为什么我会收到这个 lambda 表达式错误，我该怎么办？

Question

我对 lisp 还是很陌生；我想知道这里是否有人可以帮助我。

我有以下代码sn-p：

(defun write-lookup (binding-list pattern fact)
(cond
        ; No bindings have been stored
        ; Return the binding list with a new one!
        ((not binding-list) (cons (cons pattern fact) nil))

        ; A list of bindings is being stored
        (cond

            ; The current binding matches
            ((equal (caar binding-list) pattern)
                ; Return the binding-list if value matches, nil else
                (if (compare pattern fact) binding-list nil))

            ; Recursively search the rest of the list for the binding
            ((cdr binding-list) (write-lookup (cdr binding-list) pattern fact))

            ; The list doesn't have the binding.
            ; Return the binding-list with the added pattern
            ( T (cons (cons pattern fact) binding-list)))))

当我尝试运行它时，我得到以下信息：

*** - SYSTEM::%EXPAND-FORM: (EQUAL (CAAR BINDING-LIST) PATTERN) should be a
  lambda expression

有人可以指出我的错误吗？谢谢！

Answer 1

首先你需要正确缩进你的代码：

(defun write-lookup (binding-list pattern fact)
  (cond
        ; No bindings have been stored
        ; Return the binding list with a new one!
   ((not binding-list) (cons (cons pattern fact) nil))

        ; A list of bindings is being stored
   (cond

            ; The current binding matches
    ((equal (caar binding-list) pattern)
                ; Return the binding-list if value matches, nil else
     (if (compare pattern fact) binding-list nil))

            ; Recursively search the rest of the list for the binding
    ((cdr binding-list) (write-lookup (cdr binding-list) pattern fact))

            ; The list doesn't have the binding.
            ; Return the binding-list with the added pattern
    (T (cons (cons pattern fact) binding-list)))))

一个典型的 Lisp 编辑器会在你击键时为你做这件事。

现在您可以很容易地发现第一个 COND 缺少 T 子句。让我补充一下：

(defun write-lookup (binding-list pattern fact)
  (cond
        ; No bindings have been stored
        ; Return the binding list with a new one!
   ((not binding-list) (cons (cons pattern fact) nil))

        ; A list of bindings is being stored
   (t (cond

            ; The current binding matches
       ((equal (caar binding-list) pattern)
                ; Return the binding-list if value matches, nil else
        (if (compare pattern fact) binding-list nil))

            ; Recursively search the rest of the list for the binding
       ((cdr binding-list) (write-lookup (cdr binding-list) pattern fact))

            ; The list doesn't have the binding.
            ; Return the binding-list with the added pattern
       (T (cons (cons pattern fact) binding-list))))))

我也会将注释从代码中移出：

(defun write-lookup (binding-list pattern fact)
  (cond ((not binding-list)                          ; No bindings have been stored
         (cons (cons pattern fact) nil))             ; Return the binding list with a new one!
        (t                                           ; A list of bindings is being stored
         (cond ((equal (caar binding-list) pattern)  ; The current binding matches
                (if (compare pattern fact)           ; Return the binding-list if value matches, nil else
                    binding-list
                  nil))   
               ((cdr binding-list)                   ; Recursively search the rest list for the binding
                (write-lookup (cdr binding-list) pattern fact))
               (T                                    ; The list doesn't have the binding.
                (cons (cons pattern fact)            ; Return the binding-list adding the pattern
                      binding-list)))))) 

Answer 2

您对 cond 的嵌套使用看起来很可疑。您可以使用 if 尝试以下形式：

(defun write-lookup (binding-list pattern fact) ;未存储任何绑定 ;用新的绑定列表返回！ （如果（不是绑定列表） （缺点（缺点模式事实）无） （条件 ;当前绑定匹配 （（（等于（caar绑定列表）模式） ;如果值匹配，则返回绑定列表，否则返回 nil （如果（比较模式事实）绑定列表 nil）） ;递归搜索列表的其余部分以查找绑定 ((cdr binding-list) (write-lookup (cdr binding-list) 模式事实)) ;该列表没有绑定。 ;返回具有添加模式的绑定列表 (T (cons (cons pattern fact) binding-list)))))

对于格式的细微变化，我们深表歉意； emacs 喜欢把 cmets 放在右边。