memoizing w/sub-class - returning a new class

memoizing_a_class.rb

class Discounter
  def discount(*skus)
    expensive_calc(skus)
  end

  private
  def expensive_calc(*skus)
    puts "in expensive calc for #{skus}"
    skus.inject{|m,n| m + n}
  end
end

def memoize(cls, method)
  Class.new(cls) do
    memory = {}
    define_method(method) do |*args|
      memory ||= {}
      memory[args] = super unless memory.has_key?(args)
      memory[args]
    end
  end
end

d = memoize(Discounter, :discount).new
d.discount(1,2,3)
d.discount(1,2,3)
d.discount(4,2,3)
d.discount(4,2,3)