decorator.rb

module Decorator
  def initialize(decorated)
    @decorated = decorated
  end

  def method_missing(method,*args)
    # empty? is a method; ?: is a tri-operator
    args.empty? ? @decorated.send(method):@decorated.send(method,args)
  end

  # Decorator does not change the nature of object it decorates;
  def kind_of?(name)
    @decorated.kind_of?name
  end
end

class Coffee
  def cost
    2
  end

  def taste
    p "taste good!"
  end
end

class Milk
  include Decorator
  def cost
    @decorated.cost+0.4
  end
end

class Whip
  include Decorator
  def cost
    @decorated.cost+0.2
  end
end

my_coffee = Whip.new(Milk.new(Coffee.new))
p my_coffee.cost;
p my_coffee.kind_of?Coffee
my_coffee.taste