From: "David A. Black" <dblack@...>
Date: 2003-12-01T02:33:30+09:00
Subject: Re: Class-level readers and writers

Hi --

On Mon, 1 Dec 2003, T. Onoma wrote:

> I'm actually starting to get dizzy 8-o.  It would help, I think, if we lay 
> this down cold for all to understand. Could you fix the following yummy 
> class, to the best of your knowledge:
> 
> class ClassVariableSoup
>   class_local_var                       # scope is ?
>   def variables
>     local_var                              # scope is method
>     @class_instance_var             # scope is instance/ancestry

I think the definitive category is really just @instance_var, with the
question of whether the instance is a Class or not being irrelevant
(at least pre-2.0... see below).  

>     @_local_class_instance_var   # scope is instance
>     @@class_var                        # scope is class+instance/ancestry
>     @@_local_class_var              # scope is class+instance
>     # others ?
>   end
>   $global_var                             # scope if everywhere
> end

My current understanding, subject to correction, is:

local      =>  current lexical scope
@instance  =>  visible only when 'self' is what 'self' was at
               the time the variable was created
@_instance =>  (2.0) visible to original 'self' (as above) and,
                     if original 'self' was a Class, to subclasses
                     of that class
@@class    =>  class hierarchy, including inside instance method
                     definitions
           =>  (2.0) local to class where it's defined; still visible
                     inside instance method definitions but not in
                     subclasses

(Note: for 2.0, which is @var and which is @_var hasn't been decided.)

My least favorite of all of the proposed changes is definitely the
@var/@_var thing.  It means that, for the first time, the concept of
an instance variable will not be completely unified.  At the very
least, I don't think something should be called an "instance variable"
if it isn't private to the instance.  (And then there's that
underscore.... :-)


David

-- 
David A. Black
dblack@wobblini.net