From: "ara.t.howard" <ara.t.howard@...>
Date: 2007-10-21T01:42:03+09:00
Subject: Re: Class instance variable idiom


On Oct 19, 2007, at 7:27 PM, David A. Black wrote:

>
> But then they learn about self and instance variables and they stop
> expecting it :-) But I'm not sure I'm following exactly what you mean
> here -- i.e., what your vision is of how it should behave as opposed
> (?) to how it does behave.
>

it's not a vision - it's production code, gem released for several  
years:

   http://codeforpeople.com/lib/ruby/traits/traits-0.9.2/README

you can ignore all the features except inheritance.  here's an  
example of how it works:


cfp:~ > cat a.rb
require 'traits'

class C
   class << self
     trait 'a' => 42
   end
end

class B < C
end

class A < B
end

B.a 42.0

p A.a
p B.a
p C.a


cfp:~ > ruby a.rb
42
42.0
42



since writing traits i've grown to prefer my attributes library,  
which is < 100 lines of code and allows one to setup 'smart' class  
variable inheritance in only a few lines of code


cfp:~ > cat a.rb
require 'attributes'

class C
   class << self
     attribute('a'){
       catch(:value){
         (ancestors - [self]).each do |ancestor|
           break unless self <= ancestor
           ancestor.module_eval{ throw :value, @a if defined? @a }
         end
         default = 42
       }
     }
   end
end

class D < C; end
class E < D; end
class F < E; end


p D.a
E.a = 42.0
p F.a


cfp:~ > ruby a.rb
42
42.0


i think either for of class variable inheritance is preferable to the  
@@semantics.  it's up in the air whether references or copies should  
be taken though.

regards.

a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama