我得到了“方案申请不是程序"在函数的最后一次递归调用中 [英] I got "scheme application not a procedure" in the last recursive calling of a function

问题描述

代码如下:

(define (time-prime-test n)
  (newline)
  (display n)
  (start-prime-test n (runtime)))

(define (start-prime-test n start-time)
  (if (prime? n)
      (report-prime (- (runtime) start-time))))

(define (report-prime elapsed-time)
  (display " *** ")
  (display elapsed-time))

(define (search-for-primes n m)
  (if (< n m) 
      ((time-prime-test n)
       (search-for-primes (+ n 1) m))
      (display " calculating stopped. ")))
(search-for-primes 100000 100020)

在计算停止"后我收到了这个错误.已显示.如下图:

and i got this error after "calculating stopped." has been displayed. like below:

100017100018100019 * 54 计算停止...应用程序:不是程序；期望一个可以应用于参数的过程
给定:#
参数...:
#

100017 100018 100019 * 54 calculating stopped. . . application: not a procedure; expected a procedure that can be applied to arguments
given: #<void>
arguments...:
#<void>

推荐答案

您打算在 if 的后续部分内执行两个表达式，但是 if 只允许一个在结果中表达，在替代中表达.

You intend to execute two expressions inside the consequent part of the if, but if only allows one expression in the consequent and one in the alternative.

将两个表达式括在括号之间(如您所做的那样)将不起作用:结果表达式将被评估为第一个表达式的函数应用程序，第二个表达式作为其参数，产生错误 "application: not一个过程；期望一个可以应用于参数的过程......"，因为 (time-prime-test n) 不计算为过程，它计算为 #.

Surrounding both expressions between parenthesis (as you did) won't work: the resulting expression will be evaluated as a function application of the first expression with the second expression as its argument, producing the error "application: not a procedure; expected a procedure that can be applied to arguments ...", because (time-prime-test n) does not evaluate to a procedure, it evaluates to #<void>.

您可以使用 cond 来解决问题:

You can fix the problem by either using a cond:

(define (search-for-primes n m)
  (cond ((< n m)
         (time-prime-test n)
         (search-for-primes (+ n 1) m))
        (else
         (display " calculating stopped. "))))

或者一个begin:

(define (search-for-primes n m)
  (if (< n m)
      (begin
        (time-prime-test n)
        (search-for-primes (+ n 1) m))
      (display " calculating stopped. ")))

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