From: Ken Bloom <kbloom@...>
Date: 2008-05-02T12:10:03+09:00
Subject: Re: DSL - class-level methods that affect instances

On Thu, 01 May 2008 19:55:45 -0500, Daniel Waite wrote:

> Ken Bloom wrote:
>> On Thu, 01 May 2008 18:07:11 -0500, Daniel Waite wrote:
>> 
>>> class C
>>> 
>>> I have no idea how to break the barrier between class and instance. :(
>> 
>> Use class variables (the @@ variety).
>> 
>> class Translator::Base
>>   def self.translate word, params
>>     @@translations ||= {}
>>     @@translations[word]=params[:to]
>>   end
>>   def translations
>>     @@translations ||= {}
>>   end
>> end
>> 
>> class C < Translator::Base
>>   translate 'a', :to => 'b'
>>   translate 'c', :to => 'd'
>> end
>> 
>> C.new.translations #=> {"a"=>"b", "c"=>"d"}
> 
> That works! Except it shares translations across all subclasses.
> 
> module Translator
>   class Base
> 
>     def self.translate word, params
>       @@translations ||= {}
>       @@translations[word]=params[:to]
>     end
> 
>     def translations
>       @@translations ||= {}
>     end
> 
>   end
> end
> 
> class C < Translator::Base
> 
>   translate 'a', :to => 'b'
>   translate 'c', :to => 'd'
> 
> end
> 
> class D < Translator::Base
> 
>   translate 'this', :to => 'that'
> 
> end
> 
> irb(main):028:0> c = C.new
> => #<C:0x4d4e0>
> irb(main):029:0> c.translations
> => {"a"=>"b", "c"=>"d", "this"=>"that"}
> 
> While I understand the behavior, its undesirable. I'm going to dust off
> Ruby for Rails...

I couldn't remember why I didn't use @@ variables for another very 
similar metaprogramming problem I had this week. Now you've reminded why.

This version uses instance variables on the singleton class (which is 
different from using @@ class variables)

class Translator::Base
  def self.translate word, params
    @translations ||= {}
    @translations[word]=params[:to]
  end
  def translations
    self.class.instance_variable_get(:@translations) || {}
  end
end

class C < Translator::Base
  translate 'a', :to => 'b'
  translate 'c', :to => 'd'
end

C.new.translations #=> {"a"=>"b", "c"=>"d"}

class D < Translator::Base
  translate 'e', :to => 'f'
end

D.new.translations #=> {"e"=>"f"}
C.new.translations #=> {"a"=>"b", "c"=>"d"}

#But beware:
class E < C
  translate 'g', :to => 'h'
end

E.new.translations #=> {"g"=>"h"}

There are ways to solve this by explicitly asking for your parent class's 
translations, if you'd like to have that functionality.

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/