# test_helper.exs
ExUnit.start()

# my_super_module_test.exs
defmodule MySuperModuleTest do
  use ExUnit.Case

  test "add two numbers" do
    result = MySuperModule.add 1, 2

    assert result == 3
  end
end

# mix.exs (Elixir 1.4)
def deps do
  [{:mix_test_watch, "~> 0.5", only: :dev, runtime: false}]  
end

# mix.exs (Elixir 1.3 and earlier)
def deps do
  [{:mix_test_watch, "~> 0.5", only: :dev}]
end

# Domain
module PoetryReader do
  @poetry_library Application.get_env(:poetry_library)

  def give_me_some_poetry do
    @poetry_library.get_a_poem()
    |> do_some_other_transformation
  end
end

module PoetryLibrary do
  @callback get_a_poem() :: {:ok, String.t()}
end

# Infrastructure
module PoetryLibrary.Http do
  @behaviour MyApp.PoetryLibrary
  
  def get_a_poem() do
    url = "https://some-domain.com/poems/random"
    headers = [{"Accept", "application/json"}]

    HTTPoison.get!(url, headers)
    |> Map.get(:body)
    |> decode_response_into_string
  end
end

# Application setup
config :poetry_library, PoetryLibrary.Http

def deps do
  [{:mox, "~> 0.3", only: :test}]  
end

# test/test_helper.ex
Mox.defmock(PoetryLibrary.Mock, for: PoetryLibrary)
Application.put_env(:poetry_library, PoetryLibrary.Mock)

# As a stub
PoetryLibrary.Mock
  |> stub(:get_a_poem, fn() -> {:ok, "Some verses for you"} end)

# As a mock
PoetryLibrary.Mock
  |> expect(:get_a_poem, fn() -> {:ok, "Some verses for you"} end)