Tried Elixir "Mocks-as-noun" tests of seminal http://blog.plataformatec.com.br/2015/10/mocks-and-explicit-contracts/ fame. Hated them. They require a lot of boilerplates and force duplications. Besides controllers aren't well designed for unit testing. Looks like isolated unit testing is not very compatible with Phoenix controllers :(

mock_as_noun_testing_in_elixir_controllers_is_a_pain.ex

defmodule MyApp.AuthController do
  use MyApp.Web, :controller

  # 1. I need that \\ default arg here. I guess I can live with this. Explicit deps, pure functions, why not.
  def callback(%{assigns: %{ueberauth_auth: auth}} = conn, _params, auth_service \\ MyApp.Auth) do
    case auth_service.sign_up_or_sign_in(auth) do
      {:ok, user} ->
        conn
        # 0. More of a problem with controller unit tests than with mocks, but still related as it's about isolation unit tests:
        #    `#put_session` won't work here in my unit test
        #    because it requires the session to have been initialized and fetched in an upstream Plug.
        |> put_session(:current_user, user)
        |> redirect(to: page_path(Endpoint, :index))
      {:error, reason} ->
        conn
        |> put_flash(:error, gettext("Authentication failed: %{reason}", reason: reason))
        |> redirect(to: auth_path(Endpoint, :sign_in))
    end
  end
end

defmodule MyApp.Auth do
  # 2. I need all this noisy behaviour just my mock to be "verifying" (ensure mocked methods exist int the desired object) in my test
  # It's useful nowhere else, forces me to burden the code because of the tests
  # Do I really need to do this for every module that might end up in another's unit test? (likely all modules period)
  defmodule Behaviour do
    @module "Behaviour for testing mocks consistency"
    @callback sign_up_or_sign_in(Ueberauth.Auth.t) :: {:ok, User.t} | {:error, reason :: String.t}
  end

  @behaviour Behaviour

  def sign_up_or_sign_in(_auth) do
  end
end


defmodule MyAppTest do
  test "GET /callback, with successful auth", %{conn: conn} do
    defmodule SuccessfulAuthTest do
      @behaviour Auth.Behaviour
      def sign_up_or_sign_in(_auth) do
        # 3. No access to the test context here so:
        #   3.1. I cannot make any assertion here on the params passed (no access to #assert)
        #   3.2. I can't use a var from the test and need to redefine the return val (%User{}) both here and in the assertion: Not DRY
        {:ok, %User{}}
      end
    end

    successful_conn = Map.put(conn, :assigns, %{ueberauth_auth: %{}})

    result = MyApp.AuthController.callback(successful_conn, %{}, SuccessfulAuthTest)

    assert get_session(result, :user) == %User{}
    assert redirected_to(result, page_path(MyApp.Endpoint, :index))
  end
end

@bigfive: Wow, dynamic module resolution, that's pretty cool :) Looks like it solves both the optional arg and the behaviour boilerplate. Seems like it would make a very useful open-source package :)

I guess you could even make the Dependencies resolver look someplace else than the app config (like a mere Map) in tests so you don't have to update/restore the config.

Lots of food for thoughts, thanks :D