Question

When an instance of an object extends a module and extended is called on the module does the base already extend module?

module M
  def self.extended base
    # when this is called has base extended
  end

  def self.some_method
    # that does something special ;)
  end
end

obj = Object.new
obj.extend M

Update: Okay, so an Object and a String works, but why doesn’t numbers work? I get TypeError: can't define singleton

Answer 1

You cannot define a singleton on a Fixnum in ruby, because there is really only one of them (for each Fixnum). For example (in IRB):

1.object_id
=> 3
1.object_id
=>3

Unlike strings, for example:

"panda".object_id
=> 78405050
"panda".object_id
=> 78383110

Obviously the specific numbers are going to vary on your system.

This makes sense, since there is no way for a particular “instance” of 73 to be different from any other “instance” of 73 (and I use the word “instance” loosely because it isn’t really an instance – they are all the same object).

http://ruby-doc.org/core-1.9.3/Fixnum.html