game.rb

require "data_bank"

class Node < Struct.new(:question, :yes, :no)
  def call(*)
    puts question
    answer = gets

    case answer.chomp
    when ?y
      yes.call(self)
    when ?n
      no.call(self)
    else
      throw :quit
    end
  end

  def replace(yes_or_no)
    puts "What is it?"
    answer = Thing.new(gets.chomp)

    puts "Give me a question that would be yes for #{yes_or_no}, and no for #{answer}"
    question = gets.chomp

    node = Node.new(question, yes_or_no, answer)
    self.yes = node if yes_or_no == yes
    self.no = node if yes_or_no == no
  end
end

Thing = Struct.new(:name) do
  def call(parent)
    puts "Is it a #{name}?"
    answer = gets

    case answer.chomp
    when ?y
      puts "Got it!"
    when ?n
      parent.replace(self)
    end
  end

  def to_s
    name
  end
end

fish = Thing.new("fish")
tree = Thing.new("tree")

db = DataBank.new(".", "juliet.data")
head = db.withdraw || Node.new("Is it an animal?", fish, tree)

until false do
  head.call
  db.deposit head
  puts "="*100
end

catch :quit do
  puts "Thanks for playing"
end