Note: the foo object in ruby diagram also has an corresponding singleton class. And the
extend key word is a short-cut to open the singleton class.
moddule m // here is the module // do whatever you want end foo.exend m
foo.instance_eval do // here the singleton class of object foo is open // do whatever you want end
By the way, Metaprogramming Ruby 2 is really great a book for rubyists.