From: "David A. Black" <dblack@...>
Date: 2005-05-16T01:11:07+09:00
Subject: Re: infinite number of singleton_classes

Hi --

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?

[...]

> What do you think of it? Isn't it worth an RCR?

No; it's just a consequence of the underlying design principles.  Yes,
there are a few objects that can't have a singleton class (Fixnums and
Symbols are the ones I can think of).  But Class objects can, and
singleton classes are Class objects.  So you'd be asking for
special-case treatment for a subset of classes.

It would be a bit like saying:  Surely no one will ever use a
singleton class as a hash key, so let's remove the #hash method from
it.  There's just no reason to constrain the language and its usage
that way.

Also, I don't see any harm or danger here.  I suppose one could
accidentally write a loop that kept creating singleton classes, but
one could also write a loop that kept creating *non*-singleton classes
-- or arrays, or hashes, or MyClass objects, and so on.


David

-- 
David A. Black
dblack@wobblini.net