From: Jeff Cohen <cohen.jeff@...>
Date: 2006-10-16T14:37:25+09:00
Subject: Ruby for Rails p.462-464 - why include vs. extend?

Making my way through Ruby for Rails (excellent book, David), but am 
puzzled by the explanation of why Rails jumps through hoops to turn 
Module instance methods into class methods.

On p. 463, David represents the situation sans Rails:

module A
  module M
   module ClassMethods
     def some_method
       #...
     end

   def included(c)
     c.extend(ClassMethods)
   end
  end
end

and then a class later that does this:

class B
  include A::M
end

The goal as for M::ClassMethods to become class methods of B.  Whew.

Here are my questions:

1. Why can't class B just extend A::M::ClassMethods?  Why use include 
and therefore necessitate this whole indirect approach?  Must be some 
advantage that I'm not seeing?

In other words, why couldn't this have worked?

class B
  extend A::M::ClassMethods
end

and then there's no need M::included() needed at all?

2. I guess because M::included() was overridden, class B did not get any 
instance methods - which I think would have normally been the case with 
an include statement.  SO what if you wanted the "normal" inclusion 
behavior, but also wanted to do something "extra" when your module is 
included?

Thanks!
Jeff

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