为什么我会收到此 body 错误？

Question

每次我运行这段代码，我都会收到错误消息： "define: expected only one expression for the function body, but found 1 extra part." 我已经一次又一次地尝试解决这个问题，但我还没有找到解决方案。有谁知道我该如何解决它？抱歉，代码太长了，我想我应该把它全部包括在内，否则就没有意义了。谢谢！

(define MT (empty-scene 50 50))

; A Polygon is one of: 
; – (list Posn Posn Posn)
; – (cons Posn Polygon)

; A NELoP is one of: 
; – (cons Posn empty)
; – (cons Posn NELoP)

; Polygon -> Image 
; adds an image of p to MT
(define (render-polygon p)
  (local 
    [;Polygon -> Posn
     ; extracts the last item from p
     (define (last p) 
       (cond
         [(empty? (rest (rest (rest p)))) (third p)]
         [else (last (rest p))]))]
    [;Image Posn Posn -> Image
     (define (render-line im p q)
       (add-line
        im (posn-x p) (posn-y p) (posn-x q) (posn-y q) "red"))]
    [;NELop -> Image
     ;connects the posns in p in an image
     (define (connect-dots p)
       (cond
         [(empty? (rest p)) MT]
         [else
          (render-line
           (connect-dots (rest p)) (first p) (second p))]))])
  (render-line (connect-dots p) (first p) (last p)))

新代码（仍然无效）：

; Polygon -> Image 
; adds an image of p to MT
(define (render-polygon p)
  (local 
    [;Polygon -> Posn
     ; extracts the last item from p
     (define (last p) 
       (cond
         [(empty? (rest (rest (rest p)))) (third p)]
         [else (last (rest p))]))
    ;Image Posn Posn -> Image
     (define (render-line im p q)
       (add-line
        im (posn-x p) (posn-y p) (posn-x q) (posn-y q) "red"))
    ;NELop -> Image
     ;connects the posns in p in an image
     (define (connect-dots p)
       (cond
         [(empty? (rest p)) MT]
         [else
          (render-line
           (connect-dots (rest p)) (first p) (second p))]))
  (render-line (connect-dots p) (first p) (last p))]))

Answer 1

您的 render-line 表达式必须在内部 local 形式，而不是在它之后。此外，您所有的 define 都应该在 local 的一个子表单中，而不是每个都在其自己的子表单中。所以，它应该看起来像：

(local [(define (last p)
          ...)
        (define (render-line im p q)
          ...)
        (define (connect-dots p)
          ...)]
  (render-line ...))

为什么我会收到此 body 错误？

Why am I getting this body error?

scheme

racket