From: Robert Klemme <bob.news@...>
Date: 2005-04-15T20:34:36+09:00
Subject: Re: class locals as class methods


"Trans" <transfire@gmail.com> schrieb im Newsbeitrag
news:1113499156.837299.173260@z14g2000cwz.googlegroups.com...
> So I find myself once again creating some classes that have
> attributes/state. I have two options, either the class defines
> a class method for the info, or in a superclass I define a method
> that will set that info to a class var when called.
>
>   # Example A
>   class Thing1 < TheseThings
>     def self.mystate ; "what have you" ;  end
>   end
>
> or
>
>   # Example B
>   class Thing1 < TheseThings
>     mystate "what have you"
>   end
>
> The first is rather, well, ugly. Although it makes sense, it's
> what one does at the instance level all the time (i.e. defining
> methods to fit the interface). The second is nice and dsl-ish,
> but requires a method to be predefined in TheseThings which
> actually creates a number of oddities, like how does one read
> the info out, and still be able to set it to nil? If that's an
> issue one is lead to instead define two class methods, and
> having to use self.maystate = "blah blah", instead.
>
> Now everytime I go about this kind of thing I actually find
> myself writing:
>
>   # Example C
>   class Thing1 < TheseThings
>     mystate = "what have you"
>   end
>
> as that just feels natural. But then I have to go back and fix
> it, of course. So last night it hits me, why not? Why not have
> class locals define class methods? So in this case, Example C
> would basically be like doing Example A. Yes, I know there's
> namespace overlap, but class locals are rare enough that I don't
> think that's a show stopper. Maybe there are  other problems I'm
> not seeing, perhaps there's some OOP formalism that this rubs the
> wrong way. I don't know. But I've given it more than "15 minutes"
> and it's actually growing on me. Am I crazy?

Probably - but then again, aren't we all? :-)

Ok, here's what I'd probably do:

class TheseThings
  module ThingClassProperties
    attr_accessor :mystate

    # needed for further inheritance levels
    def inherited(cl) cl.extend ThingClassProperties end
  end

  extend ThingClassProperties
end

class Thing1 < TheseThings
  self.mystate = "t1 state"
end

class Thing2 < Thing1
  self.mystate = "t2 state"
end

p Thing1.mystate
p Thing2.mystate

13:28:13 [source]: ruby /c/temp/ruby/meta-prog.rb
"t1 state"
"t2 state"

You can make that even smarter by defining property accessors on the fly
etc.

Cheers

    robert