[plt-scheme] Class methods and class fields

I think I see what you're saying: that module variables can accomplish what 
class variables would be used for in other languages, by putting the 
class(es) in its/their own module, or even replacing each class with an 
equivalent module. To take it further, a sort-of inheritance can be 
accomplished by re-providing items from a "parent" module (or is that taking 
the analogy too far?).

I guess the mental barrier is seeing modules as more than just a way to 
organize source or object code (sort of like C include-files or .NET 
assemblies), but as an intrinsic part of the language at the same level as 
classes and structures.  That and overcoming the C-based aversion to what 
seem like global variables.

Thanks again, and let me know if I've misunderstood you.
-Eddie

----- Original Message ----- 
From: "Robby Findler" <robby at cs.uchicago.edu>
To: "Eddie Sullivan" <eddieSull at hotmail.com>
Cc: <plt-scheme at list.cs.brown.edu>
Sent: Tuesday, April 29, 2008 9:31 AM
Subject: Re: [plt-scheme] Class methods and class fields


>I think that inheritance is mismatched to what you want and that kind
> of thing only comes about because languages like you mentioned earlier
> don't have anything that isn't attached to a class somehow.
>
> For the examples you've described, you really just want lexical scope
> (plus a module system), IMO. Even for the code you write below, it
> seems to me that you just want to put my-class and my-derived-class
> into one module and have db-connection at the top-level of that
> module.
>
> Robby
>
> On Tue, Apr 29, 2008 at 8:29 AM, Eddie Sullivan <eddieSull at hotmail.com> 
> wrote:
>> That gives some of the functionality, but doesn't specifically associate 
>> the
>> variable with the class, and doesn't allow for inheritance the way 
>> structure
>> type properties do. To add to your example, I can envision something 
>> like:
>>
>>
>>  (define my-class%
>>   (class object%
>>    ;; Just initialized once ever:
>>    (static-field [db-connection (init-db)])
>>
>>    (define/public (talk-to-db stuff) ... db-connection ...)
>>    (super-new)))
>>
>>  (define my-derived-class%
>>   (class my-class%
>>    ;; Provides access to superclass's static field:
>>    (inherit-static-field db-connection)
>>    (define/public (more-db-talking stuff) ... db-connection ...)
>>    (super-new)))
>>
>>  (let ([dbc (get-static-field db-connection my-derived-class%)])
>>   ... dbc ...)
>>
>>  ----- Original Message ----- From: "Robby Findler" 
>> <robby at cs.uchicago.edu>
>>  To: "Eddie Sullivan" <eddieSull at hotmail.com>
>>  Cc: <plt-scheme at list.cs.brown.edu>
>>  Sent: Tuesday, April 29, 2008 8:43 AM
>>  Subject: Re: [plt-scheme] Class methods and class fields
>>
>>
>>
>>
>>
>> > The PLT Scheme class system is embedded in an ordinary functional
>> > language, so you can just define functions or database connections
>> > outside the class an refer to those variables lexically, eg:
>> >
>> > (define db-connection (init-db))
>> > (define my-class%
>> >  (class object%
>> >   (define/public (talk-to-db stuff) ... db-connection ...)
>> >   (super-new)))
>> >
>> > Robby
>> >
>> > On Tue, Apr 29, 2008 at 7:38 AM, Eddie Sullivan <eddieSull at hotmail.com>
>> wrote:
>> >
>> > > Hi.
>> > >  I'm another long-time programmer trying out scheme. I have been 
>> > > working
>> > > with PLT's scheme/class module because I am used to
>> > >  the object oriented way of thinking about programming.
>> > >
>> > >  In every other language I have used that has classes (Python and the
>> C++
>> > >  family, also [incr tcl] but my memory of that is vague), there is 
>> > > the
>> > >  concept of "class variables", that is, variables that are associated
>> with
>> > >  the class itself rather than with any particular instance, and the
>> similar
>> > >  concept of "class methods." (They're also often called "static", but
>> that
>> > >  term can be confusing, IMO.)
>> > >
>> > >  It's easy to imagine scenarios where these concepts would be very
>> useful,
>> > > such as a
>> > >  shared resource like a database connection, or simply a usage 
>> > > counter.
>> > >
>> > >  Perhaps I'm misreading the documentation, but I can't find anything
>> like
>> > > that in scheme/class. Structures have "structure type properties", so
>> the
>> > > idea must be acknowledged to be useful in theory.
>> > >
>> > >  Is it there and I'm missing it? If not, is there a simple way to
>> implement
>> > > this functionality that I haven't figured out? Or is there a
>> philosophical
>> > > reason why this was considered a bad idea?
>> > >
>> > >  Thanks!
>> > >  -Eddie Sullivan
>> > >
>> > >
>> > >  _________________________________________________
>> > >  For list-related administrative tasks:
>> > >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>> > >
>> > >
>> >
>> >
>>
>>
>