From: transfire@...
Date: 2006-06-14T14:24:19+09:00
Subject: Re: Why the lack of mixing-in support for Class methods?

Well, I hate to throw a wrench in the works at this point, but (by
God's grace) I may have managed a better solution. It can be
implemented in either of two ways. Either by extending module with some
special methods like so:

  class Module

    def propagating_extensions
      @propagating_extensions ||= []
    end

    def propagate( *mods )
      propagating_extensions.concat mods
    end

    def extend_and_propagate( *mods )
      propagate *mods
      extend *mods
    end

    alias_method :append_features_without_propagation, :append_features

    def append_features( base )
      unless propagating_extensions.empty?
        base.extend_and_propagate *propagating_extensions
      end
      append_features_without_propagation( base )
    end

  end

Or alternatively sepearating the capability into a subclass of module:

  class Capsule < Module

    def extensions
      @extensions ||= []
    end

    def propagate( *mods )
      extensions.concat mods
    end

    def extend( *mods )
      propagate *mods
      super
    end

    def append_features( base )
      unless extensions.empty?
        base.extend *extensions
      end
      super
    end

  end

The later is a little more elegant in implementation but requires the
use of the specialized class to continue the propogating behavior. The
former doesn't need the special class, but does require an additional
special method.

Here's an example of using the first implementation.

  module Foo
    def foo ; "foo" ; end
  end

  module Bar
    extend_and_propagate Foo
  end

  class X
    include Bar
  end

  p X.foo      #=> "foo"

And here's an example of the second:

  module Foo
    def foo ; "foo" ; end
  end

  Bar = Capsule.new do
    extend Foo
  end

  class X
    include Bar
  end

  p X.foo       #=> "foo"

T.