;; product with break and O(1) case for a zero operand

;; call/cc
(define (product-cc ls)
  (call-with-current-continuation
   (lambda (break)
     (cond ((null? ls) 1)
           ((= (car ls) 0) (break 0))
           (else (* (car ls) (product-cc (cdr ls))))))))

;; continuation-passing style
(define (product-cps ls k break)
  (cond ((null? ls) (k 1))
        ((= (car ls) 0) (break 0))
        (else (product-cps (cdr ls) (lambda (x) (k (* (car ls) x))) break))))

(define (product-cps2 ls k)
  (let ((break k))
    (let f ((ls ls) (k k))
      (cond ((null? ls) (k 1))
            ((= (car ls) 0) (break 0))
            (else (f (cdr ls)
                     (lambda (x)
                       (k (* (car ls) x)))))))))

(define (product ls)
  (product-cps ls (lambda (x) x) (lambda (x) x)))