表達式樹評估

Question

我試圖評估表達式樹。這裏是我的代碼：

(define (eval-tree expr-tree) 
    (eval-treeaux eval-tree expr-tree)) 

(define (eval-treeaux f expr-tree) 
    (if (null? expr-tree) 
     0 
     (cond ((equal? '+ (operator expr-tree)) (+ (left-op expr-tree) (right-op expr-tree))) 
      ((equal? '- (operator expr-tree)) (- (left-op expr-tree) (right-op expr-tree))) 
      ((equal? '* (operator expr-tree)) (* (left-op expr-tree) (right-op expr-tree))) 
      ((equal? '/ (operator expr-tree)) (/ (left-op expr-tree) (right-op expr-tree))) 
      (else (f eval-treeaux expr-tree))))) 

當我運行這個(eval-tree '((6 * 3) + (4 - 2)))，它給了我這個錯誤：

+: expects type <number> as 1st argument, given: (6 * 3); other arguments 
    were: (4 - 2) 

誰能告訴我是什麼問題，我該如何解決？

---

這是我的新代碼：

(define (eval-tree expr-tree) 
    (if (null? expr-tree) 
     0 
     ((eval-treeaux eval-tree (car expr-tree)) (eval-tree (cdr expr-tree))))) 

(define (eval-treeaux f expr-tree) 
    (cond ((null? expr-tree) '()) 
     ((not (isExpression expr-tree)) (list expr-tree)) 
     (else (cond ((equal? '+ (operator expr-tree)) (+ (left-op expr-tree) (right-op expr-tree))) 
        ((equal? '- (operator expr-tree)) (- (left-op expr-tree) (right-op expr-tree))) 
        ((equal? '* (operator expr-tree)) (* (left-op expr-tree) (right-op expr-tree))) 
        ((equal? '/ (operator expr-tree)) (/ (left-op expr-tree) (right-op expr-tree))) 
        (else (f eval-treeaux expr-tree)))))) 

但是，當我運行這個(eval-tree '((6 * 3) + (4 - 2)))，我得到這個錯誤：

procedure application: expected procedure, given: 2; arguments were: 0 

我真的很困惑。任何人都可以幫助我嗎？ 感謝

Answer 1

表達((6 * 3) + (4 - 2))似乎是由2子表達式：

(6 * 3) ; e_1 
(4 - 2) ; e_2 

您需要遞歸評估子表達式，然後才能評估父表達。