Messages and timeouts

# Prepare one reference for process exit and another for the reply
def call(server, request, timeout \ 1_000) do
  monitor_ref = Process.monitor(server)
  request_ref = make_ref()
  # Tell the server both the return address and request number
  send(server, {:call, self(), request_ref, request})

  receive do
    # A normal reply must match the request reference
    {:reply, ^request_ref, response} ->
      # Stop watching and remove a DOWN message that may already be waiting
      Process.demonitor(monitor_ref, [:flush])
      {:ok, response}

    # A monitor reports exit in one direction; the caller chooses the result
    {:DOWN, ^monitor_ref, :process, ^server, reason} ->
      {:error, {:server_down, reason}}
  after
    # A timeout stops waiting; it does not pull back the request
    timeout ->
      Process.demonitor(monitor_ref, [:flush])
      {:error, :timeout}
  end
end