In CLOS, instance remorphing considered useless in practice?

Didier Verna didier at
Wed Dec 9 10:01:21 UTC 2020

Jean-Claude Beaudoin <jean.claude.beaudoin at> wrote:

> My main stance here is to state that I have yet to see, in a
> significant application, any use of functions
> #'cl:make-instance-obsolete and
> #'cl:update-instance-for-redefined-class and of the underlying
> machinery that support the remorphing of instances following a class
> redefinition (through a subsequent cl:defclass most likely). I can
> state the same thing about #'cl:update-instance-for-different-class
> and #'cl:change-class.

  At least from a theoretical point of view, CHANGE-CLASS is a simple,
  natural, and elegant solution to situations where you can't come up
  with a design that gets behavioral subtyping (LSP) right in all

  Simple example: the famous square/rectangle (or circle/ellipse)

(defclass rectangle ()
  ((width :initarg :width :accessor width)
   (height :initarg :height :accessor height)))

(defun make-rectangle (width height)
  (if (= width height)
    (make-instance 'square :width width)
    (make-instance 'rectangle :width width :height height)))

(defmethod (setf width) :after (width (rectangle rectangle))
  (declare (ignore width))
  (when (= (width rectangle) (height rectangle))
    (change-class rectangle 'square)))

(defmethod (setf height) :after (height (rectangle rectangle))
  (declare (ignore height))
  (when (= (width rectangle) (height rectangle))
    (change-class rectangle 'square)))

(defclass square ()
  ((width :initarg :width :reader width :reader height :accessor side)))

(defun make-square (width)
  (make-instance 'square :width width))

(defmethod (setf width) (width (square square))
  (let ((side (side square)))
    (unless (= width side)
      (change-class square 'rectangle :width width :height side)))

(defmethod (setf height) (height (square square))
  (unless (= height (side square))
    (change-class square 'rectangle :height height))

I would hate to see it go. I have 450 new students every year that learn

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