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Re: slot-value-using-class to remap "virtual" slots & closer-mop

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* In <m3ldebnkqu.fsf@pison.robolove.meer.net> :
I Wrote on Sat, 25 Apr 2026 20:35:29 +0530:

> I had occasion to want to have certain slot-value accesses "redirect" to
> other slots, so accessing one would seem to affect the other.
>
> (defclass foo ()
>   ((slot-1 :initform 10))
>   (#+lispworks :optimize-slot-access #+lispworks nil))
>
> this is the behaviour which is required
>
> (setq $f (make-instance 'foo))
> (slot-value $f 'slot-1) ; 30
;; should be 10


> (setf (slot-value $f 'slot-3) 20)

Apparently this only works on lispworks. all other lisps I tried fail at
this step i.e. accessing the "virtual slot" slot-3, saying the slot does
not exist.

since is not found in (class-slots (find-class 'foo)) and the suggested
definition of slot-value involves finding the slot in class-slots before
passing it to s-v-u-c

but the goal was to have slot-3 be a "virtual" slot. So I'm back to
square 1 in trying to implement these "virtual slot redirects"

> (slot-value $f 'slot-1) ; => 30
>
> here is the expanded boilerplate which maps 'slot-2' and 'slot-3' to
> 'slot-1'
>
> (defmethod c2mop:slot-value-using-class
>     ((class standard-class) (obj foo) slot)
>   (let ((slot-name
>          (etypecase slot
>            (symbol slot)
>            (c2mop:standard-effective-slot-definition
>             (c2mop:slot-definition-name slot)))))
>     (case slot-name
>       (slot-2 (slot-value obj 'slot-1))
>       (slot-3 (slot-value obj 'slot-2))
>       (otherwise (call-next-method)))))
>
> (defmethod (setf c2mop:slot-value-using-class)
>     (value (class standard-class) (obj foo) slot)
>   (let ((slot-name
>          (etypecase slot
>            (symbol slot)
>            (c2mop:standard-effective-slot-definition
>             (c2mop:slot-definition-name slot)))))
>     (case slot-name
>       (slot-2 (setf (slot-value obj 'slot-1) value))
>       (slot-3 (setf (slot-value obj 'slot-2) value))
>       (otherwise (call-next-method)))))
>
> any pitfalls comments or alternative implementations
>
> i'm using standard-class but maybe i should be using
> c2mop:standard-class (and c2mop:defmethod) instead, i couldn't spot any
> documentation on closer-mop on how it is to be used, the objects are
> slightly different, and i'm not able to assess if there is an impact
> across implementaions.

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slot-value-using-class to remap "virtual" slots & closer-mop Madhu <enometh@meer.net> - 2026-04-25 20:35 +0530
  Re: slot-value-using-class to remap "virtual" slots & closer-mop tfb <no_email@invalid.invalid> - 2026-04-25 18:27 +0000
  Re: slot-value-using-class to remap "virtual" slots & closer-mop Madhu <enometh@meer.net> - 2026-05-07 05:22 +0530
    Re: slot-value-using-class to remap "virtual" slots & closer-mop Stefan Monnier <monnier@iro.umontreal.ca> - 2026-05-07 15:14 -0400
      Re: slot-value-using-class to remap "virtual" slots & closer-mop Madhu <enometh@meer.net> - 2026-05-08 09:48 +0530

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