From: transfire@...
Date: 2006-06-11T15:09:51+09:00
Subject: Re: Why the lack of mixing-in support for Class methods?

Yukihiro Matsumoto wrote:

> But I am still not sure if we need to provide the way to inject module
> methods to a class's singleton class at the same time instance methods
> are inherited to the class.  Could somebody elaborate?  It seems that
> having two separate modules (one to include for instance methods and
> another to extend for class methods) is more natural for me.

I think the clearest use is with DSLs that also has useful instance
methods. Currently DSLs of this sort can only be provided via
subclasses. As you say, you can split it into two modules but if the
methods go together inextrobaly, then this is very inconvenient --and
unacceptably so when building frameworks and asking others to make use
of such tools. But sometimes it goes beyond mere inconvenience b/c an
instance method might call on a class method. The most basic example of
this kind of inter-relationship looks like this:

  module M
    class << self
      attr_accessor :foo
    end
    def foo
      self.class.foo
    end
  end

  class X
    self << M
  end

It's a class level setting, but with a way to easily access it from the
instance level. To split this up into two modules is clearly a forced
concession.

  module Mc
      attr_accessor :foo
  end

  module Mi
    def foo
      self.class.foo
    end
  end

  class X 
    extend Mc
    include Mi
  end

T.