>.

<p>In older versions, compilation was thread-safe due to a global
<em>lock</em> in <a id="idx:loadfiles2:519"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
and the code dealing with
<em>autoloading</em> (see <a class="sec" href="autoload.html">section 
2.13</a>). Besides unnecessary stalling when multiple threads trap 
unrelated undefined predicates, this easily leads to deadlocks, notably 
if threads are started from an
<a id="idx:initialization1:520"></a><a class="pred" href="consulting.html#initialization/1">initialization/1</a> 
directive.<sup class="fn">38<span class="fn-text">Although such goals 
are started after loading the file in which they appear, the calling 
thread is still likely to hold the `load' lock because it is compiling 
the file from which the file holding the directive is loaded.</span></sup>

<p>Starting with version 5.11.27, the autoloader is no longer locked and 
multiple threads can compile files concurrently. This requires special 
precautions only if multiple threads wish to load the same file at the 
same time. Therefore, <a id="idx:loadfiles2:521"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
checks automatically whether some other thread is already loading the 
file. If not, it starts loading the file. If another thread is already 
loading the file, the thread blocks until the other thread finishes 
loading the file. After waiting, and if the file is a module file, it 
will make the public predicates available.

<p>Note that this schema does not prevent deadlocks under all 
situations. Consider two mutually dependent (see <a class="sec" href="consulting.html">section 
4.3.2.1</a>) module files <var>A</var> and <var>B</var>, where thread&nbsp;1 
starts loading <var>A</var> and thread&nbsp;2 starts loading
<var>B</var> at the same time. Both threads will deadlock when trying to 
load the used module.

<p>The current implementation does not detect such cases and the 
involved threads will freeze. This problem can be avoided if a mutually 
dependent collection of files is always loaded from the same start file.

<p><h4 id="sec:reload"><a id="sec:4.3.2.3"><span class="sec-nr">4.3.2.3</span> <span class="sec-title">Reloading 
running code</span></a></h4>

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

<p>This section discusses <em>not re-</em>loading of code. Initial 
loading of code is discussed in <a class="sec" href="consulting.html">section 
4.3.2.2</a>.

<p>As of version 5.5.30, there is basic thread-safety for reloading 
source files while other threads are executing code defined in these 
source files. Reloading a file freezes all threads after marking the 
active predicates originating from the file being reloaded. The threads 
are resumed after the file has been loaded. In addition, after 
completing loading the outermost file, the system runs
<a id="idx:garbagecollectclauses0:522"></a><a class="pred" href="consulting.html#garbage_collect_clauses/0">garbage_collect_clauses/0</a>.

<p>What does that mean? Unfortunately it does <em>not</em> mean we can 
`hot-swap' modules. Consider the case where thread <var>A</var> is 
executing the recursive predicate <var>P</var>. We `fix' <var>P</var> 
and reload. The already running goals for <var>P</var> continue to run 
the old definition, but new recursive calls will use the new definition! 
Many similar cases can be constructed with dependent predicates.

<p>It provides some basic security for reloading files in multithreaded 
applications during development. In the above scenario the system does 
not crash uncontrolled, but behaves like any broken program: it may 
return the wrong bindings, wrong truth value or raise an exception.

<p>Future versions may have an `update now' facility. Such a facility 
can be implemented on top of the <em>logical update view</em>. It would 
allow threads to do a controlled update between processing independent 
jobs.

<p><h3 id="sec:qlf"><a id="sec:4.3.3"><span class="sec-nr">4.3.3</span> <span class="sec-title">Quick 
load files</span></a></h3>

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

<p>SWI-Prolog supports compilation of individual or multiple Prolog 
source files into `Quick Load Files'. A `Quick Load File' (<code>.qlf</code> 
file) stores the contents of the file in a precompiled format.

<p>These files load considerably faster than source files and are 
normally more compact. They are machine-independent and may thus be 
loaded on any implementation of SWI-Prolog. Note, however, that clauses 
are stored as virtual machine instructions. Changes to the compiler will 
generally make old compiled files unusable.

<p>Quick Load Files are created using <a id="idx:qcompile1:523"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>. 
They are loaded using
<a id="idx:consult1:524"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
or one of the other file-loading predicates described in
<a class="sec" href="consulting.html">section 4.3</a>. If <a id="idx:consult1:525"></a><a class="pred" href="consulting.html#consult/1">consult/1</a> 
is given an explicit <code>.pl</code> file, it will load the Prolog 
source. When given a <code>.qlf</code> file, it will load the file. When 
no extension is specified, it will load the
<code>.qlf</code> file when present and the <code>.pl</code> file 
otherwise.

<dl class="latex">
<dt class="pubdef"><a id="qcompile/1"><strong>qcompile</strong>(<var>:File</var>)</a></dt>
<dd class="defbody">
Takes a file specification as <a id="idx:consult1:526"></a><a class="pred" href="consulting.html#consult/1">consult/1</a>, 
etc., and, in addition to the normal compilation, creates a <em>Quick 
Load File</em> from <var>File</var>. The file extension of this file is <code>.qlf</code>. 
The basename of the Quick Load File is the same as the input file.

<p>If the file contains `<code>:- consult(<var>+File</var>)</code>', `<code>:- 
[<var>+File</var>]</code>' or `<code>:- load_files(<var>+File</var>, 
[qcompile(part), ...])</code>' statements, the referred files are 
compiled into the same <code>.qlf</code> file. Other directives will be 
stored in the
<code>.qlf</code> file and executed in the same fashion as when loading 
the
<code>.pl</code> file.

<p>For <a id="idx:termexpansion2:527"></a><a class="pred" href="consulting.html#term_expansion/2">term_expansion/2</a>, 
the same rules as described in
<a class="sec" href="compilation.html">section 2.10</a> apply.

<p>Conditional execution or optimisation may test the predicate
<a id="idx:compiling0:528"></a><a class="pred" href="consulting.html#compiling/0">compiling/0</a>.

<p>Source references (<a id="idx:sourcefile2:529"></a><a class="pred" href="consulting.html#source_file/2">source_file/2</a>) 
in the Quick Load File refer to the Prolog source file from which the 
compiled code originates.</dd>
<dt class="pubdef"><a id="qcompile/2"><strong>qcompile</strong>(<var>:File, 
+Options</var>)</a></dt>
<dd class="defbody">
As <a id="idx:qcompile1:530"></a><a class="pred" href="consulting.html#qcompile/1">qcompile/1</a>, 
but processes additional options as defined by
<a id="idx:loadfiles2:531"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.<sup class="fn">bug<span class="fn-text">Option 
processing is currently incomplete.</span></sup>
</dd>
</dl>

<p></body></html>
swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / consulting.html
