wallet_with_pocket.rb

# ポケットモジュールを定義
module Pocket
  def insert_card(card)
    if @pocket
      raise "already exist in pocket"
    end

    @pocket = card
  end

  def pull_card
    if not @pocket
      return nil
    end

    card = @pocket
    @pocket = nil
    return card
  end
end

# Wallet に Pocket をつける
class Wallet
  include Pocket

  def initialize
    @amount = 0
  end

  def put(money)
    @amount += money
  end

  def pull_out(money)
    if @amount < money
      raise "be short of the needful"
    end

    @amount -= money
  end

  def get_balance
    return @amount
  end
end

wallet = Wallet.new
wallet.put 10000
puts wallet.get_balance # => 10000
wallet.pull_out 500
puts wallet.get_balance # => 9500
wallet.insert_card "visa card"
puts wallet.pull_card # => visa card