Asp Forum - question re: modules and class variable scope

Justin

9/21/2007 10:53:00 PM

I spent some time trying to find an answer to this but haven't found an
explanation I understand. Could someone tell me what is going on here:

module Mod
def self.append_features(base)
base.extend ClassMethods
super
end
module ClassMethods
def growl
puts @@donkey
end
end
end

class Animal
include Mod
@@donkey = "heyhaw"

def self.working_growl
puts @@donkey
end
end

# uninitialized class variable @@donkey in Mod::ClassMethods
Animal.growl

# works fine
Animal.working_growl

Now, in my limited understanding calling Animal.extend (or base in this
case) ClassMethods should have added growl as a class method to Animal,
equivalent to working_growl in scope (as it turns out self in both
methods is Animal which confuses me even more). Clearly this is not the
case. Any help would be appreciated. Thanks!
--
Posted via http://www.ruby-....

10 Answers

Gary Wright

9/22/2007 12:36:00 AM

On Sep 21, 2007, at 6:53 PM, Got Ascii wrote:

> I spent some time trying to find an answer to this but haven't
> found an
> explanation I understand. Could someone tell me what is going on
> here:
>
> module Mod
> def self.append_features(base)
> base.extend ClassMethods
> super
> end
> module ClassMethods
> def growl
> puts @@donkey
> end
> end
> end
>
> class Animal
> include Mod
> @@donkey = "heyhaw"
>
> def self.working_growl
> puts @@donkey
> end
> end
>
> # uninitialized class variable @@donkey in Mod::ClassMethods
> Animal.growl
>
> # works fine
> Animal.working_growl
>
> Now, in my limited understanding calling Animal.extend (or base in
> this
> case) ClassMethods should have added growl as a class method to
> Animal,
> equivalent to working_growl in scope (as it turns out self in both
> methods is Animal which confuses me even more). Clearly this is
> not the
> case. Any help would be appreciated. Thanks!

Class variables are lexically scoped--not dynamically scoped. This
means that @@donkey as it appears in Mod::ClassMethods#growl is
associated
with Mod::ClassMethods and not with Animal even after you have
extended Animal and are executing Animal.growl. Since you have never
assigned a value to Mod::ClassMethods/@@donkey, Ruby complains.

This is very different from instance variables which are dynamically
scoped (i.e. resolved relative to 'self' at the time of execution).

So despite the fact that class variables and instance variables both
use a similar syntactical sigil (@@ vs @), they are scoped in vastly
different ways.

class variables: resolved at parse time relative to lexical scope
instance variables: resolved at execution time relative to 'self'

My advice is to just stay away from class variables entirely. Simply
use
class instance variables instead.

Gary Wright

Morton Goldberg

9/22/2007 12:39:00 AM

David A. Black

9/22/2007 9:42:00 AM

Todd Benson

9/22/2007 1:16:00 PM

On 9/22/07, David A. Black <dblack@rubypal.com> wrote:
> Hi --
>
> On Sat, 22 Sep 2007, Gary Wright wrote:
>
> > So despite the fact that class variables and instance variables both
> > use a similar syntactical sigil (@@ vs @), they are scoped in vastly
> > different ways.
>
> I honestly wish that class variables looked like $$this, rather than
> @@this. They're so completely unrelated to, and radically different
> from, instance variables, whereas they're much more akin to globals.
> The @/@@ thing has caused more confusion than anything else in Ruby, I
> believe.

I'm in no place to disagree with David, but I should point out that $$
suggests superiority over $ globals.

My one cent.

