memoize.rb

module Memoize
  def memoize(method_name)
    ivar_name = :"@#{method_name}"
    aliased_method_name = :"__unmemoized_#{method_name}"

    alias_method aliased_method_name, method_name
    define_method method_name do
      return instance_variable_get(ivar_name) if instance_variable_defined?(ivar_name)

      instance_variable_set(ivar_name, send(aliased_method_name))
    end
  end
end