CHANGE-CLASS on specialized metaclass

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2 total messages Started by Len Charest Wed, 11 Nov 1992 01:02

#292

Author: Len Charest
Date: Wed, 11 Nov 1992 01:02

45 lines
2914 bytes

Now that the discussion on class-prototype has died down, I thought I'd throw
this on the fire:

STANDARD-CLASS has the interesting property that it is an instance of itself. 
Suppose I want to make a specilized metaclass that also has this property:

(defclass my-class (standard-class) 
  ((arbitrary-slot :initform (gensym)))  ;to keep things interesting
  )

;;; now make my-class be an instance of itself
(change-class (find-class 'my-class) (find-class 'my-class))

;;; test the result (if any...)
(slot-value (find-class 'my-class) 'arbitrary-slot)

Should this work in a correct implementation of CLOS? Should it matter whether
MY-CLASS has been finalized before the call to CHANGE-CLASS? My intuition is that
the above code should perform as expected--even in the absence of a MOP since the
functions involved (DEFCLASS, CHANGE-CLASS and SLOT-VALUE) are all part of the
'regular' CLOS programmer interface. My experience shows otherwise:

Lisp         Platform            Result
----         --------            ------
Lucid 4.0.2  Sparc2/SunOS 4.1.1  Throws into debugger during CHANGE-CLASS.
                                 "Expanding stack due to overflow..."
                                 FINALIZE-INHERITANCE not implemented.
PCL July 92  Sparc2/SunOS 4.1.1  Works as expected regardless of whether
on Lucid                         MY-CLASS is finalized.
Allegro 4.1  Sparc2/SunOS 4.1.1  Throws into debugger during CHANGE-CLASS.
                                 "No methods applicable for gf: CLOS:CLASS-DIRECT-
                                 SLOTS with args (#<CLOS:FORWARD-REFERENCED-CLASS 
                                 STANDARD-CLASS...> of classes (CLOS:FORWARD-
                                 REFERENCED-CLASS)"
                                 Same error occurs if attempt to finalize MY-CLASS
                                 before CHANGE-CLASS.
MCL 2.0      Quadra 700/Sys 7    Throws into debugger during CHANGE-CLASS, but
                                 will work if MY-CLASS has no local slots.
                                 FINALIZE-INHERITANCE not implemented.

Comments?
..................................................
                                  Len Charest, Jr.
                 JPL Artificial Intelligence Group
                          charest@aig.jpl.nasa.gov

Re: CHANGE-CLASS on specialized metaclass

#294

Author: Steve Haflich
Date: Tue, 10 Nov 1992 13:01

58 lines
3020 bytes

   From: charest@Aig.Jpl.Nasa.Gov (Len Charest)

   STANDARD-CLASS has the interesting property that it is an instance of itself. 
   Suppose I want to make a specilized metaclass that also has this property:

   Should this work in a correct implementation of CLOS? Should it matter whether
   MY-CLASS has been finalized before the call to CHANGE-CLASS? My intuition is that
   the above code should perform as expected--even in the absence of a MOP since the
   functions involved (DEFCLASS, CHANGE-CLASS and SLOT-VALUE) are all part of the
   'regular' CLOS programmer interface. My experience shows otherwise:

   Allegro 4.1  Sparc2/SunOS 4.1.1  Throws into debugger during CHANGE-CLASS.
				    "No methods applicable for gf: CLOS:CLASS-DIRECT-
				    SLOTS with args (#<CLOS:FORWARD-REFERENCED-CLASS 
				    STANDARD-CLASS...> of classes (CLOS:FORWARD-
				    REFERENCED-CLASS)"
				    Same error occurs if attempt to finalize MY-CLASS
				    before CHANGE-CLASS.
   Comments?

I agree it should work, and that it should not matter whether the
class has yet been finalized.  So I tried to reproduce the failure,
but it worked just fine for me:

   Allegro CL 4.1 [SPARC; R1] (4/13/92 21:39)
   ...
   (1): (defclass my-class
	    (standard-class)
	    ((arbitrary-slot :initform (gensym))))
   #<STANDARD-CLASS MY-CLASS @ #x8977de>
   (2): (change-class (find-class 'my-class) (find-class 'my-class))
   #<MY-CLASS MY-CLASS @ #x8977de>
   (3): (find-class 'my-class)
   #<MY-CLASS MY-CLASS @ #x8977de>
   (4): (slot-value (find-class 'my-class) 'arbitrary-slot)
   #:G42

   Allegro CL 4.1 [SPARC; R1] (4/13/92 21:39)
   ...
   (1): (defclass my-class
	    (standard-class)
	    ((arbitrary-slot :initform (gensym))))
   #<STANDARD-CLASS MY-CLASS @ #x8977de>
   (2): (defclass your-class () () (:metaclass my-class))
   #<MY-CLASS YOUR-CLASS @ #x899dfe>	; finalizes inheritance
   (3): (change-class (find-class 'my-class) (find-class 'my-class))
   #<MY-CLASS MY-CLASS @ #x8977de>
   (4): (slot-value (find-class 'my-class) 'arbitrary-slot)
   #:G43

These two examples were in fresh images with no patches loaded.  I
can't be sure why the example didn't work for you, but the appearance
of STANDARD-CLASS as a FORWARD-REFERENCED-CLASS suggests a package
problem -- that you were working in some package in which the symbol
COMMON-LISP:STANDARD-CLASS is not present by inheritance.

I'll contact you privately from bugs@franz.com to pursue this without
bothering the entire common-loops list.

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