Todd

Todd Benson

9/22/2007 1:29:00 PM

On 9/22/07, Todd Benson <caduceass@gmail.com> wrote:
> On 9/22/07, David A. Black <dblack@rubypal.com> wrote:
> > Hi --
> >
> > On Sat, 22 Sep 2007, Gary Wright wrote:
> >
> > > So despite the fact that class variables and instance variables both
> > > use a similar syntactical sigil (@@ vs @), they are scoped in vastly
> > > different ways.
> >
> > I honestly wish that class variables looked like $$this, rather than
> > @@this. They're so completely unrelated to, and radically different
> > from, instance variables, whereas they're much more akin to globals.
> > The @/@@ thing has caused more confusion than anything else in Ruby, I
> > believe.
>
> I'm in no place to disagree with David, but I should point out that $$
> suggests superiority over $ globals.
>
> My one cent.

Unless, of course, a person understands two symbols together to
represent a subset (like b has superiority over bb).

The @/@@ thing was really easy for me to understand right away because
I could separate them entirely. I wouldn't mind a different symbol
entirely, though.

Todd

David A. Black

9/22/2007 1:33:00 PM

Todd Benson

9/28/2007 1:08:00 AM

On 9/22/07, David A. Black <dblack@rubypal.com> wrote:

> People in many places have disagreed with me, and I with them. Feel
> free :-)
>
> At least $$this would point the confusion away from instance
> variables, which are much more important and more deserving of not
> being made confusing than globals. Not that I think confusion is good
> in any direction. What I'd like most is for class variables to
> disappear from Ruby, but I don't think that's going to happen.
>
>
> David
>

The last sentence I agree with, because, while I was reading the
pickaxe -- well, with my lack of CS experience -- one of the biggest
questions in my mind was "why do we need class variables?". Kind of a
gut feeling thing, and not a logistical idea. Perhaps I need to read
more code to truly wrap my head around it. I wish I spoke Nihongo :-)

Todd

Justin

10/2/2007 1:28:00 PM

Gary Wright wrote:
> class variables: resolved at parse time relative to lexical scope
> instance variables: resolved at execution time relative to 'self'

Cool, thank you this makes sense. My next question, is this information
in the pickaxe...did I just miss this?
--
Posted via http://www.ruby-....

Gary Wright

10/2/2007 2:45:00 PM

On Oct 2, 2007, at 9:27 AM, Got Ascii wrote:

> Gary Wright wrote:
>> class variables: resolved at parse time relative to lexical scope
>> instance variables: resolved at execution time relative to 'self'
>
> Cool, thank you this makes sense. My next question, is this
> information
> in the pickaxe...did I just miss this?

No, I don't' think it is in the Pickaxe, at least not in the way I've
phrased it. I've found that most of the Ruby books I've looked at
do a terrible job at discussing class variable semantics.

Gary Wright

Robert Dober

10/2/2007 3:18:00 PM

On 9/22/07, David A. Black <dblack@rubypal.com> wrote:
> Hi --
>
> On Sat, 22 Sep 2007, Todd Benson wrote:
>
> > On 9/22/07, David A. Black <dblack@rubypal.com> wrote:
> >> Hi --
> >>
> >> On Sat, 22 Sep 2007, Gary Wright wrote:
> >>
> >>> So despite the fact that class variables and instance variables both
> >>> use a similar syntactical sigil (@@ vs @), they are scoped in vastly
> >>> different ways.
> >>
> >> I honestly wish that class variables looked like $$this, rather than
> >> @@this. They're so completely unrelated to, and radically different
> >> from, instance variables, whereas they're much more akin to globals.
> >> The @/@@ thing has caused more confusion than anything else in Ruby, I
> >> believe.
> >
> > I'm in no place to disagree with David, but I should point out that $$
> > suggests superiority over $ globals.
>
> People in many places have disagreed with me, and I with them. Feel
> free :-)
Really, I cannot recall ;)
Anyway I am even more radical on this one, I'd wish class variables
simply went away.

Cheers
Robert
--
what do I think about Ruby?
http://ruby-smalltalk.blo...

comp.lang.ruby

question re: modules and class variable scope

Justin

Gary Wright

Morton Goldberg

David A. Black

Todd Benson

Todd Benson

David A. Black

Todd Benson

Justin

Gary Wright

Robert Dober

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