[closer-devel] using a custom unbound slot marker

[Attila Lendvai]

>
> One trick I have used once is to just inspect the unbound value that is
> used by the CLOS implementation. You can do this in a portable way, like
> this:
>
> (defclass test ()
>   (test-slot))
>
> (defvar +unbound-slot-value+
>   (let ((test (make-instance 'test)))
>     (standard-instance-access test
>       (slot-definition-location
>         (find 'test-slot (class-slots (find-class 'test))
>               :key #'slot-definition-name)))))
>
> (setf (find-class 'test) nil)
>
> This code assumes that the value for unbound slots remains constant for
> the lifetime of an image (but I can't imagine why that would not be the
> case).
>

fyi, clisp has all kind of trouble with this. seems like even variable reads
are being checked against this marker (!?). i've managed to get a little
further by capturing the value into a lambda and going through a funcall,
but playing with this can bring down clisp and/or slime.

-- 
- attila

"- The truth is that I've been too considerate, and so became
unintentionally cruel...
- I understand.
- No, you don't understand! We don't speak the same language!"
(Ingmar Bergman - Smultronstället)
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