From: "David A. Black" <dblack@...>
Date: 2008-05-02T12:24:32+09:00
Subject: Re: DSL - class-level methods that affect instances

Hi --

On Fri, 2 May 2008, Ken Bloom wrote:

> 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)

I wish that class variables were $$ instead of @@. They're really a
kind of restricted global, and they have the same qualities of sprawl
that globals do. The @@ always makes it look like they have some kind
of kinship with instance variables, when in fact they're practically
the opposite, in that they open up a shared space for storing state
whereas instance variables specifically make it possible for objects
to control state on an individual, private (give or take the fact that
nothing is really private in Ruby) basis.


David

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