Ping Pong in elixir

ping_pong.ex

defmodule PingPong do
  def random do
    time = :rand.uniform(1000)
    :timer.sleep(time)
    time
  end

  def ping do
    receive do
      {pong_pid} ->
        IO.puts ("Ping")
        random()
        send pong_pid, { self() }
        ping()
    end
  end

  def pong do
    receive do
      {ping_pid}  ->
        IO.puts ("Pong")
        random()
        send ping_pid, { self() }
        pong()
    end
  end

  def run do
    ping_pid = spawn(PingPong, :ping, [])
    pong_pid = spawn(PingPong, :pong, [])
    send ping_pid, {pong_pid}

    receive do
      # but why?
    end
  end
end

PingPong.run