A game of ping pong between two Elixir processes

ping_pong.exs

defmodule Player do
  def start(p1, p2) do
    # start their game p1 sends a ping to p2
    send(p2, {:ping, p1})
  end

  def play do
    receive do
      {input_message, _from = opponent_pid} ->
        respond(opponent_pid, input_message)
        play()
    end
  end

  defp respond(opponent_pid, input_message) do
    IO.puts("RECV #{input_message} FROM #{inspect(opponent_pid)}")
    Process.sleep(1000) # sleep for easier reading of output
    send(opponent_pid, {response_for(input_message), _my_pid = self()})
  end

  defp response_for(:ping), do: :pong
  defp response_for(:pong), do: :ping
end

p1 = spawn(Player, :play, [])
p2 = spawn(Player, :play, [])
Player.start(p1, p2)