From: ara.t.howard@...
Date: 2006-06-09T00:07:45+09:00
Subject: Re: Why the lack of mixing-in support for Class methods?

On Thu, 8 Jun 2006 transfire@gmail.com wrote:

> I think one would need to set that up oneself using a class variable.
> Rather I would expext this it to behave like it does, just as with a class:
>
>  class X
>    def self.x
>      @x
>    end
>    @x = 10
>  end
>
>  class Z < X
>  end
>
>  X.x  #=> 10
>  Z.x  #=> nil

right.  it's just that 'inheritence' with object generally follows this
pattern

   class B
     attr :b
     def initialize() @b = 'bar' end
   end

   class C < B
   end

   p C.new.b #=> 'bar'

which is to say there are mechanisms, namely initialize and super, for
propagating state.  no such mechanisms exist for class based state.  if one is
proposing an 'inherit' method (which i think is a great idea) then i think
it's important to also toss around ideas for things like Class.class_init
and/or Module.module_init - eg hooks that are provided to accomplish this.  my
preferred approach now is this

   module M
     module ClassMethods
       attr 'a'
       attr 'b'
     end
     module InstanceMethods
     end
     def self.included other
       other.extend ClassMethods
       other.module_eval{ include InstaneMethods }
       init other
     end
     def self.init other
       other.a = 42
       other.b = 'forty-two'
     end
   end

if we don't also consider this then the situation you describe in Nitro, where
there a muliple ways of implimenting class method mixins, will more or less
remain if any of those methods require state - that is to say it'd be a shame
to make every ruby developer roll his is own way of initializing the required
state for the module methods he could mix into his classes so easily.

> You bring up your traits library all the time ;)

yeah, not on purpose though: it just fills a lot of meta-programming and class
inheritence niches which seems to come up often on this list.

> Actually all-in-all I think it's pretty good. Unfortuately I have this one
> nagging problem with it. I think the term 'traits' is a terrible misnomer.
> That may seem silly but having done some study of prototype-base OOPS,
> especially Self, traits are just a totlatlly different concept to me.

well - you can use the 'has' interface

   class C
     has 'c' => 42
     class_has 'b'
   end

it's just an alias - but if you hate the name...

cheers.

-a
-- 
suffering increases your inner strength.  also, the wishing for suffering
makes the suffering disappear.
- h.h. the 14th dali lama