5  Modules

5 Modules

Erlang code is divided into modules. A module consists of a sequence of attributes and function declarations, each terminated by period (.).

Example:

-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

For a description of function declarations, see Function Declaration Syntax.

A module attribute defines a certain property of a module.

A module attribute consists of a tag and a value:

-Tag(Value).

Tag must be an atom, while Value must be a literal term. As a convenience in user-defined attributes, if the literal term Value has the syntax Name/Arity (where Name is an atom and Arity a positive integer), the term Name/Arity is translated to {Name,Arity}.

Any module attribute can be specified. The attributes are stored in the compiled code and can be retrieved by calling Module:module_info(attributes), or by using the module beam_lib(3) in STDLIB.

Several module attributes have predefined meanings. Some of them have arity two, but user-defined module attributes must have arity one.

Pre-defined module attributes is to be placed before any function declaration.

-module(Module).

Module declaration, defining the name of the module. The name Module, an atom, is to be same as the file name minus the extension .erl. Otherwise code loading does not work as intended.

This attribute is to be specified first and is the only mandatory attribute.

-export(Functions).

Exported functions. Specifies which of the functions, defined within the module, that are visible from outside the module.

Functions is a list [Name1/Arity1, ..., NameN/ArityN], where each NameI is an atom and ArityI an integer.

-import(Module,Functions).

Imported functions. Can be called the same way as local functions, that is, without any module prefix.

Module, an atom, specifies which module to import functions from. Functions is a list similar as for export.

-compile(Options).

Compiler options. Options is a single option or a list of options. This attribute is added to the option list when compiling the module. See the compile(3) manual page in Compiler.

-vsn(Vsn).

Module version. Vsn is any literal term and can be retrieved using beam_lib:version/1, see the beam_lib(3) manual page in STDLIB.

If this attribute is not specified, the version defaults to the MD5 checksum of the module.

-on_load(Function).

This attribute names a function that is to be run automatically when a module is loaded. For more information, see Running a Function When a Module is Loaded.

-nifs(Functions).

Specifies which of the functions, defined within the module, that may be loaded as NIFs with erlang:load_nif/2.

Functions is a list [Name1/Arity1, ..., NameN/ArityN], where each NameI is an atom and ArityI an integer.

While not strictly necessary, it is recommended to use -nifs() attribute in any module that load NIFs, to allow the compiler to make better decisions regarding optimizations.

There is no need to add -nifs([]) in modules that do not load NIFs. The lack of any call to erlang:load_nif/2, from within the module, is enough for the compiler to draw the same conclusion.

Change

The special meaning for the -nifs() attribute was introduced in Erlang/OTP 25.0. In previous releases, the -nifs() was accepted, but had no special meaning.

It is possible to specify that the module is the callback module for a behaviour:

-behaviour(Behaviour).

The atom Behaviour gives the name of the behaviour, which can be a user-defined behaviour or one of the following OTP standard behaviours:

  • gen_server
  • gen_statem
  • gen_event
  • supervisor

The spelling behavior is also accepted.

The callback functions of the module can be specified either directly by the exported function behaviour_info/1:

behaviour_info(callbacks) -> Callbacks.

or by a -callback attribute for each callback function:

-callback Name(Arguments) -> Result.

Here, Arguments is a list of zero or more arguments. The -callback attribute is to be preferred since the extra type information can be used by tools to produce documentation or find discrepancies.

Read more about behaviours and callback modules in OTP Design Principles.

The same syntax as for module attributes is used for record definitions:

-record(Record,Fields).

Record definitions are allowed anywhere in a module, also among the function declarations. Read more in Records.

The same syntax as for module attributes is used by the preprocessor, which supports file inclusion, macros, and conditional compilation:

-include("SomeFile.hrl").
-define(Macro,Replacement).

Read more in Preprocessor.

The same syntax as for module attributes is used for changing the pre-defined macros ?FILE and ?LINE:

-file(File, Line).

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.

A similar syntax as for module attributes is used for specifying types and function specifications: 

-type my_type() :: atom() | integer().
-spec my_function(integer()) -> integer().

Read more in Types and Function specifications.

The description is based on EEP8 - Types and function specifications, which is not to be further updated.

5.3  The feature directive

While not a module attribute, but rather a directive (since it might affect syntax), there is the -feature(..) directive used for enabling and disabling features.

The syntax is similar to that of an attribute, but has two arguments: 

-feature(FeatureName, enable | disable).

Note that the feature directive can only appear in a prefix of the module.

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.

The compiler automatically inserts the two special, exported functions into each module:

  • Module:module_info/0
  • Module:module_info/1

These functions can be called to retrieve information about the module.

The module_info/0 function in each module, returns a list of {Key,Value} tuples with information about the module. Currently, the list contain tuples with the following Keys: module, attributes, compile, exports, md5 and native. The order and number of tuples may change without prior notice.

The call module_info(Key), where Key is an atom, returns a single piece of information about the module.

The following values are allowed for Key:

module

Returns an atom representing the module name.

attributes

Returns a list of {AttributeName,ValueList} tuples, where AttributeName is the name of an attribute, and ValueList 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.

The list of attributes becomes empty if the module is stripped with the beam_lib(3) module (in STDLIB).

compile

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 beam_lib(3) module (in STDLIB).

md5

Returns a binary representing the MD5 checksum of the module.

exports

Returns a list of {Name,Arity} tuples with all exported functions in the module.

functions

Returns a list of {Name,Arity} tuples with all functions in the module.

nifs

Returns a list of {Name,Arity} tuples with all NIF functions in the module.

native

Return true if the module has native compiled code. Return false otherwise. In a system compiled without HiPE support, the result is always false

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