From: "Hal E. Fulton" <hal9000@...>
Date: 2003-05-09T09:33:23+09:00
Subject: Re: Can a global be a constant?

----- Original Message -----
From: "Jim Freeze" <jim@freeze.org>
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Thursday, May 08, 2003 7:22 PM
Subject: Re: Can a global be a constant?


> On Friday,  9 May 2003 at  9:16:35 +0900, dblack@superlink.net wrote:
> > Hi --
> >
> > On Fri, 9 May 2003, Jim Freeze wrote:
> >
> > > On Friday,  9 May 2003 at  8:57:15 +0900, dblack@superlink.net wrote:
> > >
> > > > You can use instance variables (of the object State) instead of
class
> > > > variables, and then State becomes:
> > > >
> > > >   class State
> > > >     class << self
> > > >       attr_accessor :debug, :verbose
> > > >     end
> > > >   end
> > >
> > > Why does this not define the class method State.debug to
> > > set the instance variable @debug?
> >
> > It does; it creates #debug and #debug= in State's singleton class, and
> > those methods get/set State's instance variable @debug.
>
> So, this is something I don't understand. State is never instantiated.
> How does it have an instance variable?

It's not an instance variable in State, it's an instance
variable in the singleton class of State.

Or have I confused myself again? :)

Note that a class X *cannot* have a class method that
references an instance variable of X. Think about it.
A class-level method knows nothing about any particular
instantiation of X, so it can't get/set instance-level
data.

But class X itself is an object, and you can add a
singleton method to an object. When the object you
add a singleton method to is a class, the method
is then a class method.

"Cogito cogito, ergo cogito sum." (I think I think,
therefore I think I am.)

Cheers,
Hal