+#+(or allegro scl)
+(progn
+ ;; One entry for each class with lazy readers defined. The value is a plist mapping
+ ;; slot-name to a lazy reader, each of which is a list of a function and slot-names.
+ (defvar *lazy-readers* (make-hash-table))
+
+(defmethod slot-unbound :around ((class hyperobject-class) instance slot-name)
+ (let ((lazy-reader (loop for super in (mop:class-precedence-list class)
+ as lazy-reader = (getf (gethash super *lazy-readers*) slot-name)
+ when lazy-reader return it)))
+ (if lazy-reader
+ (setf (slot-value instance slot-name)
+ (apply (car lazy-reader)
+ (loop for arg-slot-name in (cdr lazy-reader)
+ collect (slot-value instance arg-slot-name))))
+ ;; No lazy reader -- defer to regular slot-unbound handling.
+ (call-next-method))))
+
+ ;; The reader is a function and the reader-keys are slot names. The slot is lazily set to
+ ;; the result of applying the function to the slot-values of those slots, and that value
+ ;; is also returned.
+ (defun ensure-lazy-reader (class-name slot-name reader &rest reader-keys)
+ (setf (getf (gethash (find-class class-name) *lazy-readers*) slot-name)
+ (list* reader (copy-list reader-keys))))
+
+ (defun remove-lazy-reader (class-name slot-name)
+ (setf (getf (gethash (find-class class-name) *lazy-readers*) slot-name)
+ nil))
+
+ ) ;; #+(or allegro scl)
+
+