memoize.rb

module Memoize
  def memoize(*names)
    names.each do |name|
      target = instance_method(name)
      wrapped = lambda do |arg|
        @_cache_ ||= {}
        @_cache_[name] ||= {}
        @_cache_[name][arg] || @_cache_[name][arg] = target.bind(self).call(arg)
      end
      define_method name, &wrapped
    end
  end
end

# Usage
class Foo
  extend Memoize

  def fib(n)
    n <= 1 ? 1 : fib(n -1) + fib(n - 2)
  end

  def fib2(n)
    n <= 1 ? 1 : fib2(n - 1) + fib2(n - 2)
  end

  memoize :fib2
end