Define and implement a behaviour in the same module

[Jack Rowlingson]

I had attempted to the following usage:

iex(1)> defmodule MyApp.Calculator do
...(1)>   @behaviour __MODULE__
...(1)>   @callback add(integer(), integer()) :: integer()
...(1)>   def add(x, y), do: x + y
...(1)> end
warning: behaviour MyApp.Calculator is undefined

After some discussion with Muskala (micmus)/Milde (lostkobrakai) it seemed that use case may be possibly be valid. That is, the ability to define and implement a behaviour in the same module.

For my part, I had wanted to locate the documentation and @callback along with the default implementation. I have been diligently factoring my modules into separate files per http://blog.plataformatec.com.br/2015/10/mocks-and-explicit-contracts/. With this approach, however, I am finding that there has been a bit of an explosion with the number of files.

It was suggested that the following is considered an idiomatic way to associate doc with an implementation:

defmodule MyApp.Calculator do
  @callback add(integer(), integer()) :: integer()

  defmacro __using__ do
    quote do
      def add(x, y), do: x + y
    end
  end
end

defmodule MyApp.CalculatorImpl do
  use MyApp.Calculator
  @behaviour MyApp.Calculator
end

But here too I need to define a second file for the default implementation (assuming one module definition per file is desired). I find that there is also a bit of additional complexity here which makes it slightly more difficult to reason about.

Note: Related? https://groups.google.com/forum/#!topic/elixir-lang-core/8jrwjECcFTw

Jack

[José Valim]

Since behaviours are also validated by the Erlang compiler and tools like dialyzer, I don't believe we can make this work without first adding support for this directly to Erlang/OTP.

José Valim

www.plataformatec.com.br

Founder and 

Director of R&D

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