The game that I'm working on in my spare time has a lot of state machines in it compiled to CLOS, letting me use method calls as a means of directly sending se

The game that I'm working on in my spare time has a lot of state machines in it compiled to CLOS, letting me use method calls as a means of directly sending sending events. Rather than write all those methods myself, with `case` forms on the state, I spent an afternoon writing this [0] (defmacro defdelta (class &body body) (let* ((body `(,@body (- enter - - -) (- leave - - -))) (acc (group-by 1 (let ((-q0) (-e) (-w) (-q) (-a)) (loop for (q0 e w q a) in (cdr body) do (setf -q0 (tc q0 -q0) -e (tc e -e) -w (tc w -w) -q (tc q -q) -a (tc a -a)) collect (list -q0 -e -w -q -a)))))) (push (cons class acc) *delta*) `(progn ,@(loop for (e . clauses) in acc collecting `(progn (defevent ,e) (defmethod ,e ((me ,class) &rest args) (when (member *@* args) (apply #'emit ',e (state me) me args)) (case (state me) ,@ (loop for (q0 . deltas) in (group-by 0 clauses) collecting `(,q0 (random-case ,@ (loop for (- -- w q a) in deltas collecting `((,@(or w 1)) ,@(when (and q (not (eq q0 q))) `((change-state me ',q args))) ,@(when a `((apply ',a me args))))))))))))))) which allows me to write, for example, this (defdelta door ( q0 event - q action ) ;;-------------------------------------------------- ( locked challenge - - test-lock ) ( ^ unlock - unlocked - ) ( ^ enter - - to-locked ) ;;-------------------------------------------------- ( unlocked enter - - to-unlocked ) ( ^ challenge - open - ) ( ^ lock - locked - ) ;;-------------------------------------------------- ( open enter - - to-open ) ( ^ arrival - blocked - ) ( ^ shut - unlocked - ) ;;-------------------------------------------------- ( blocked departure - open - ) ( ^ enter - - to-blocked )) along with some support code (e.g., the action functions) to define the auto-closing, auto-locking behavior of doors. --- [0] Some helpers have been omitted. `defevent` defines a generic function with the given name and a particular set of arguments, and a default method that does nothing. `change-state` handles the mechanics of updating the state variable, sending a `leave` event just before the transition and `enter` just after.