Re: closures for GOOPS privates

[Clark McGrew <mcgrew at ale dot physics dot sunysb dot edu>]

Hello,

I've been using yasos, but since goops is The Guile
Object System I figured I'd start using it.

Yasos is a "classless" system (check out the slib documentation for
more, I'm not an expert) so each object has a specific instantiation
routine.  One of the tricks is to wrap private data fields up into a
closure.

For instance (a silly example).  This yasos object contains 2 private
slots are used to calculate the value of "a-b".  They can be set, but
not accessed individually.

(define make-mine
  (let ((a 1) (b 2))
    (lambda ()
       (object
         ((a-b self) (- a b)
	 ((a! self val)	(set! a val))
	 ((b! self val) (set! b val)))))))

The reason I asked is there has been a claim that goops doesn't
support private fields.  But, if I understand your second example,
then I do this in goops like so:

(define-generic a-b)
(define-generic a!)
(define-generic b!)
(define <a-b>
   (let ((local-a (make-accessor 'a))
         (local-b (make-accessor 'b)))
     (define-method a-b ((obj <a-b>) (- (a obj) (b obj)))
     (define-method a! ((obj <a-b>) (v <number>)) (set! (a obj) v))
     (define-method b! ((obj <a-b>) (v <number>)) (set! (b obj) v))
     (class ()
       (local-a #:accessor a #init-value 1)
       (local-b #:accessor b #init-value 2))))

This means that goops does have data hiding independent of any module
system.  That is great.  I guess the data can still be reached with
slot-ref, but who cares...

I can see I need to spend a few hours and run goops through it's
paces.

Thanks,

Clark