Re: closures for GOOPS privates

[Mikael Djurfeldt <mdj at thalamus dot nada dot kth dot se>]

Mikael Djurfeldt <mdj@thalamus.nada.kth.se> writes:

> Clark McGrew <mcgrew@ale.physics.sunysb.edu> writes:
> 
> > In some scheme object systems it's possible to use closures to
> > associate private data with an object.  Is it possible to do this with
> > GOOPS?  I haven't had much time to play with GOOPS it doesn't appear
> > that it can be done. 
> 
> What is private data?
> 
> If "private" means data that only can be accessed by a certain group
> of methods, then the most straightforward method to give them private
> data is to put the data together with this dedicated group in one
> module.

Hmm...  This was quite a bit unclear.  I didn't really mean "put the
data" but "puta accessors for the data" into a module.

In the (strange) example below `d' is a "private" slot used for
memoization of the length of the vector (abs v).

(define-module (math 2D-vector)
  :use-module (oop goops))

(export <2D-vector> x y) ; x and y are "public" slots.

(define-class <2D-vector> ()
  (x #:accessor x' #:init-value 0)
  (y #:accessor y' #:init-value 0)
  (d #:accessor d #:init-value #f))

(define-method x ((v <2D-vector>))
  (x' v))

(define-method (setter v) ((v <2D-vector>) (value <real>))
  (set! (x' v) value)
  (set! (d v) #f))

;; ...similar for y...

;; Uses memoization
(define-method abs ((v <2D-vector>))
  (if (d v)
      d
      (let ((res (sqrt (* (x' v) (x' v)) (* (y' v) (y' v)))))
        (set! (d v) res)
        res)))

Note also the following possibility:

(define-accessor x)
(define-accessor y)

(define <2D-vector>
  (let ((d (make-accessor 'd)))
    ;; ...
    ;; Here you can do all sorts of things which will belong to a
    ;; "private" environment.  Note that the accessor `d' only exists
    ;; within this environment.
    ;; ...
    (class ()
      (x #:accessor x #:init-value 0)
      (y #:accessor y #:init-value 0)
      (d #:accessor d #:init-value #f))))