otp_2.ex

defmodule Calculator do
  use GenServer

  ###
  # External API

  def start_link(initial \\ 0) do
    GenServer.start_link(__MODULE__, initial, name: __MODULE__)
  end

  def plus(number) do
    GenServer.call(__MODULE__, {:plus, number})
  end

  def minus(number) do
    GenServer.call(__MODULE__, {:minus, number})
  end

  def times(number) do
    GenServer.call(__MODULE__, {:times, number})
  end

  def divided_by(number) do
    GenServer.call(__MODULE__, {:divided_by, number})
  end

  ###
  # GenServer implementation

  def handle_call({:plus, number}, _from, current_number) do
    new_number = current_number + number
    {:reply, new_number, new_number}
  end

end