From: Ara.T.Howard@...
Date: 2005-05-16T00:55:30+09:00
Subject: Re: infinite number of singleton_classes

On Sun, 15 May 2005, Lionel Thiry wrote:

> Hello!
>
> ----8<----
> a = Object.new
>
> b = class << a
> class << self
>  class << self
>   class << self
>    # "class << self".times(n)
>     self
>    # "end".times(n)
>   end
>  end
> end
> end
> # => #<Class:#<Class:#<Class:#<Class:#<Object:0x2aa8348>>>>>
> ----8<----
>
> How many levels can we reach? It seems there is no limit.
>
> I suppose those singleton_class exists only when needed? Otherwise, I really
> wonder where does ruby hide all those singleton_classes...
>
> But, sincerely, is it needed to have the singleton_class of a singleton_class?
> Isn't it a non-sense? Shouldn't this raise an error?
>
> ---8<---
> a = Object.new
>
> begin
> b = class << a
>  class << self
>   self
>  end
> end
> rescue NewError
> puts "Sincerely, you don't need the singleton_class of the singleton_class!"
> end
> ---8<---
>
> What do you think of it? Isn't it worth an RCR?


i've found them useful in the following situation:

   harp:~ > cat a.rb
   class C
     class << self
       class << self
         def class_method_generator name
           module_eval <<-code
             def #{ name }; @#{ name }; end
             alias #{ name }? #{ name }
             def #{ name }= value; @#{ name } = value; end
           code
         end
       end
       %w( a b c ).each{|meth| class_method_generator meth}
     end
   end

   class B < C
   end

   C.a = 42
   p C.a

   B.a = 'forty-two'
   p B.a

   C.class_method_generator :x


   harp:~ > ruby a.rb
   42
   "forty-two"
   a.rb:25: undefined method `class_method_generator' for C:Class (NoMethodError)

granted, you could do this with a private class method of C - but this is what
singleton methods are for : defining a method on 'this' instance right here
and right now that only that instance should be able to do.  in this case the
instance happens to be a singleton of a singleton.

in any case i don't think one can say when/if that construct would be useful a
priori and think that having that notion enforced by ruby itself ( __i__ know
when need a singleton class and __you__ don't ) is the kind of exception that
reminds of of a 'p' language.  ruby programmers are continually pushing the
boundries of oo coding precisely because matz has given them the tools to do
so and it would be a shame to take a tool away just because someone hasn't
figured out something indespesible to do with that particular tool __yet__.

kind regards.

-a
-- 
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