macros.ex

defmodule Mozzie.Service.DSL do
  ...

  defmacro action(p1, p2, p3, do: block) do
    p1 = to_list(p1)
    p2 = to_list(p2)

    quote do
      def handle_action(unquote(p1), unquote(p2), unquote(p3)) do
        case unquote(block) do
          {:ok, val} -> {:ok, val}
          {:error, val} -> {:error, val}
          _ -> :ok
        end
      end
    end
  end
end

defmodule Mozzie.Service.Spotify do
  use Mozzie.Service.DSL
  
  action("start", "song", _opts), do: play_song() # This doesn't work

  action("start", "song", _opts) do # This works
    play_song()
  end
end