otp_1.ex

defmodule MyServer do
  use GenServer

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

end

{:ok, pid} = GenServer.start_link(MyServer, 10, name: :calculator)

GenServer.call(pid, {:plus, 10})
# 20

GenServer.call(:calculator, :plus)
# 30