(define (shuffle a)
  (let* ((len (length a))
     (i len) (ret (list->vector a))
     (r 0) (tmp null))
    (let loop ()
      (if (> i 1)
      (begin
        (set! r (random i))
        (set! i (sub1 i))
        (set! tmp (vector-ref ret i))
        (vector-set! ret i (vector-ref ret r))
        (vector-set! ret r tmp)
        (loop))))
    (vector->list ret)))

;; Test

(printf "~a~n" (shuffle '(1 2 3 4 5 6)))
(printf "~a~n" (shuffle '(#\h #\e #\l #\l #\o #\, #\w #\o #\r #\l #\d)))

;; Shuffle a whole deck of cards!
(printf "~a~n" (shuffle (list 'clubs-ace 'clubs-2 'clubs-3 'clubs-4 

                  'clubs-5 'clubs-6 'clubs-7

                  'clubs-8 'clubs-9 'clubs-10

                  'clubs-jack 'clubs-queen 'clubs-king

                  'diamonds-ace 'diamonds-2 'diamonds-3 'diamonds-4 

                  'diamonds-5 'diamonds-6 'diamonds-7

                  'diamonds-8 'diamonds-9 'diamonds-10

                  'diamonds-jack 'diamonds-queen 'diamonds-king

                  'hearts-ace 'hearts-2 'hearts-3 'hearts-4 

                  'hearts-5 'hearts-6 'hearts-7

                  'hearts-8 'hearts-9 'hearts-10

                  'hearts-jack 'hearts-queen 'hearts-king

                  'spades-ace 'spades-2 'spades-3 'spades-4 

                  'spades-5 'spades-6 'spades-7

                  'spades-8 'spades-9 'spades-10

                  'spades-jack 'spades-queen 'spades-king)))