; Continuation Passing Sytle (CPS) Quicksort algorithm 

(define cps-qsort
  (lambda (pivot ls k)  ;; k is the "continuation" procedure
    (if (null? ls)
        (k '() '())
        (cps-qsort pivot (cdr ls)     ;; tail call
                   (lambda (left right)
                     (let ((x (car ls)))
                           (if (< x pivot)
                               (k (cons x left) right)
                               (k left (cons x right)))))))))

; quicksort "driver" procedure that creates the
; continuation to send to "cps-qsort"

(define qsort
  (lambda (ls)
    (if (null? ls) 
        '()
        (let ((pivot (car ls)))
          (cps-qsort pivot 
                     (cdr ls)
                     (lambda (left right) ; "continuation" function
                       (append (qsort left) (list pivot) (qsort right)))
                     )))))

(qsort '())
(qsort '(0))
(qsort '(1 2 3 4 5))
(qsort '(5 4 3 2 1))
(qsort '(0 -1 1 -2 2 -3 3 -4 4 -5 5))
(qsort '(-99 0 1024 202 -1 77 18 -34 -33 2048 19 1 9 7 2 3 -3 191))