One of the nice things about posting code into your blog is there are a wealth of people on the 'net that will keep you honest. After posting my automaton code yesterday, Tim Moore wrote me with a style tip. Basically, Tim suggested I clean up the CONS/APPEND nightmare that existed in the innermost LOOP form. I had been thinking the same thing the night before when I did the post, so the suggestion was spot on. In particular, Tim said that his own personal rule is that when he sees lots of CONS/APPEND/LIST action, he takes it as a sign to replace the mess with a backquoted form.

Without further delay...

(defmacro define-automaton (name states &key (stop 'stop) (debug nil))
  (let ((event-func (gensym "func")))
    `(defun ,name (,event-func)
      (tagbody
	 ,@(loop for (state-name . transitions) in states
		 appending 
		 (list state-name
		       `(case (funcall ,event-func)
 			 ,@(loop for (match next . actions) in transitions
				 collecting `(,match
					      ,@actions
					      ,@(when debug
						      `((format t "Matched ~A. Transitioning to state ~A.~%" ',match ',next)))
					      (go ,next))))
		       `(go ,state-name)))
	 ,stop))))

Ahhhh... much better.