From: Robert Klemme <shortcutter@...>
Date: 2007-09-11T17:55:17+09:00
Subject: Re: Singleton Modules rather than Singleton Classes

2007/9/1, Trans <transfire@gmail.com>:
> This recently came up in the thread entitled "Python-style
> Decorators", so I thought it a fair idea to "formally" put it before
> the Ruby community. Here's the deal...
>
> Singleton classes already have more in common with modules than
> classes by the very behaviors that distinguish a module from a class --
> they cannot be instantiated and they can not be inherited. So why
> exactly do we deem them classes at all? If instead we took them to be
> actual modules, and used as such, it would open up some really nice
> possibilities. For example:
>
>   class A
>     def self.x
>       "x"
>     end
>   end
>
>   class B
>     extend (class << A; self; end)
>   end
>
>   B.x  #=> "x"
>
> This makes it dead simple to pass along module class-level methods in
> the class hierarchy. The immediate use of this change is apparent --no
> more ClassMethods/included callback hacks required.

You do not need that, because the functionality is there already:

$ ruby <<XXX
>  class A
>    def self.x
>      "x"
>    end
>  end
> class B < A; end
> p B.x
> XXX
"x"

> One thing to note about this example, the notation "(class << A; self;
> end)" becomes a bit of a misnomer in light of the suggested change.
> "A.singleton" or some other method name, would be far better.
>
> Another possibility, which derives from the aforementioned thread, are
> method decorators using singleton def notation:
>
>   class Example
>
>     def memoized.foo
>       ...
>     end
>
>   end
>
> It is not possible to meta-code this currently b/c the definition of
> foo gets locked away in a singleton class where it can not be reused.
> If that singleton could be included into a class (in this case
> Example) then presto, problem solved, and a powerful new notation is
> opened up to the Ruby programmer.

I'd generally prefer to define modules explicitly and extend classes
with them as Ara suggested.  IMHO that's a cleaner way to achieve what
you want and it also documents things more nicely (especially you can
look at inheritance etc.). My 0.02EUR

Kind regards

robert