Provides functions to deal with modules during compilation time.

It allows a developer to dynamically add, delete and register attributes, attach documentation and so forth.

After a module is compiled, using many of the functions in this module will raise errors, since it is out of their scope to inspect runtime data. Most of the runtime data can be inspected via the __info__(attr) function attached to each compiled module.

Module attributes

Each module can be decorated with one or more attributes. The following ones are currently defined by Elixir:

• @after_compile

  A hook that will be invoked right after the current module is compiled. Accepts a module or a tuple {<module>, <function atom>}. See the “Compile callbacks” section below.

• @before_compile

  A hook that will be invoked before the module is compiled. Accepts a module or a tuple {<module>, <function/macro atom>}. See the “Compile callbacks” section below.

• @behaviour (notice the British spelling)

  Behaviours can be referenced by modules to ensure they implement required specific function signatures defined by @callback.

  For example, you can specify the URI.Parser behaviour as follows:

  defmodule URI.Parser do
    @doc "Parses the given URL"
    @callback parse(uri_info :: URI.t) :: URI.t
  
    @doc "Defines a default port"
    @callback default_port() :: integer
  end
  
  And then a module may use it as:
  
  defmodule URI.HTTP do
    @behaviour URI.Parser
    def default_port(), do: 80
    def parse(info), do: info
  end

  If the behaviour changes or URI.HTTP does not implement one of the callbacks, a warning will be raised.

  Specifies an OTP or user-defined behaviour.

  Example

  defmodule M do
    @behaviour :gen_event
  
    # ...
  end

• @callback, @macrocallback, and @optional_callbacks

  These attributes are used to define a behaviour (as shown in the documentation for @behaviour above). @callback defines a function callback, @macrocallback defines a macro callback, and @optional_callbacks specifies which callbacks and macrocallbacks are optional.

• @compile

  Defines options for module compilation. This is used to configure both Elixir and Erlang compilers, as any other compilation pass added by external tools.

  Multiple uses of @compile will accumulate instead of overriding previous ones. See the “Compile options” section below.

  Example

  defmodule M do
    @compile {:inline, myfun: 1}
  
    def myfun(arg) do
      to_string(arg)
    end
  end

• @doc

  Provides documentation for the function or macro that follows the attribute.

  Accepts a string (often a heredoc) or false where @doc false will make the function/macro invisible to the documentation extraction tools like ExDoc.

  Can be invoked more than once.

  Example

  defmodule M do
    @doc "Hello world"
    def hello do
      "world"
    end
  
    @doc """
    Sums `a` to `b`.
    """
    def sum(a, b) do
      a + b
    end
  end

• @dialyzer

  Defines warnings to request or suppress when using a version of :dialyzer that supports module attributes.

  Accepts an atom, a tuple, or a list of atoms and tuples.

  For the list of supported warnings, see :dialyzer module.

  Multiple uses of @dialyzer will accumulate instead of overriding previous ones.

  Example

  defmodule M do
    @dialyzer {:nowarn_function, myfun: 1}
  
    def myfun(arg) do
      M.not_a_function(arg)
    end
  end

• @external_resource

  Specifies an external resource to the current module.

  Many times a module embeds information from an external file. This attribute allows the module to annotate which external resources have been used.

  Tools like Mix may use this information to ensure the module is recompiled in case any of the external resources change.

• @file

  Changes the filename used in stacktraces for the function or macro that follows the attribute.

  Accepts a string. Can be used more than once.

  Example

  defmodule M do
    @doc "Hello world"
    @file "hello.ex"
    def hello do
      "world"
    end
  end

• @moduledoc

  Provides documentation for the current module.

  Accepts a string (which is often a heredoc) or false where @moduledoc false will make the module invisible to the documentation extraction tools like ExDoc.

  Example

  defmodule M do
    @moduledoc """
    A very useful module
    """
  end

• @on_definition

  A hook that will be invoked when each function or macro in the current module is defined. Useful when annotating functions.

  Accepts a module or a tuple {<module>, <function atom>}. See the “Compile callbacks” section below.

• @on_load

  A hook that will be invoked whenever the module is loaded.

