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<h1>4 Modules</h1>
<h3><a name="id68326">4.1
Module Syntax</a></h3>
<p>Erlang code is divided into <strong>modules</strong>. A module consists
of a sequence of attributes and function declarations, each
terminated by period (.). Example:</p>
<div class="example"><pre>
-module(m). % module attribute
-export([fact/1]). % module attribute
fact(N) when N>0 -> % beginning of function declaration
N * fact(N-1); % |
fact(0) -> % |
1. % end of function declaration</pre></div>
<p>See the <span class="bold_code"><a href="functions.html">Functions</a></span> chapter
for a description of function declarations.</p>
<h3><a name="id68358">4.2
Module Attributes</a></h3>
<p>A <strong>module attribute</strong> defines a certain property of a
module. A module attribute consists of a tag and a value.</p>
<div class="example"><pre>
-Tag(Value).</pre></div>
<p><span class="code">Tag</span> must be an atom, while <span class="code">Value</span> must be a literal
term. As a convenience in user-defined attributes, the literal term
<span class="code">Value</span> the syntax <span class="code">Name/Arity</span>
(where <span class="code">Name</span> is an atom and <span class="code">Arity</span> a positive integer)
will be translated to <span class="code">{Name,Arity}</span>.</p>
<p>Any module attribute can be specified. The attributes are stored
in the compiled code and can be retrieved by calling
<span class="code">Module:module_info(attributes)</span> or by using
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#chunks-2');">beam_lib(3)</a></span>.</p>
<p>There are several module attributes with predefined meanings,
some of which have arity two, but user-defined module
attributes must have arity one.</p>
<h4>Pre-Defined Module Attributes</h4>
<p>Pre-defined module attributes should be placed before any
function declaration.</p>
<dl>
<dt><strong><span class="code">-module(Module).</span></strong></dt>
<dd>
<p>Module declaration, defining the name of the module.
The name <span class="code">Module</span>, an atom, should be the same as
the file name minus the extension <span class="code">erl</span>. Otherwise
<span class="bold_code"><a href="code_loading.html#loading">code loading</a></span> will
not work as intended.</p>
<p>This attribute should be specified first and is the only
attribute which is mandatory.</p>
</dd>
<dt><strong><span class="code">-export(Functions).</span></strong></dt>
<dd>
<p>Exported functions. Specifies which of the functions
defined within the module that are visible outside
the module.</p>
<p><span class="code">Functions</span> is a list
<span class="code">[Name1/Arity1, ..., NameN/ArityN]</span>, where each
<span class="code">NameI</span> is an atom and <span class="code">ArityI</span> an integer.</p>
</dd>
<dt><strong><span class="code">-import(Module,Functions).</span></strong></dt>
<dd>
<p>Imported functions. Imported functions can be called
the same way as local functions, that is without any module
prefix.</p>
<p><span class="code">Module</span>, an atom, specifies which module to import
functions from. <span class="code">Functions</span> is a list similar as for
<span class="code">export</span> above.</p>
</dd>
<dt><strong><span class="code">-compile(Options).</span></strong></dt>
<dd>
<p>Compiler options. <span class="code">Options</span>, which is a single option
or a list of options, will be added to the option list when
compiling the module. See <span class="code">compile(3)</span>.</p>
</dd>
<dt><strong><span class="code">-vsn(Vsn).</span></strong></dt>
<dd>
<p>Module version. <span class="code">Vsn</span> is any literal term and can be
retrieved using <span class="code">beam_lib:version/1</span>, see
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#version-1');">beam_lib(3)</a></span>.</p>
<p>If this attribute is not specified, the version defaults
to the MD5 checksum of the module.</p>
</dd>
<dt><strong><span class="code">-on_load(Function).</span></strong></dt>
<dd>
<p>Names a function that should be run automatically when a
module a loaded. See <span class="bold_code"><a href="code_loading.html#on_load">
code loading</a></span> for more information.</p>
</dd>
</dl>
<h4>Behaviour Module Attribute</h4>
<p>It is possible to specify that the module is the callback
module for a <strong>behaviour</strong>:</p>
<div class="example"><pre>
-behaviour(Behaviour).</pre></div>
<p>The atom <span class="code">Behaviour</span> gives the name of the behaviour,
which can be a user defined behaviour or one of the OTP
standard behaviours <span class="code">gen_server</span>, <span class="code">gen_fsm</span>,
<span class="code">gen_event</span> or <span class="code">supervisor</span>.</p>
<p>The spelling <span class="code">behavior</span> is also accepted.</p>
<p>Read more about behaviours and callback modules in OTP Design
Principles.</p>
<h4>Record Definitions</h4>
<p>The same syntax as for module attributes is used by
for record definitions:</p>
<div class="example"><pre>
-record(Record,Fields).</pre></div>
<p>Record definitions are allowed anywhere in a module,
also among the function declarations.
