What is a didatic example of the with-slots macro in CLOS?

Question

The Common Lisp HyperSpec covers the with-slots macro. However, the example is hard to grasp.

Is there an easier and more didactic example about it?

Answer 1

To better understand with-slots, one should look into defclass first.

No reader or writer functions are defined by default; their generation must be explicitly requested. However, slots can always be accessed using slot-value.

It means that, unless specific request, defclass don't create any accessor for the slots:

> (defclass point ()
  (x y))

(let ((new-point (make-instance 'point)))
  (setf (point-x new-point) 1))

Error: The function (COMMON-LISP:SETF COMMON-LISP-USER::POINT-X) is undefined.

In this case, one must use the slot-value function to access or modify the slot value.

(defclass point ()
  (x y))

(let ((new-point (make-instance 'point)))
  (setf (slot-value new-point 'x) 1))

Obviously, when there are several slots to update, the code become a little cumbersome:

(defmethod translate ((point-instance point) delta-x delta-y)
  (setf (slot-value point-instance 'x)
        (+ (slot-value point-instance 'x) delta-x))
  (setf (slot-value point-instance 'y)
        (+ (slot-value point-instance 'y) delta-y)))

For that reason, the with-slots macro can make the code easier to read:

(defmethod translate ((point-instance point) delta-x delta-y)
  (with-slots (x y) point-instance
    (setf x (+ x delta-x))
    (setf y (+ y delta-y))))