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<h1>5 Modules</h1>
<h3><a name="idp282174676">5.1
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 (.).</p>
<p><strong>Example:</strong></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>For a description of function declarations, see
<span class="bold_code"><a href="functions.html">Function Declaration Syntax</a></span>.</p>
<h3><a name="idp282177108">5.2
Module Attributes</a></h3>
<p>A <strong>module attribute</strong> defines a certain property of a
module.</p>
<p>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, if the literal term
<span class="code">Value</span> has 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),
the term <span class="code">Name/Arity</span> is 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 the module
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#chunks-2');">beam_lib(3)</a></span>
in STDLIB.</p>
<p>Several module attributes have predefined meanings.
Some of them have arity two, but user-defined module
attributes must have arity one.</p>
<h4>Pre-Defined Module Attributes</h4>
<p>Pre-defined module attributes is to 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, is to be 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> does
not work as intended.</p>
<p>This attribute is to be specified first and is the only
mandatory attribute.</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 from 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. 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>.</p>
</dd>
<dt><strong><span class="code">-compile(Options).</span></strong></dt>
<dd>
<p>Compiler options. <span class="code">Options</span> is a single option
or a list of options.
This attribute is added to the option list when
compiling the module. See the <span class="bold_code"><a href="javascript:erlhref('../../','compiler','compile.html');">
compile(3)</a></span> manual page in Compiler.</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 the
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#version-1');">beam_lib(3)</a></span>
manual page in STDLIB.</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>This attribute names a function that is to be run
automatically when a
module is loaded. For more information, see
<span class="bold_code"><a href="code_loading.html#on_load">
Running a Function When a Module is Loaded</a></span>.</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 following OTP
standard behaviours:</p>
<ul>
<li><span class="code">gen_server</span></li>
<li><span class="code">gen_fsm</span></li>
<li><span class="code">gen_event</span></li>
<li><span class="code">supervisor</span></li>
</ul>
<p>The spelling <span class="code">behavior</span> is also accepted.</p>
<p>The callback functions of the module can be specified either
directly by the exported function <span class="code">behaviour_info/1</span>:</p>
<div class="example"><pre>
behaviour_info(callbacks) -> Callbacks.</pre></div>
<p>or by a <span class="code">-callback</span> attribute for each callback
function:</p>
<div class="example"><pre>
-callback Name(Arguments) -> Result.</pre></div>
<p>Here, <span class="code">Arguments</span> is a list of zero or more arguments.
The <span class="code">-callback</span> attribute is to be preferred since the
extra type information can be used by tools to produce
documentation or find discrepancies.</p>
<p>Read more about behaviours and callback modules in
<span class="bold_code"><a href="javascript:erlhref('../../','doc/design_principles','spec_proc.html#behaviours');">
OTP Design Principles</a></span>.</p>
<h4>Record Definitions</h4>
<p>The same syntax as for module attributes is used
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>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">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 is generated by another tool.
It also indicates the correspondence of source files to lines of
the original user-written file, from which the source program
is 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 is not to be further updated.
</p>
<h3><a name="idp282208324">5.3
Comments</a></h3>
<p>Comments can be placed anywhere in a module except within strings
and quoted atoms. A comment begins with the character "%",
continues up to, but does not include the next end-of-line, and
has no effect. Notice that the terminating end-of-line has
the effect of white space.</p>
<h3><a name="idp282209284">5.4
module_info/0 and module_info/1 functions</a></h3>
<p>The compiler automatically inserts the two special, exported
functions into each module:</p>
<ul>
<li><span class="code">Module:module_info/0</span></li>
<li><span class="code">Module:module_info/1</span></li>
</ul>
<p>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">module</span>, <span class="code">attributes</span>, <span class="code">compile</span>,
<span class="code">exports</span>, <span class="code">md5</span> and <span class="code">native</span>.
The order and number of tuples
may change without prior notice.</p>
<h4>module_info/1</h4>
<p>The call <span class="code">module_info(Key)</span>, where <span class="code">Key</span> 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">module</span></strong></dt>
<dd>
<p>Returns an atom representing the module name.</p>
</dd>
<dt><strong><span class="code">attributes</span></strong></dt>
<dd>
<p>Returns 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. Notice that a given
attribute can 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 becomes empty if
the module is stripped with the
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#strip-1');">beam_lib(3)</a></span>
module (in STDLIB).</p>
</dd>
<dt><strong><span class="code">compile</span></strong></dt>
<dd>
<p>Returns a list of tuples with information about
how the module was compiled. This list is empty if
the module has been stripped with the
<span class="bold_code"><a href="javascript:erlhref('../../','stdlib','beam_lib.html#strip-1');">beam_lib(3)</a></span>
module (in STDLIB).</p>
</dd>
<dt><strong><span class="code">md5</span></strong></dt>
<dd>
<p>Returns a binary representing the MD5 checksum of the module.
If the module has native code loaded, this will be the MD5 of the
native code, not the BEAM bytecode.</p>
</dd>
<dt><strong><span class="code">exports</span></strong></dt>
<dd>
<p>Returns 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>Returns a list of <span class="code">{Name,Arity}</span> tuples with
all functions in the module.</p>
</dd>
<dt><strong><span class="code">native</span></strong></dt>
<dd>
<p>Return <span class="code">true</span> if the module has native compiled code.
Return <span class="code">false</span> otherwise. In a system compiled without HiPE
support, the result is always <span class="code">false</span></p>
</dd>
</dl>
</div>
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