From: Robert Klemme <bob.news@...>
Date: 2004-09-16T07:39:49+09:00
Subject: Re: Singleton method on object via define_method?


"Mauricio Fern�ndez" <batsman.geo@yahoo.com> schrieb im Newsbeitrag 
news:20040915170634.GA25725@student.ei.uni-stuttgart.de...
> On Wed, Sep 15, 2004 at 09:49:50PM +0900, Robert Klemme wrote:
>> > > >> o = Object.new
>> > > => #<Object:0x10184d40>
>> > > >> class << o; class << self; def to_s ; "foo" end end end
>> > > => nil
>> > > >> class << o; class << self;to_s end end
>> > > => "foo"
>> > > >> class << o; class << self; class << self; end end end
>> > > => nil
>> > > >> class << o; class << self; to_s end end
>> > > => "#<Class:foo>"
>> >
>> > It's using Module#to_s because the klass of the singleton class is
>> > no longer itself... Module#to_s ends up calling
>> >  class << o; self.to_s end
>> > which you defined before.
>>
>> Well yes, but the fact remains that there is a singleton we don't know 
>> of,
>> do we?  Probably I don't see clearly your point so I'd appreciate it if
>> you'd elaborate it.
>
> It is I who doesn't understand what the above is supposed to prove :)

Utter confusion on all sides. :-)))

My understanding: someone (I?) made a remark about the fact (well, assumed 
fact) that you can't test whether an instance has a singleton class.  You 
can access it, but you will not know whether it was created for this access 
or whether it existed before.  You then presented the example with a 
NameError due to undefined method foo.  I thought, you use the NameError for 
detecting the singleton class, hence I used to_s which I know is always 
defined and thus would not trigger the exception.  The idea being to 
disprove the test.  Maybe I got something wrong here...

> Before creating the singleton class of the singleton of o's singleton
> class, to_s returned "foo", afterwards it returns "#<Class:foo>"...
> And anyway, nobody forces us to use to_s
>
> batsman@tux-chan:/tmp$ cat dfg.rb
> class Class
> # only works for singletons of singletons, will always return
> # true otherwise
>    def has_singleton?
>        m = "__magic_#{Time.new.to_i}_#{rand(100000)}"
>        class_eval do
>            remove_method(m) rescue nil
>            define_method(m) { }
>            begin
>                send m
>            rescue NameError
>                return true
>            ensure
>                remove_method m
>            end
>        end
>        false
>    end
> end

I'm not sure whether I fully understand what the test should do.  But it's 
late and I'll better go to bed now.  A brain with some sleep thinks 
better...

> o = Object.new
> klass = class << o; class << self; self end end
> p klass.has_singleton?
> class << klass; end
> p klass.has_singleton?
>
> batsman@tux-chan:/tmp$ ruby dfg.rb
> false
> true

Hm, but what does this tell me:

>> o = Object.new
=> #<Object:0x10194858>
>> klass = class << o; self end
=> #<Class:#<Object:0x10194858>>
>> klass.has_singleton?
=> true

The answer is definitely wrong, because the singleton class of o does not 
have a singleton klass.  Hm...

For the moment I'll stick with my statement that it's generally not possible 
to determine whether an instance has a singleton or not other than by using 
some kind of extension. :-)

Kind regards

    robert