exercism/elixir/take-a-number/lib/take_a_number.ex

defmodule TakeANumber do
  def start() do
    spawn(fn -> loop(0) end)
  end

  defp loop(number) do
    new_state = receive do
      :stop -> exit(:ok)
      {:report_state, pid} -> send(pid, number)
      {:take_a_number, pid} -> send(pid, number + 1)
      _ -> number
    end
    loop(new_state)
  end
end