Behaviours in

About Behaviours

Behaviours allow us to define interfaces (sets of functions and macros) in a behaviour module that can be later implemented by different callback modules. Thanks to the shared interface, those callback modules can be used interchangeably.

Defining behaviours

To define a behaviour, we need to create a new module and specify a list of functions that are part of the desired interface. Each function needs to be defined using the @callback module attribute. The syntax is identical to a function typespec (@spec). We need to specify a function name, a list of argument types, and all the possible return types.

defmodule Countable do
  @callback count(collection :: any) :: pos_integer
end

Implementing behaviours

To add an existing behaviour to our module (create a callback module) we use the @behaviour module attribute. Its value should be the name of the behaviour module that we're adding.

Then, we need to define all the functions (callbacks) that are required by that behaviour module. If we're implementing somebody else's behaviour, like Elixir's built-in Access or GenServer behaviours, we would find the list of all the behaviour's callbacks in the documentation on hexdocs.pm.

A callback module is not limited to implementing only the functions that are part of its behaviour. It is also possible for a single module to implement multiple behaviours.

To mark which function comes from which behaviour, we should use the module attribute @impl before each function. Its value should be the name of the behaviour module that defines this callback.

defmodule BookCollection do
  @behaviour Countable

  defstruct [:list, :owner]

  @impl Countable
  def count(collection) do
    Enum.count(collection.list)
  end

  def mark_as_read(collection, book) do
    # other function unrelated to the Countable behaviour
  end
end

Default callback implementations

When defining a behaviour, it is possible to provide a default implementation of a callback. This implementation should be defined in the quoted expression of the __using__/1 macro. To make it possible for users of the behaviour module to override the default implementation, call the defoverridable/1 macro after the function implementation. It accepts a keyword list of function names as keys and function arities as values.

defmodule Countable do
  @callback count(collection :: any) :: pos_integer

  defmacro __using__(_) do
    quote do
      @behaviour Countable
      def count(collection), do: Enum.count(collection)
      defoverridable count: 1
    end
  end
end

Note that defining functions inside of __using__/1 is discouraged for any other purpose than defining default callback implementations, but you can always define functions in another module and import them in the __using__/1 macro.