Why Common Lisp CLOS matches on a method where the arguments are the wrong class?

Question

I am trying to learn CLOS and bumped into this surprise. I have those 3 action methods that seemingly incorrectly match on my arguments. When I run tryme function, why the action marked with the last argument being ':c' does not trigger the expected error? What am I doing wrong? Or is it an SBCL bug?

(eval-when (:compile-toplevel :load-toplevel :execute)
  (ql:quickload '(alexandria serapeum defclass-std)))

;; (load "~/AAA//clos-turnstile.lisp")

(in-package "CL-USER")

(shadowing-import 'defclass-std::defclass/std)

;;; -----------------------------------------------------------------------------------------

(defclass/std current-state ()              ())
(defclass/std locked        (current-state) ())
(defclass/std unlocked      (current-state) ())

(defclass/std input ()      ())
(defclass/std icoin  (input) ())
(defclass/std ipush  (input) ())

(defclass/std turnstile ()
  ((state :std (make-instance 'locked))))

(defgeneric action (turnstile state i msg)
  (:documentation "action for our FSM"))

(defmethod action ((turnstile turnstile) (any-state T) (any-input T) (msg T))
  (error "0 unmatched action for ~S ~S ~a" any-state any-input msg))

(defmethod action ((turnstile turnstile) (locked current-state) (i icoin) (msg T))
  (warn "1 lock coin unlock ~S" msg))

(defmethod action ((turnstile turnstile) (unlocked current-state) (i ipush) (msg T))
  (warn "2 unlock push lock ~s" msg))

(defparameter *turnstile* (make-instance 'turnstile))

(defun show-me ()
  (format t "=== ~S~%" *turnstile*))

(defun tryme ()
  (show-me)
  (action *turnstile* (make-instance 'locked) (make-instance 'icoin) :a)
  (show-me)
  (action *turnstile* (make-instance 'unlocked) (make-instance 'ipush) :b)
  (show-me)

  ;; why this does not give the error? I have swapped the argument types.
  (action *turnstile* (make-instance 'unlocked)  (make-instance 'icoin) :c)
  (show-me)

  (action *turnstile* (make-instance 'locked) (make-instance 'icoin) :d)
  (show-me)
  (action *turnstile* (make-instance 'unlocked) (make-instance 'ipush) :e)
  (show-me)
  *turnstile*)

Suspicion

I suspect I should have: (locked locked) instead of (locked current-state) in the method definition, but how do I avoid such mistakes?

Answer 1

With the modified arguments of the method definitions, I get the expected error.

(defmethod action ((turnstile turnstile) (any-state T) (any-input T) (msg T))
  (error "0 unmatched action for ~S ~S ~a" any-state any-input msg))

(defmethod action ((turnstile turnstile) (locked locked) (i icoin) (msg T))
  (warn "1 lock coin unlock ~S" msg))

(defmethod action ((turnstile turnstile) (unlocked unlocked) (i ipush) (msg T))
  (warn "2 unlock push lock ~s" msg))

Answer 2

In what you expected to be an error, you were calling action on objects of classes turnstile, unlocked and icoin (and :c which is a keyword, but none of your methods specializes on this argument). But unlocked is a subclass of current-state ! So the call matches

(defmethod action ((turnstile turnstile) (locked current-state) (i icoin) (msg T))
  (warn "1 lock coin unlock ~S" msg))

Remember that in a defmethod lambda-list, each specialized argument has the form (var-name class-name) (or some other combinations, e.g. using eql or classes themselves rather than their name ...)

Hence,

(defmethod action ((turnstile turnstile) (locked current-state) (i icoin) (msg T))
  (warn "1 lock coin unlock ~S" msg))

(defmethod action ((turnstile turnstile) (unlocked current-state) (i ipush) (msg T))
  (warn "2 unlock push lock ~s" msg))

only differ on their third argument (ipush/icoin), and not the current-state.