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

Yes. This (great) tutorial of 2003 has a good one from the geometry domain.

Create a class to represent points in 3-dimensions:

(defclass point ()
    (x y z))

Create a variable to instantiate the class and a function to set the values:

(defvar my-point
  (make-instance 'point))

(defun set-point-values (point x y z)
  (setf (slot-value point 'x) x
        (slot-value point 'y) y
        (slot-value point 'z) z))

In the REPL, do:

CL-USER 17 > (set-point-values my-point 3 4 12)
12

Now, think about a function to compute the distance between points. A brute force way would be:

(defun brute-force-distance-from-origin (point)
  (let ((x (slot-value point 'x))
        (y (slot-value point 'y))
        (z (slot-value point 'z)))
    (sqrt (+ (* x x)
             (* y y)
             (* z z)))))

Using the with-slots macro:

(defun distance-from-origin (point)
  (with-slots (x y z) point (sqrt (+ (* x x)
                                     (* y y)
                                     (* z z)))))

Calling the function in the REPL works as expected:

CL-USER> (distance-from-origin my-point)
13.0