From: David Masover <ninja@...>
Date: 2008-06-09T13:02:41+09:00
Subject: Re: Singleton object vs. enhancing singleton class

On Sunday 08 June 2008 22:35:25 Paul McMahon wrote:
> In ruby, as class are singleton objects, does anyone have any ideas when
> a a singleton object should be used vs. simply adding methods to the
> class?

I always include Singleton, but I see what you mean. A better way might be to 
do it with modules:

module Foo
  def self.bar
  end
end

One example is mixins, though. For example, here:

> class Foo
>   def self.bar
>   end
> end

A mixin would almost certainly be expecting to be mixed via "include" here, 
not "extend". This might not always matter, but some mixins will define 
things like self.included, and won't work at all if you try to extend them 
instead.

However, a mixin which expected "extend" would still work on a Singleton 
object:

class Foo
  include Singleton
  def bar
  end
end
Foo.instance.extend SomeMixin

I think the main reason for doing Singleton, though, is that it expresses 
intent better, and it prevents the class from being instantiated -- sure, 
there are ways around it, but you're not going to accidentally call Foo.new 
without getting an error.