Scheme if expression returns '#<void>' 如果主体包含表达式

Question

所以我目前正在阅读 SICP 并且我被困在练习 1.22 中，因为我不明白为什么我的程序没有按照我预期的方式工作。这是代码

#lang sicp
; the given function to time the search for a prime
(define (timed-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))

; finds the smallest natural number that can divide n without any remainder
(define (smallest-divisor n)
    (define (square x)
        (* x x))
    (define (divides? a b)
        (= (remainder a b) 0))
    (define (find-divisor n test-divisor)
        (cond ((> (square test-divisor) n) n)
              ((divides? n test-divisor) test-divisor)
              (else (find-divisor n (+ test-divisor 1)))))
    (find-divisor n 2))

; returns true if the given number n is prime
(define (prime? n)
    (= n (smallest-divisor n)))

; start searching at start and found keeps track of the amount of
; primes found, if it equals 3 return found
(define (search-for-primes start found)
    (if (= found 3)
        found   ; after finding 3 primes above start return
        ((timed-prime-test start)    ; if not continue search with start + 1
         (search-for-primes (+ start 1) (if (not (prime? start))
                                            found
                                            (+ found 1))))))

(search-for-primes 1000 0)

问题是，当我运行这个程序时，它工作正常，直到它找到一个质数。我使用的解释器是球拍，程序终止于：

application: not a procedure;
 expected a procedure that can be applied to arguments
  given: #<void>
  arguments...:
   3
1019 *** 0

如果我对解释器的理解正确，那么它应该根据应用顺序求值原则来求值这个表达式，对吧？那么为什么要将 if 表达式作为过程传递给我的 search-for-primes 过程呢？我在这里错过了什么？

Answer 1

问题出在search-for-primes，如果你在if的一条分支中有多个表达式，那么你需要将它们放在begin块中-用 () 包围它是行不通的。这应该可以解决问题：

(define (search-for-primes start found)
  (if (= found 3)
      found
      (begin ; add this!
        (timed-prime-test start)
        (search-for-primes (+ start 1) (if (not (prime? start))
                                           found
                                           (+ found 1))))))

Scheme if expression returns '#<void>' 如果主体包含表达式

Scheme if expression returns '#<void>' if the body contains an expression

scheme

sicp