Read more in <span class="bold_code"><a href="records.html">Records</a></span>.</p>
<h4>The Preprocessor</h4>
<p>The same syntax as for module attributes is used by
the preprocessor, which supports file inclusion, macros,
and conditional compilation:</p>
<div class="example"><pre>
-include("SomeFile.hrl").
-define(Macro,Replacement).</pre></div>
<p>Read more in <span class="bold_code"><a href="macros.html">The Preprocessor</a></span>.</p>
<h4>Setting File and Line</h4>
<p>The same syntax as for module attributes is used for
changing the pre-defined macros <span class="code">?FILE</span> and <span class="code">?LINE</span>:</p>
<div class="example"><pre>
-file(File, Line).</pre></div>
<p>This attribute is used by tools such as Yecc to inform the
compiler that the source program was generated by another tool
and indicates the correspondence of source files to lines of
the original user-written file from which the source program
was produced.</p>
<h4>Types and function specifications</h4>
<p>A similar syntax as for module attributes is used for
specifying types and function specifications.
</p>
<div class="example"><pre>
-type my_type() :: atom() | integer().
-spec my_function(integer()) -> integer().
</pre></div>
<p>Read more in <span class="bold_code"><a href="typespec.html">Types and Function specifications</a></span>.
</p>
<p>
The description is based on
<span class="bold_code"><a href="http://www.erlang.org/eeps/eep-0008.html">EEP8 -
Types and function specifications</a></span>
which will not be further updated.
</p>
<h3><a name="id74337">4.3
Comments</a></h3>
<p>Comments may be placed anywhere in a module except within strings
and quoted atoms. The comment begins with the character "%",
continues up to, but does not include the next end-of-line, and
has no effect. Note that the terminating end-of-line has
the effect of white space.</p>
<h3><a name="id74350">4.4
The module_info/0 and module_info/1 functions</a></h3>
<p>The compiler automatically inserts the two special, exported
functions into each module: <span class="code">Module:module_info/0</span> and
<span class="code">Module:module_info/1</span>. These functions can be called to
retrieve information about the module.</p>
<h4>module_info/0</h4>
<p>The <span class="code">module_info/0</span> function in each module returns
a list of <span class="code">{Key,Value}</span> tuples with information about
the module. Currently, the list contain tuples with the following
<span class="code">Key</span>s: <span class="code">attributes</span>, <span class="code">compile</span>,
<span class="code">exports</span>, and <span class="code">imports</span>. The order and number of tuples
may change without prior notice.</p>
<div class="warning">
<div class="label">Warning</div>
<div class="content"><p><p>The <span class="code">{imports,Value}</span> tuple may be removed in a future
release because <span class="code">Value</span> is always an empty list.
Do not write code that depends on it being present.</p></p></div>
</div>
<h4>module_info/1</h4>
<p>The call <span class="code">module_info(Key)</span>, where key is an atom,
returns a single piece of information about the module.</p>
<p>The following values are allowed for <span class="code">Key</span>:</p>
<dl>
<dt><strong><span class="code">attributes</span></strong></dt>
<dd>
<p>Return a list of <span class="code">{AttributeName,ValueList}</span> tuples,
where <span class="code">AttributeName</span> is the name of an attribute,
and <span class="code">ValueList</span> is a list of values. Note: a given
attribute may occur more than once in the list with different
values if the attribute occurs more than once in the module.</p>
<p>The list of attributes will be empty if
the module has been stripped with
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#strip-1');">beam_lib(3)</a></span>.</p>
</dd>
<dt><strong><span class="code">compile</span></strong></dt>
<dd>
<p>Return a list of tuples containing information about
how the module was compiled. This list will be empty if
the module has been stripped with
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#strip-1');">beam_lib(3)</a></span>.</p>
</dd>
<dt><strong><span class="code">imports</span></strong></dt>
<dd>
<p>Always return an empty list. The <span class="code">imports</span> key may not
be supported in future release.</p>
</dd>
<dt><strong><span class="code">exports</span></strong></dt>
<dd>
<p>Return a list of <span class="code">{Name,Arity}</span> tuples with
all exported functions in the module.</p>
</dd>
<dt><strong><span class="code">functions</span></strong></dt>
<dd>
<p>Return a list of <span class="code">{Name,Arity}</span> tuples with
all functions in the module.</p>
</dd>
</dl>
</div>
<div class="footer">
<hr>
<p>Copyright © 2003-2012 Ericsson AB. All Rights Reserved.</p>
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