Hello!
In article <m3k83r7e3z.fsf@localhost.localdomain>,
Lieven Marchand <mal@wyrd.be> wrote:
>[... evolved from a discussion of declaring methods on make in Dylan,
> specialized on single classes ...]
>BTW, why does LispWorks complain about the equivalent CL code? The
>Hyperspec specifically allows users to augment the MAKE-INSTANCE
>generic function.
>CL-USER 8 > (defmethod make-instance ((class (eql (find-class
>'abstract))) &rest args)
> (apply #'make-instance 'concrete args))
>Error: Defining method #<STANDARD-METHOD MAKE-INSTANCE NIL ((EQL
>#<STANDARD-CLASS ABSTRACT 21151714>)) 204D8444> visible from packages
>COMMON-LISP.
> 1 (continue) Define it anyway.
> 2 Discard the new method.
> 3 (abort) Return to level 0.
> 4 Return to top loop level 0.
>It works as expected when you choose restart 1.
Dunno. sbcl accepts the equivalent of that:
(defclass a () ())
(defclass b () ())
(defmethod make-instance ((class (eql (find-class 'a))) &key &allow-other-keys
&rest args)
(apply #'make-instance 'b args))
(It spits out many optimization warnings though)
Now,
(let ((x (make-instance 'a)))
(class-of x))
yields
#<STANDARD-CLASS A>
while
(let ((x (make-instance (find-class 'a))))
(class-of x))
yields
#<STANDARD-CLASS B>
So to achieve what was intended, you'd also have to specify
(defmethod make-instance ((class (eql 'a)) ...)
...)
Looks like an application for a small macro: define a generic
where one argument position is specialized eql a symbol *and*
the corresponding class.
Kind regards,
Hannah.