Brian Denheyer writes:
>
> Here is a class :
>
> (define-class <foo> ()
> ((read-only :accessor read-only)
> (value :accessor value)))
>
> Let's say that I want to control the way value is accessed based on
> the value of read-only, so that if it is true and the program tries to
> set a slot it won't work. It seems to me the proper way to do this is
> to make value a virtual slot and modify the accessors :
>
> (value :accessor value
> :allocation :virtual
> :slot-ref
> (lambda (self) (slot-ref self 'value)))
>
> whoops ! the accessor is now infinitely recursive.
>
> Obviously I can get around this by using define-method to create
> read/writer methods, but it seems more elegant to do it the way I want
> to. Is it possible ?
>
Yes.
In fact STklos define a particular kind of slots called active
slots. An active slot is a slot whose access can be controlled before
or after reading (as well as before/after writing).
For that,you give a function for each of the 4 hook that you want to
set. In the case of the "before-slot-set!" function, the value that it
returns is used to know if the slot must be set (the slot is not set
if it returns #f).
So your class can be written as
(define-class <foo> ()
((read-only :accessor read-only :init-form #t)
(value :accessor value
:allocation :active
:before-slot-set! (lambda(o v)
(not (slot-ref o 'read-only))))
:metaclass <active-metaclass>)
The slot is active because of its allocation scheme (:active here) and
only the before-slot-set! function is given. Here the function just
forbid to set the value slot if read-only slot is not false. Of course
you can add error message or other processing if needed.
Hope it helps.
-- Erick
Received on Fri Feb 26 1999 - 15:42:58 CET