Racket 中的嵌套 'for' 循环显示错误 'not a procedure'

Question

我有以下代码行可以在 Racket 中进行冒泡排序。我发现语法正确，但仍然显示错误：

(define List (list 1 3 5 7 9 2 4 6 8 0))
(define Left  0)
(define Right 0)

(for ([I (range (- (length List) 1))])
  (for ([J (range (+ I 1) (length List))])
    (set! Left  (list-ref List I))
    (set! Right (list-ref List J))

    (when (> Left Right)
      [(set! List (list-set List I Right))
       (set! List (list-set List J Left ))]
    )
  ) 
)

错误是：

application: not a procedure;
 expected a procedure that can be applied to arguments
  given: #<void>
  arguments...:
   #<void>
  context...:
   /home/jdoodle.rkt:6:2: for-loop
   /home/jdoodle.rkt:5:0: for-loop
   top-level: [running body]
   eval-one-top12
   begin-loop
   loop

猜不出来为什么。我在 https://www.jdoodle.com/execute-racket-online 上测试了它，但没有用。

Answer 1

我找到了答案，整理出来了。我必须在 when 之后为多个语句添加表达式 begin。不是双括号。

(for ([I (range (- (length List) 1))])
  (for ([J (range (+ I 1) (length List))])
    (set! Left  (list-ref List I))
    (set! Right (list-ref List J))

    (when (> Left Right)
      (begin
        (set! List (list-set List I Right))
        (set! List (list-set List J Left ))
))))

或者when之后的一系列表格：

(for ([I (range (- (length List) 1))])
  (for ([J (range (+ I 1) (length List))])
    (set! Left  (list-ref List I))
    (set! Right (list-ref List J))

    (when (> Left Right)
      (set! List (list-set List I Right))
      (set! List (list-set List J Left ))
)))

Racket 中的嵌套 'for' 循环显示错误 'not a procedure'

Nested 'for' loop in Racket shows error 'not a procedure'

lisp

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