#lang racket
(require racket/trace)
;;; Using call/cc to implement a return functionality.
;;; Since product-return is recursive, this does not
;;; buy us anything that we do not already have.

(define product-return
  (lambda (ls)
    (call/cc (lambda (return)
	       (cond [(null? ls) 1]
		     [(zero? (car ls)) (return 0)]
		     [else (* (car ls) (product-return (cdr ls) ))])))))
(trace product-return)

;;; Using call/cc to implement break functionality.
;;; This is more useful.

(define product
  (lambda (ls)
    (call/cc (lambda (break) (product-break ls break)))))
(trace product)

(define product-break
  (lambda (ls break)
    (cond [(null? ls) 1]
	  [(zero? (car ls)) (break 0)]
	  [else (* (car ls) (product-break (cdr ls) break))])))
(trace product-break)

;;; This is realy nice code.

(define product2
  (lambda (ls)
    (call/cc (lambda (break) 
	       (letrec ([product-return
			 (trace-lambda (ls)
			   (cond [(null? ls) 1]
				 [(zero? (car ls)) (break 0)]
				 [else (* (car ls) (product-return (cdr ls)))]))])
		 (product-return ls))))))