I've made a simple Tic-Tac-Toe game in Common Lisp using the Ltk library.
One of the things I wanted to do was to support CPU vs CPU, human vs human, human vs CPU, and CPU vs human. Also, possibly multiple types of CPU algorithms. The ideal way to do this (for me at least) is to have a scheduler that gives control to Player 1, who chooses a move, and passes control back to the scheduler with the move he chose to make; the scheduler then passes control to Player 2, who chooses a move, and passes control back to the scheduler with the move he chose to make. And then the scheduler rotates back to Player 1. The job of the scheduler is also to update the GUI whenever a player makes a move, and to check whether the Game Over condition has been satisfied. Ideally, the way to implement this would be to use coroutines, but since Common Lisp doesn't support coroutines, I ended up using CPS.
I find the solution to be somewhat ugly. Is there a cleaner way of doing this?
(defun create-board () (loop for i from 1 to 3 collect (loop for j from 1 to 3 collect nil))) (defun at-position (i j board) (nth j (nth i board))) (defun cpu-choose (board scheduler) (let ((choice-i (random 3)) (choice-j (random 3))) (if (not (illegal-choice choice-i choice-j board)) (funcall scheduler choice-i choice-j) (cpu-choose board scheduler)))) (defun illegal-choice (i j board) (at-position i j board)) (defun set-at-position (row-index col-index mat val) (loop for i from 0 to (length mat) for row in mat collect (loop for j from 0 to (length row) for entry in row collect (if (and (= i row-index) (= j col-index)) val entry)))) (defun add-mark (choice board mark) (set-at-position (car choice) (cadr choice) board mark)) (defun wonp (current-symbol board) (let ((rows board) (columns (apply #'mapcar #'list board)) (diagonal1 (list (at-position 0 0 board) (at-position 1 1 board) (at-position 2 2 board))) (diagonal2 (list (at-position 0 2 board) (at-position 1 1 board) (at-position 2 0 board)))) (member t (mapcar (all-equal-to current-symbol) (cons diagonal1 (cons diagonal2 (append columns rows))))))) (defun drewp (board) (and (not (wonp 'O board)) (not (wonp 'X board)) (every #'(lambda (row) (not (member nil row))) board))) (defun all-equal-to (current-symbol) (lambda (row) (every #'(lambda (v) (equal v current-symbol)) row))) (defun circular (items) (setf (cdr (last items)) items) items) (defun human-choose (board scheduler) ;; pass to event handler nil) (defun noughts-and-crosses (&key (players (list #'human-choose #'cpu-choose))) (with-ltk () (let ((board (create-board)) (buttons (make-array '(3 3) :initial-element nil)) (scheduler-resume nil)) (labels ((scheduler (board turns symbols) (let ((current-symbol (car symbols)) (current-player (car turns))) (setf scheduler-resume #'(lambda (i j) (if (illegal-choice i j board) (scheduler board turns symbols) (progn (setf board (set-at-position i j board current-symbol)) (setf (text (aref buttons i j)) (string current-symbol)) (cond ((wonp current-symbol board) (do-msg (format nil "~A won!" current-symbol)) (return-from noughts-and-crosses)) ((drewp board) (do-msg "Draw!") (return-from noughts-and-crosses)) (t (scheduler board (cdr turns) (cdr symbols)))))))) (funcall current-player board scheduler-resume)))) (loop for i from 0 to 2 do (loop for j from 0 to 2 do (let ((this-i i) (this-j j)) (setf (aref buttons i j) (make-instance 'button :text "" :command #'(lambda () (funcall scheduler-resume this-i this-j)))) (grid (aref buttons i j) i j)))) (scheduler board (circular players) (circular '(X O)))))))