`((defun (setf ,from) (value object)
(setf (,to object) value))))))
+(export 'define-on-demand-slot)
+(defmacro define-on-demand-slot (class slot (instance) &body body)
+ "Defines a slot which computes its initial value on demand.
+
+ Sets up the named SLOT of CLASS to establish its value as the implicit
+ progn BODY, by defining an appropriate method on `slot-unbound'."
+ (with-gensyms (classvar slotvar)
+ `(defmethod slot-unbound
+ (,classvar (,instance ,class) (,slotvar (eql ',slot)))
+ (declare (ignore ,classvar))
+ (setf (slot-value ,instance ',slot) (progn ,@body)))))
+
;;;--------------------------------------------------------------------------
;;; CLOS hacking.