From: Alex Young <alex@...>
Date: 2006-07-26T08:50:59+09:00
Subject: Re: Automagic class prototyping

Aleks Kissinger wrote:
> Anybody know any slick techniques for automatically prototyping
> classes so they can interact with each other in top-level code?  I've
> run through two methods, but I'm not real wild about either of them.
> 
> Say for example, classes represent some security label and I want to
> allow things with one label to access those of another label.  So,
> these hypothetical classes all descent from Sec....
> 
> class Sec
>  class <<self
>   def allow(c)
>    (@allowed_classes||=[]) <<c
>   end
>  end
> end
> 
> 
> And I want to ultimately just indescriminately run stuff like this:
> 
> class Documents < Sec
>  allow Downloads
> end
> 
> class Downloads < Sec
>  allow Documents
> end
> 
> But this is a run-time error unless the classes are prototyped somehow.  
> Like:
> 
> class Downloads < Sec; end
> 
> class Documents < Sec
>  allow Downloads
> end
> 
> class Downloads < Sec
>  allow Documents
> end
> 
> As a framework, this would be so much nicer for the end-user if it
> does that automatically, so she doesnt have to thing about declaration
> order.  So thats the problem.  Possible solutions I've tried:
> 
> 1. Grep-prototyping:
> File.grep out all class defs and eval them before I actually require
> the code.  I don't like this because it chokes on classes defined
> inside of modules or other classes.  Grep has no idea about the
> structure of the ruby code.
> 
> 2. Delayed execution of the 'allow' statements.
> I dump allow statements off into proc's then after all the definitions
> are loaded, I run them.  This works great, but syntactically its
> clunky.

3:
class Sec
   class <<self
     def allow(c)
       begin
         Kernel.const_get(c)
       rescue NameError #is there a better way to do this?
         class c < Sec; end
       end
       (@allowed_classes||=[]) <<c
     end
   end
end

There's probably stuff in there to trip you up, but you get the idea. 
I'm not too keen on using an exception like that, but I can't seem to 
find a better or more succinct way to test for the existence of a class.

-- 
Alex