A Common Lisp solution for the Mars Rover exercise

rover.lisp

;;; Mars Rover

(defun move (rover cmd)
  (let ((compass '(NORTH EAST SOUTH WEST)))
    (let ((x (caar rover))
          (y (cadar rover))
          (h (cadr rover)))
      (let ((dd (case cmd ('F 1) ('B -1) (otherwise 0)))
            (dh (case cmd ('R 1) ('L -1) (otherwise 0)))
            (dx (case h ('EAST 1) ('WEST -1) (otherwise 0)))
            (dy (case h ('NORTH 1) ('SOUTH -1) (otherwise 0))))
        (list 
          (list (+ x (* dd dx)) (+ y (* dd dy)))
          (nth (mod (+ (position h compass) dh) (length compass)) compass))))))

(defun instruct (rover instructions)
  (if (null instructions)
    rover
    (instruct (move rover (car instructions)) (cdr instructions))))

(defun command (rover cmd)
  (let ((convert (lambda (c) (intern (string (char-upcase c))))))
    (let ((instructions (mapcar convert (coerce cmd 'list))))
      (instruct rover instructions))))

(assert (equal '((0 0) EAST) (move '((0 0) NORTH) 'R)))
(assert (equal '((6 4) NORTH) (instruct '((4 2) EAST) '(F L F F F R F L B))))
(assert (equal '((6 4) NORTH) (command '((4 2) EAST) "FLFFFRFLB")))