[Openmcl-devel] Bug or spec change?

[Ron Garret]

The first thing to do is to see if a different CL implementation gives the same error:

➔ sbcl
This is SBCL 1.1.9, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
(defclass rectangle ()
   ((height :initform 0.0 :initarg :height)
    (width :initform 0.0 :initarg :width)))

#<STANDARD-CLASS RECTANGLE>
(defclass counted-class (standard-class)
   ((counter :initform 0)))

#<STANDARD-CLASS COUNTED-CLASS>
(make-instance 'counted-class
         :name 'counted-rectangle
         :direct-superclasses (list (find-class 'rectangle))
         :direct-slots ())

debugger invoked on a SIMPLE-ERROR:
  The class #<STANDARD-CLASS RECTANGLE> was specified as a super-class of the
  class #<COUNTED-CLASS COUNTED-RECTANGLE>, but the meta-classes
  #<STANDARD-CLASS STANDARD-CLASS> and #<STANDARD-CLASS COUNTED-CLASS> are
  incompatible.  Define a method for SB-MOP:VALIDATE-SUPERCLASS to avoid this
  error.

So this appears to be a bug in the MOP book.  And, if you think about it, the example in the book really doesn’t make sense.  COUNTED-CLASS is a metaclass, i.e. all of its instances are classes.  But RECTANGLE is not a meta-class, it is a class.  All of its instances are rectangles, which are not classes.  So it makes no sense to create an instance of COUNTED-CLASS which inherits from RECTANGLE.  The instances of such a class would have to be both classes and rectangles, and that’s not possible.

rg

On Mar 4, 2021, at 2:07 PM, Paul Krueger <plkrueger at comcast.net> wrote:

> I was trying to do a little MOP hacking and when what I was trying to do got errors I went back to “The Art of the Metaobject Protocol” and ran an example from there to see if it encountered similar errors, which it did (most of this from p. 72 of the book):
> 
> I’m running Clozure Common Lisp Version 1.11.6 (v1.11.6) DarwinX8664
> 
> ? (defclass rectangle ()
>    ((height :initform 0.0 :initarg :height)
>     (width :initform 0.0 :initarg :width)))
> #<STANDARD-CLASS RECTANGLE>
> ? (defclass counted-class (standard-class)
>    ((counter :initform 0)))
> #<STANDARD-CLASS COUNTED-CLASS>
> ? (setf (find-class 'counted-rectangle)
>        (make-instance 'counted-class
>          :name 'counted-rectangle
>          :direct-superclasses (list (find-class 'rectangle))
>          :direct-slots ()))
>> Error: The class #<STANDARD-CLASS RECTANGLE> was specified as a
>>       super-class of the class #<COUNTED-CLASS COUNTED-RECTANGLE>;
>>       but the meta-classes #<STANDARD-CLASS STANDARD-CLASS> and
>>       #<STANDARD-CLASS COUNTED-CLASS> are incompatible.
>> While executing: #<CCL::STANDARD-KERNEL-METHOD CCL::ENSURE-CLASS-INITIALIZED (CCL::SLOTS-CLASS)>, in process Listener(4).
>> Type cmd-. to abort, cmd-\ for a list of available restarts.
>> Type :? for other options.
> 1 > 
> ?
> 
> My question is whether this is a problem with CCL’s implementation or a spec change of some sort that invalidates the example from the book. It’s not clear to me how you could ever employ meta-classes without getting this sort of error in CCL, so if this isn’t a bug, what’s the work-around?
> 
> Thanks ...
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> Openmcl-devel at clozure.com
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