Elixir callbacks

callback.ex

defmodule Mod do
  use GenServer

  def start_link(_args) do
    GenServer.start_link(__MODULE__, :ok)
  end

  def init(:ok) do
    send(self(), :work)
    {:ok, :ok}
  end

  @callback handle_work() :: any

  defmacro __using__(opts) do
    quote location: :keep do
      use GenServer # Just to get the child_spec
      @behaviour Mod

      # Inject a default implementation on `use`
      def handle_work() do
        IO.puts "Not implemented!"
      end

      defoverridable [handle_work: 0]
    end
  end

  def handle_info(:work, state) do
    IO.puts "Work!"

    handle_work() # Calling the defined callback hopefully

    Process.send_after(self(), :work, 1_000)
    {:noreply, state}
  end
end

defmodule Using do
  use Mod

  def start_link(args) do
    Mod.start_link(args)
  end

  def handle_work() do
    IO.puts "Implemented!"
  end
end