20150420_defps.exs

defmodule Functions do
  # switch using `def` or `defp` in compile time
  defmacro defpt(func, body \\ nil) do
    case test_env? do
      true -> quote do: def(unquote(func), unquote(body))
      false -> quote do: defp(unquote(func), unquote(body))
    end
  end

  defp test_env? do
    case System.get_env "MIX_ENV" do
      "test" -> true
      _ -> false
    end
  end
end

defmodule Defpt do
  defmacro __using__(_) do
    quote do
      import Functions
    end
  end
end

defmodule Alexa do
  use Defpt

  # use defpt which defines as macro
  defpt hello do
    "hi"
  end
end

# Failed call Alexa.hello if MIX_ENV is not "test"
# Anyone can't find Alexa.hello is used or unused by warning in compile time
# or should cover coverage.
# iex> Me.inu
# ** (UndefinedFunctionError) undefined function Alexa.hello/0
#   Alexa.hello()
#   neko.exs:37: Me.inu/0
defmodule Me do
  def inu do
    IO.puts Alexa.hello
  end
end