From: "David A. Black" <dblack@...>
Date: 2008-05-02T10:44:16+09:00
Subject: Re: DSL - class-level methods that affect instances

Hi --

On Fri, 2 May 2008, Daniel Waite wrote:

> I'm having a problem of design. I've tried a few configurations of class
> collaboration and responsibility, but they all end up feeling messy.
>
> I'm looking at these (either or, my original attempt was with a module,
> but attr_accessor *looks* like it does what I want):
>
> class C < Translator::Base
>  translate 'this', :to => 'that'
> end
>
> class C
>  include Translator
>  translate 'this', :to => 'that'
> end
>
> With the expectation that...
>
> c = C.new
> c.translations
>
> Would yield a hash of the from and to values.
>
> I have no idea how to break the barrier between class and instance. :(

I'd say: don't think of it as any more of a barrier than exists
between any other two objects. Every object, including a class object,
has the right to maintain state, and to decide what it will and will
not expose. There's no privileged relationship between c and i just
because i is an instance of c.

(I know that sounds kind of doctrinaire, but I like to lay it on thick
since it usually takes some doing to chip away at the presumptive
class/instance special relationship :-)

As Ken suggested, class variables break through the class/instance
boundaries. That's one of the reasons I don't like them :-) You can
certainly do it that way; I'll also show you another possibility,
based on engineering all the objects concerned so that they query each
other where necessary:

module Translator
   attr_reader :translations

   def translate(target, actions)
     @translations ||= []
     @translations.push("Translating #{target} to #{actions[:to]}")
   end
end

class C
   extend Translator

   def translations
     self.class.translations
   end

   translate "this", :to => "that"
end

p C.new.translations   # ["Translating this to that"]


David

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