cached.rb

module Cached
  def cache(*method_names)
    method_names.each do |m|
      original = instance_method(m)
      results  = {}

      define_method(m) do |*a|
        results[[m, a]] ||= original.bind(self).call(*a) 
      end
    end
  end
end

## EXAMPLE USAGE:

class Numbers
  extend Cached

  def fib(n)
    raise ArgumentError if n < 0
    return n if n < 2

    fib(n - 1) + fib(n - 2)
  end

  cache :fib
end

n = Numbers.new
  
(0..100).each { |e| p [e, n.fib(e)] }