Agentで実装

stopwatchD.ex

defmodule StopwatchD do
  use Agent
  
  import System, only: [ monotonic_time: 1 ]
  
  @gclock __MODULE__
  
  def activate() do
    now = monotonic_time(100)
    Agent.start_link(fn -> now end, name: @gclock)
    :ok
  end

  def restart() do
    Agent.get_and_update(@gclock, fn last_time ->
      now = monotonic_time(100)
      {now - last_time, now}
    end)
  end
  
  def lap() do
    Agent.get(@gclock, &(monotonic_time(100) - &1))
  end
end