[plt-scheme] inheritance of private fields?

Yes, that's what I meant. Does the below working example help? (v  
4.0) -- Matthias


#lang scheme

(require scheme/class)

(define-values (fish% pond% ) ;; two mutually recursive classes
   (let () ; create a local definition scope
     (define-member-name get-depth (generate-member-key))
     (define fish%
       (class object%
         (super-new)
         (define my-depth 10)
         (define my-pond (new pond%))
         (define/public (dive amt)
           (set! my-depth
                 (min (+ my-depth amt )
                      (send my-pond get-depth)))
           my-depth)))
     (define pond%
       (class object%
         (super-new)
         (define current-depth 200)
         (define/public (get-depth) current-depth)))
     (values fish% pond%)))

(define fish (new fish%))

(send fish dive 300)

(with-handlers ((exn:fail:object? (lambda (x) (printf "method get- 
depth not found, as expected: ~e" x))))
   (send fish get-depth))




On Feb 29, 2008, at 9:47 AM, tom sgouros wrote:

>
> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>
>>
>> Take a look at the paper at
>>
>>  http://www.ccs.neu.edu/scheme/pubs/#aplas06-fff
>>
>> It shows how to generate "private" names that you can share in a way
>> that's analogous to protected in Java but you can choose the
>> restriction policy via lexical scope.
>
> Thank you for the help, and I'm sorry to be so dense, but I've been
> looking at that paper for about half an hour, and I don't see which  
> part
> is applicable.  You're not talking about the generate-member-key
> business are you?  Can you be more specific, please?
>
> My apologies,
>
>  -tom
>
>>
>>
>> On Feb 29, 2008, at 7:45 AM, Tom Sgouros wrote:
>>
>>>
>>> Hello all:
>>>
>>> I want to define a general class with some private fields that  
>>> can be
>>> used by subclass.  Here's a dumb example of what I thought would be
>>> the
>>> right way to do this, but apparently isn't:
>>>
>>> (define size%
>>>   (class* object% ()
>>>     (public print-name print-age print-weight)
>>>
>>>     (init-field (name 'Fred) (bday 070661)))
>>>
>>>     (define age (calculate-age bday))
>>>     (define weight 0)
>>>
>>>     (define print-name (display name))
>>>     (define print-age (display age))
>>>     (define print-weight (display weight))
>>>
>>>     (super-new)))
>>>
>>> Now I want to be able to define a sub-class of this, so that age and
>>> weight are available to definitions in that sub-class.  I haven't  
>>> been
>>> able to do that, so feel I must be missing something.
>>>
>>> In the example age is a calculated value that depends on other
>>> init-field values.  I'd make it public by putting it into the init-
>>> field
>>> sequence, if there's a guarantee that the init-field entries are
>>> processed in order (that it's a let* and not a let).  I can't  
>>> find any
>>> such guarantee in the documentation.  Failing that, what should I
>>> do to
>>> make it available?
>>>
>>> The weight private field should have no default value, but is  
>>> intended
>>> to be given one by the sub-class.  In my class-naif view of these
>>> things, I should really make size% an interface and force the sub-
>>> class
>>> to fill in the weight.  But I can't seem to figure out how to do  
>>> that,
>>> either.
>>>
>>> Can someone explain to me the proper way to deal with these  issues?
>>> Any
>>> advice welcome.
>>>
>>> Many thanks,
>>>
>>>  -tom
>>>
>>> -- 
>>>  ------------------------
>>>  tomfool at as220 dot org
>>>  http://sgouros.com
>>>  http://whatcheer.net
>>> _________________________________________________
>>>   For list-related administrative tasks:
>>>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>
>
>
> -- 
>  ------------------------
>  tomfool at as220 dot org
>  http://sgouros.com
>  http://whatcheer.net