I've been playing around with GOOPS and the metaobject protocol
lately. It has been fun, but I've hit a snag with redefinable classes.
Here's a contrived example program that demonstrates my problem:
(use-modules (oop goops)
(srfi srfi-111))
(define-class <meta> (<class>))
(define (boxed-slot? slot)
(get-keyword #:box? (slot-definition-options slot)))
(define-method (compute-getter-method (class <meta>) slot)
(if (boxed-slot? slot)
(make <method>
#:specializers (list class)
#:procedure (let ((slot-name (slot-definition-name slot)))
(lambda (obj)
(unbox (slot-ref obj slot-name)))))
(next-method)))
(define-method (compute-setter-method (class <meta>) slot)
(if (boxed-slot? slot)
(make <method>
#:specializers (list class <top>)
#:procedure (let ((slot-name (slot-definition-name slot)))
(lambda (obj value)
(set-box! (slot-ref obj slot-name) value))))
(next-method)))
(define-class <foo> ()
(bar #:accessor bar #:box? #t #:init-form (box 123))
#:metaclass <meta>)
(define-class <redefinable-foo> ()
(bar #:accessor bar #:box? #t #:init-form (box 123))
#:metaclass <redefinable-class>)
;; This works:
(pk (+ (bar (make <foo>)) 456))
;; This throws an error:
(pk (+ (bar (make <redefinable-foo>)) 456))
The above code introduces a metaclass, <meta>, that supports a special
slot option: #:box? The metaclass arranges for boxed slots to be
boxed/unboxed transparently via the slot's accessor methods when it
detects the presence of #:box? #t in the slot options. However, the
redefinable class implementation does a transformation of the slot
definitions in its compute-slot method and discards all non-standard
slot options, so the final line throws an error because the bar method
returns a box, not a number.
I see two possibilities:
1) Slots are not designed to be extended with custom options, but it
just happens to work in the case of <class>
2) There's a bug in compute-slots for <redefinable-class>
Is this a bug or as-designed?
Thanks,
- Dave
