プリミティブなプロセスで実装

stopwatchB.ex

defmodule StopwatchB do
  import System, only: [ monotonic_time: 1 ]
  
  @gclock __MODULE__
  
  def activate() do
    now = monotonic_time(100)
    pid = spawn(StopwatchB, :watch_loop, [now])
    Process.register(pid, @gclock)
    :ok
  end

  def watch_loop(last_time) do
    receive do
      {:restart, caller} ->
        now = monotonic_time(100)
        send caller, {:ok, now - last_time}
        watch_loop(now)
      {:lap, caller} ->
        now = monotonic_time(100)
        send caller, {:ok, now - last_time}
        watch_loop(last_time)
    end
  end
  
  def restart() do
    send @gclock, {:restart, self()}
    receive do
      {:ok, time} -> time
    end
  end
  
  def lap() do
    send @gclock, {:lap, self()}
    receive do
      {:ok, time} -> time
    end
  end
end