memoizing on object level

memoizing_singletons.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(obj, method)
  ghost = class << obj ; self; end
  ghost.class_eval do
    define_method(method) do |*args|
      @memory ||= {}
      @memory[args] = super unless @memory.has_key?(args)
      @memory
    end
  end
end
d = Discounter.new
memoize(d, :discount)

d.discount(1,2,3)
d.discount(1,2,3)
d.discount(4,2,3)
d.discount(4,2,3)