From: Intransition <transfire@...>
Date: 2010-03-23T03:44:17+09:00
Subject: Re: Singleton methods without the singleton class

On Mar 15, 2:53 pm, Charles Oliver Nutter <head...@headius.com> wrote:
> JRuby currently allows you to add singleton methods to normal Java
> objects like this:
>
> foo = java.lang.String.new('blah')
> def foo.bar
>   # do something
> end
>
> Unfortunately, this feature requires us to keep a weak table of all
> the Java objects that enter Ruby space, since we can't attache this
> singleton method directly to the object itself. I would like to
> eliminate this feature at some point, but I recognize that since
> people are using it we need an alternative.
>
> So I've hacked together a short script that provides a "Singletonizer"
> module that can simulate singleton methods without actually creating a
> new singleton class:
>
> http://gist.github.com/333174
>
> The basic idea is to go ahead and add the method to the actual class,
> but add it via a hashed lookup on a per-instance basis. Objects which
> have had the method added will have a corresponding entry in their
> attached_methods table. Objects that don't will raise NoMethodError as
> usual.
>
> Thoughts? Comments? This version is obviously not threadsafe, but it
> gets you pretty close to singleton methods without requiring singleton
> classes.

How do you handle #extend?

I've always thought it would be wise if singleton methods were added
to an anonymous module rather then directly to the object.* Perhaps
taking this approach will work for JRuby. While not exactly like MRI
it should be close enough for all practical purposes.

(* Which is why Facets extends the #extend method to do exactly that
if you supply it a block.)