universal_server.ex

defmodule UniversalServer do

  def universal_server() do
    receive do
      {:become, f} ->
        f.()
    end
  end

  def factorial_server() do
    receive do
      {from, n} ->
        send from, factorial(n)
        factorial_server()
    end
  end

  defp factorial(0) do
    1
  end

  defp factorial(n) do
    n * factorial(n-1)
  end

  def run(n) do
    pid = spawn &universal_server/0
    send pid, {:become, &factorial_server/0}
    send pid, {self(), n}

    receive do
      x -> x
    end
  end
end