-(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)))))
-