A code example of decorator pattern in Ruby. The example is taken from Head First Design Pattern book.

decorator.rb

module AbstractBeverage
  def cost
    raise Exception, %Q|You should implement the cost for #{self.class}|
  end
end

class DarkRoast
  include AbstractBeverage
  COST = 0.99
  def cost
    return COST
  end
end

class Espresso
  include AbstractBeverage
  COST = 1.99
  def cost
    return COST
  end
end

module AbstractCondiment
  def cost
    raise Exception, %Q|You should implement the cost for #{self.class}|
  end
end

class Mocha
  COST = 0.20
  include AbstractCondiment
  def initialize(beverage)
    @beverage = beverage
  end
  def cost
    return COST + @beverage.cost
  end
end

class Whip
  COST = 0.10
  include AbstractCondiment
  def initialize(beverage)
    @beverage = beverage
  end
  def cost
    return COST + @beverage.cost
  end
end


# The darkroast
dark_roast = DarkRoast.new

#Double Mocha
dark_roast = Mocha.new(dark_roast)
dark_roast = Mocha.new(dark_roast)
# and a Whip
dark_roast = Whip.new(dark_roast)

#Ooops! how much it cost ???
puts dark_roast.cost