From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-07-26T15:33:48+09:00
Subject: Re: Beginning meta-programming question

On Mon, Jul 26, 2010 at 6:48 AM, Andrew Wagner <wagner.andrew@gmail.com> wrote:
> I'm pretty new to ruby, and just playing around. I can't work out why the
> following won't work, though:
>
>   1.  module TrackInheritors
>   2.    def self.extended(parent)
>   3.      class << parent
>   4.        puts self
>   5.        @@inheritors = []
>   6.
>   7.        def self.inheritors
>   8.          @@inheritors
>   9.        end
>   10.
>   11.        def self.inherited(child)
>   12.          @@inheritors << child
>   13.        end
>   14.      end
>   15.    end
>   16.  end
>   17.
>   18.  class Parent
>   19.    extend TrackInheritors
>   20.  end
>   21.
>   22.  class Child1 < Parent
>   23.  end
>   24.
>   25.  class Child2 < Parent
>   26.  end
>   27.
>   28.  puts Parent.inheritors
>   29.
>   30.  # -:28: undefined method `inheritors' for Parent:Class
>   (NoMethodError)
>

When you open the singleton class of parent, you don't need to define
the methods for "self.", just define them directly, cause you are
already on the singleton class:

module TrackInheritors
  def self.extended(parent)
    class << parent
      puts self
      @@inheritors = []

      def inheritors
        @@inheritors
      end

      def inherited(child)
        @@inheritors << child
      end
    end
  end
end

 class Parent
   extend TrackInheritors
 end

 class Child1 < Parent
 end

 class Child2 < Parent
 end

 puts Parent.inheritors


$ ruby inheritors.rb
#<Class:Parent>
Child1
Child2

I'm not really sure where were you defining those methods, in the
singleton class of the singleton class?

Jesus.