[Sbcl-devel] Re: [Q] Dylan to Java/JVM compiler?

[Hannah S. <ha...@sc...>]

[posted to comp.lang.lisp and mailed to
 sbcl-devel at lists dot sourceforge ... because of bug analysis]

Hello!

In article <m34ruue9js.fsf@localhost.localdomain>,
Lieven Marchand  <ma...@wy...> wrote:
>ha...@sc... (Hannah Schroeter) writes:

>[...]

>> (defclass a () ())
>> (defclass b () ())

>> (defmethod make-instance ((class (eql (find-class 'a))) &key &allow-other-keys
>>                           &rest args)
>>   (apply #'make-instance 'b args))

>[...]

>> (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>

>[...]

>I'm afraid you've found a bug in sbcl ;-)

>Section 7.1.7 of the CLHS states

>[...]

Looks so.

In sbcl, make-instance has these methods by default:

1. #<FUNCTION "DEFMETHOD MAKE-INSTANCE (COMMON-LISP:CLASS)"
  {1060ABC9}> is a function.

2. #<FUNCTION "DEFMETHOD MAKE-INSTANCE (SYMBOL)" {106022D1}> is a function.

3. #<FUNCTION "DEFMETHOD MAKE-INSTANCE (CLASS)" {106023D1}> is a function.

1. and 3. seem to be distinct. Now, 2. is implemented like this:

(in-package "SB-PCL")
...
(defmethod make-instance ((class symbol) &rest initargs)
  (apply #'make-instance (find-class class) initargs))

Now, there are different cl:find-class and sb-pcl:find-class. The
method for symbol seems to use #'sb-pcl:find-class, which seems to
return a sb-pcl:class; while the (cl:)find-class in

(defmethod make-instance ((class (find-class 'a)) &rest initargs)
  (apply #'make-instance (find-class 'b) initargs))

seems to return a cl:class.

So after that defmethod,

(make-instance 'a) => (make-instance #<SB-PCL:CLASS A>) =>
create an instance of A

while

(make-instance (find-class 'a)) => (make-instance #<CL:CLASS A>) =>
(match the eql specialized method) (make-instance (find-class 'b)) =>
creates an instance of B.

Kind regards,

Hannah.