On Mar 4, 2004, at 12:04 PM, Hal Fulton wrote:

> When did this start working? :) This seems to be a good step
> toward singletons for Fixnums (unless those are already working
> also, and I haven't noticed).
>
  >> 23.instance_eval {@prime = true}
=> true
  >> 24.instance_eval {@prime = false}
=> false
  >> 23.instance_eval {@prime}
=> true
  >> 24.instance_eval {@prime}
=> false
  >> RUBY_VERSION
=> "1.6.8"

1.6.x is the earliest version I've worked with... I remember trying 
this when I first started out with ruby :) I thought it was a nifty 
trick. You still can't define singleton methods though, just add 
instance variables. Though, I suppose you could add a proc object as an 
instance variable.