🚀 go-pugleaf

Problem: given a metaclass CACHING-CLASS that maintains a list of all its direct 
instances, how can I create and use a CLASS-PROTOTYPE for a class based on
CACHING-CLASS and also, as a special case, ensure that the CLASS-PROTOTYPE does not
appear in the metaclass instance list?

I use the following implementation of CACHING-CLASS to maintain a list of all
instances of a class whose metaclass is CACHING-CLASS. (AMOP gives a related
example of a metaclass named COUNTED-CLASS on pp. 72-74)

;;; define a metaclass 'caching-class' which maintains a list
;;; of all direct instances of the class; the instances may be 
;;; retrieved and mapped over; instances are stored on the class
;;; object, not on a shared slot
(defclass caching-class (standard-class)
  ((direct-instances :accessor class-direct-instances
		     :initform nil)))

;;; automatically record instance creation on any class that uses
;;; caching-class as a metaclass
(defmethod make-instance :around ((class caching-class) &rest initargs)
  (let ((instance (call-next-method)))
    (push instance (class-direct-instances class))
    instance))

The problem with this implementation is that its impossible to know a priori
whether the instance being created in MAKE-INSTANCE is a class prototype or a
'normal' instance to be pushed on the list of CLASS-DIRECT-INSTANCES.

Any ideas. I'm using LUCID 4.0.2.
..................................................
                                  Len Charest, Jr.
                 JPL Artificial Intelligence Group
                          charest@aig.jpl.nasa.gov