  Accepts a function atom of a function in the current module. The function must have arity 0 (no arguments) and has to return :ok, otherwise the loading of the module will be aborted.

  Example

  defmodule M do
    @on_load :load_check
  
    def load_check do
      if some_condition() do
        :ok
      else
        nil
      end
    end
  
    def some_condition do
      false
    end
  end

• @vsn

  Specify the module version. Accepts any valid Elixir value.

  Example

  defmodule M do
    @vsn "1.0"
  end

The following attributes are part of typespecs and are also reserved by Elixir:

• @type - defines a type to be used in @spec
• @typep - defines a private type to be used in @spec
• @opaque - defines an opaque type to be used in @spec
• @spec - provides a specification for a function
• @callback - provides a specification for a behaviour callback
• @macrocallback - provides a specification for a macro behaviour callback
• @optional_callbacks - specifies which behaviour callbacks and macro behaviour callbacks are optional

In addition to the built-in attributes outlined above, custom attributes may also be added. A custom attribute is any valid identifier prefixed with an @ and followed by a valid Elixir value:

defmodule M do
  @custom_attr [some: "stuff"]
end

For more advanced options available when defining custom attributes, see register_attribute/3.

Compile callbacks

There are three callbacks that are invoked when functions are defined, as well as before and immediately after the module bytecode is generated.

@after_compile

A hook that will be invoked right after the current module is compiled.

Accepts a module or a tuple {<module>, <function atom>}. The function must take two arguments: the module environment and its bytecode. When just a module is provided, the function is assumed to be __after_compile__/2.

Example

defmodule M do
  @after_compile __MODULE__

  def __after_compile__(env, _bytecode) do
    IO.inspect env
  end
end

@before_compile

A hook that will be invoked before the module is compiled.

Accepts a module or a tuple {<module>, <function/macro atom>}. The function/macro must take one argument: the module environment. If it’s a macro, its returned value will be injected at the end of the module definition before the compilation starts.

When just a module is provided, the function/macro is assumed to be __before_compile__/1.

Note: unlike @after_compile, the callback function/macro must be placed in a separate module (because when the callback is invoked, the current module does not yet exist).

Example

defmodule A do
  defmacro __before_compile__(_env) do
    quote do
      def hello, do: "world"
    end
  end
end

defmodule B do
  @before_compile A
end

@on_definition

A hook that will be invoked when each function or macro in the current module is defined. Useful when annotating functions.

Accepts a module or a tuple {<module>, <function atom>}. The function must take 6 arguments:

• the module environment
• kind: :def, :defp, :defmacro, or :defmacrop
• function/macro name
• list of quoted arguments
• list of quoted guards
• quoted function body

Note the hook receives the quoted arguments and it is invoked before the function is stored in the module. So Module.defines?/2 will return false for the first clause of every function.

If the function/macro being defined has multiple clauses, the hook will be called for each clause.

Unlike other hooks, @on_definition will only invoke functions and never macros. This is because the hook is invoked inside the context of the function (and nested function definitions are not allowed in Elixir).

When just a module is provided, the function is assumed to be __on_definition__/6.

Example

defmodule H do
  def on_def(_env, kind, name, args, guards, body) do
    IO.puts "Defining #{kind} named #{name} with args:"
    IO.inspect args
    IO.puts "and guards"
    IO.inspect guards
    IO.puts "and body"
    IO.puts Macro.to_string(body)
  end
end

defmodule M do
  @on_definition {H, :on_def}

  def hello(arg) when is_binary(arg) or is_list(arg) do
    "Hello" <> to_string(arg)
  end

  def hello(_) do
    :ok
  end
end

Compile options

The @compile attribute accepts diverse options that is used by both Elixir and Erlang compilers. Some of the common use cases are documented below:

• @compile :debug_info - includes :debug_info regardless of the setting in Code.compiler_options

• @compile {:debug_info, false} - disables :debug_info regardless of the setting in Code.compiler_options

• @compile {:inline, some_fun: 2, other_fun: 3} - inlines the given name/arity pairs

• @compile {:autoload, false} - disables automatic loading of modules after compilation. Instead, the module will be loaded after it is dispatched to

You can see a handful more options used by the Erlang compiler in the documentation for the :compile module.