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>}. 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

• @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

• @compile

  Defines options for module compilation that are passed to the Erlang compiler.

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

  For the list of supported options, see Erlang’s :compile module.

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

  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

• @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>}. 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

• @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

• @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.

• @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

The following attributes are part of typespecs and are also reserved by Elixir (see Kernel.Typespec for more information about typespecs):

• @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

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.

Runtime information about a module

It is possible to query a module at runtime to find out which functions and macros it defines, extract its docstrings, etc. See __info__/1.