From: Sebastian <shfriedrich@...>
Date: 2007-01-25T09:55:09+09:00
Subject: Class Variables and Singelton Mix-Ins

I'm trying to find a way to declare class variables in a module that 
can be mixed into class methods and retain scope for only that 
particular class instead of being defacto module-variables.

Here's a simplified example (albeit somewhat silly) of what i'm trying to do:

module Foo
  module Bar
      #...some instance methods

    def self.included(klass)
      klass.extend(BarClassMethods)
    end

  end

  module BarClassMethods

    def greeting
      @@greeting ||= self.determine_greeting
    end

    def determine_greeting
      hello
    end

  end
end

class A
  include Foo::Bar

     def self.hello
       "hello from A"
     end

end

class B
  include Foo::Bar

  def self.hello
     "hello from B"
   end

end

puts A.greeting
#=> 'hello from A'

puts B.greeting
#=> 'hello from A'

The result i would hope for is "hello from A" and "hello from B" 
respectively. But since @@greeting is really scoped module-wide it is 
only intialized once. How can i force the class variable to pertain 
only to the singelton class that mixes it in?