Testing Common Lisp Object System's method dispatch

clos-test.lisp

(defgeneric thing (x y))

(defclass foo () ())

;; BAR inherits FOO
(defclass bar (foo) ())

(defmethod thing ((x foo) (y foo))
  (format t "Both are of type FOO~%"))

(defmethod thing ((x bar) (y foo))
  (format t "X is a BAR, Y is a FOO. Next...~%")
  (call-next-method))

(defmethod thing ((x foo) (y bar))
  (format t "X is a FOO, Y is a BAR. Next...~%")
  (call-next-method))

(defun test-dispatch ()
  (let ((x (make-instance 'bar))
        (y (make-instance 'bar)))
    (thing x y)))

;;; Output:
;;; X is a BAR, Y is a FOO. Next...
;;; X is a FOO, Y is a BAR. Next...
;;; Both are of type FOO