cache_annotation.rb

module Cacheable
  def cache(method_name)
    orig_name = :"__orig__#{method_name}"
    alias_method orig_name, method_name
    define_method method_name do
      __send__(orig_name).tap do |res|
        self.class.class_eval do
          undef_method method_name
          undef_method orig_name
          define_method method_name do
            res
          end
        end
      end
    end
  end
end

class A
  extend Cacheable

  cache def foo
    sleep 1
    :foo
  end
end

a = A.new
p a.foo
p a.foo