Re: stklos question

From: Erick Gallesio <>
Date: Tue, 13 Jan 1998 10:07:13 +0100

Andrew Dorrell writes:
> Hi,
> I am frequently wanting to use virtual slots to "hide" the
> implementation details of certain class attributes.
> In order to do this I would like to be able to have variables whose
> scope is limited to but includes both slot-ref and slot-set! functions.
> I have not worked out a simple way of doing this however.
> Can anyone help?
I must admit that I'm quite sure I have not understood your problem.
Anyway, I will try to respond...

The slot-ref and slot-set! functions are normal closures. So you can use the
standard Scheme techniques to hide a value.
For instance:
     (define setter! #f)
     (define getter #f)

     (let ((var 0))
       (set! getter (lambda (o) var))
       (set! setter! (lambda (o v) (set! var v))))

     (define-class <foo> ()
       ((slot :allocation :virtual :slot-set! setter! :slot-ref getter)))

Here the setter and getter functions capture the variable var which is not
visible outside the getter and setter functions. And we have:
     (define foo (make <foo>))
     (slot-ref foo 'slot) ; ==> 0
     (slot-set! foo 'slot 1)
     (slot-ref foo 'slot) ; ==> 1

As I said, I'm not sure I respond to your question.....
                -- Erick
Received on Tue Jan 13 1998 - 12:44:45 CET

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