elixir GenServer issue

gistfile1.txt

defmodule MyApp.MainWorker do
  use GenServer
  # Client
  def start_link(default) do
    GenServer.start_link(__MODULE__, default)
  end

  def add(pid, num) do
    GenServer.call(pid, {:add, num})
  end

  def subtract(pid, num) do
    GenServer.call(pid, {:subtract, num})
  end

  def get_state(pid) do
    GenServer.call(pid, :get_state)
  end

  # Server (callbacks)
  def handle_call({:add, num}, state) do
    {:reply, num + state}
  end

  def handle_call({:subtract, num}, state) do
    {:reply, num - state}
  end

  def handle_call(:get_state, _from, current_state) do
    {:reply, current_state, current_state}
  end

  def handle_call(request, from, state) do
    super(request, from, state)
  end
end