From: Alder Green <alder.green@...>
Date: 2006-06-08T17:02:35+09:00
Subject: Re: Why the lack of mixing-in support for Class methods?

On 6/8/06, Phil Tomson <rubyfan@gmail.com> wrote:
> On 6/7/06, Alder Green <alder.green@gmail.com> wrote:
> > Hi
> >
> > I'm confused as to why class methods aren't include'd by default along
> > with instance methods when mixing-in modules.
> >
> > It obstructs the important orthoganility between modules and classes,
> > and - among other problems - thus undermines the position of mixins as
> > an alternative to class-based multiple inheritance.
> >
> > In short, I can see a lot of poor consequence to that lack of support,
> > while on the other hand no good reason for it. Maybe someone would
> > care to elighten me?
> >
> > (I'm aware of the various hacks for fudging such support, and in fact
> > use some of them in my code. But there are readability and
> > maintainability issues with those hacks for something that's supposed
> > to be a core feature).
> >
>
> The main reason is because a different 'self' is involved, I would guess.
> You can use extend to mixin class methods.  Also, there's a nice idiom
> for getting include to mixin both class and instance methods of a
> module (though the class methods are actually in another sub-module):
>
> module A
>   def self.included receiver
>     receiver.extend ClassMethods
>   end
>
>   module ClassMethods
>     def foo
>       "foo"
>     end
>   end
>   def inst_method
>     "instance method"
>   end
> end
>
> class B
>   include A
> end
>
> B.foo #=> "foo"
> B.new.inst_method #=> "instance method"
>
>

Yes, I'm using this idiom in my code. Note, however, that if you want
to use A'd "class methods" from A itself, you have to add the  extra
line:

self.extend(ClassMethods)

after A::ClassMethods's definition.

Still, it's unclear to me why this very natural functionality isn't
built into the default append_features. It hinders the ease of going
from a module to a class or vice-versa, which Ruby seems to encourage.
In any case, every other aspect of switching between those two is
well-handled, so why not this one as well?  Are there plans to add it
for Ruby 2?

-- 
-Alder