继续CPS

(define (non-number-value-error x)
  (display "Value error: ")
  (display  x)
  (display " is not number.")
  (newline)
  'error)


(define (kproduct ls k k-value-error)
  (let ((break k))
    (let loop ((ls ls) (k k))
      (cond
       ((null? ls) (k 1))
       ((not (number? (car ls))) (k-value-error (car ls)))
       ((zero? (car ls)) (break 0))
       (else (loop (cdr ls) (lambda (x) (k (* (car ls) x)))))))))


;;; valid
(kproduct '(2 4 7) 
      (lambda (x) (k+ x 100 return)) 
      non-number-value-error)
;Value: 156

;;; invalid
(kproduct '(2 4 7 hoge) 
      (lambda (x) (k+ x 100 return)) 
      non-number-value-error)
Value error: hoge is not number.
;Value: error

从这个例子中可以看出，CPS风格的程序类似尾递归，需要的中间状态都融合在不停构建的新函数中。比如下例就不是尾递归，因为它除了正常该有的error消息外，还有解释器打出的异常信息：

(define (fact n)
  (if (= n 1)
      1
      (if (< n 3)
          (non-number-value-error n)
          (* n (fact (- n 1))))))

(fact 6)

继续阅读，发现下面总结了一下Continuation：

You may grasp what is continuation by the above explanation.Continuation

1. exists whole computional processes, and   

2. it can be treated explicitly by a functional programming language and CPS.

In addition, examples above show that it is a chain of closure.