game.ex

defmodule Game do
  # Client

  def start do
    spawn &Game.play/0
  end

  def move(pid, request) do
    send(pid, {:request, self, request})
    receive do
      {:response, ^pid, response} -> response
    end
  end

  # Server

  def listen do
    receive do
      {:request, pid, request} ->
        Process.put(:last_pid, pid)
        request
    end
  end

  def reply(response) do
    pid = Process.get(:last_pid)
    send(pid, {:response, self, response})
  end

  def play do
    case listen do
      name -> reply "Hello #{name}. You are in a maze. You can go :left or :right."
        case listen do
          :left  -> reply "You are eaten by a grue. Game over, #{name}."
          :right -> reply "You see an old man. He asks your favorite color."
            case listen do
              :blue -> reply "You win #{name}!"
              _     -> reply "He tosses you into a pit. Game over, #{name}."
            end
        end
    end
  end
end

iex

iex(1)> import Game
nil
iex(2)> g = start
#PID<0.69.0>
iex(3)> move g, "Mario"
"Hello Mario. You are in a maze. You can go :left or :right."
iex(4)> move g, :right
"You see an old man. He asks your favorite color."
iex(5)> move g, :blue
"You win Mario!"