使用Common LISP运行爬山搜索时出现的问题

Question

这是我的书中的代码，我需要使用它来使用我们预定义的节点运行爬山搜索。我还成功地运行了其他一些搜索功能，比如Best-first-- (“看起来”很相似)。在运行这个爬山搜索时，它无法运行。我是lisp的新手，所以我不明白为什么会发生这种情况。顺便说一句，我正在运行Allegro CL。函数如下；

    ;; A simple hill-climbing search example adapted from Winston & Horn

(setf (get 's 'neighbors) '(a b)
      (get 'a 'neighbors) '(s c d)
      (get 'b 'neighbors) '(s e)
      (get 'c 'neighbors) '(a d e)
      (get 'd 'neighbors) '(a c e)
      (get 'e 'neighbors) '(b c)
      (get 'f 'neighbors) '(d))
;; I added this for distance assuming that it uses straight line distance in order to 
   search.. not sure if that is correct    
(setf (get 's 'distance) 65
      (get 'a 'distance) 50
      (get 'b 'distance) 48
      (get 'c 'distance) 45
      (get 'd 'distance) 30
      (get 'e 'distance) 28
      (get 'f 'distance) 0)

(defun straight-line-distance (node-1 node-2)
    (let ((coordinates-1 (get node-1 'coordinates))
              (coordinates-2 (get node-2 'coordinates)))
           (sqrt (+ (expt (- (first coordinates-1)
                             (first coordinates-2))
                           2)
                     (expt (- (second coordinates-1)
                              (second coordinates-2))
                           2)))))

(defun closerp (path-1 path-2 target-node)
    (< (straight-line-distance (first path-1) target-node)
           (straight-line-distance (first path-2) target-node)))

(defun hill-climb (start finish &optional
                       (queue (list (list start))))
     (cond ((endp queue) nil)
       ((equal finish (first (first queue)))
           (reverse (first queue)))
       (t (hill-climb start finish
                      (append (sort (extend (first queue))
                                   #'(lambda (p1 p2) (closerp p1 p2 finish)))
                              (rest queue))))))

(defun extend (path)
    (print (reverse path))
    (mapcar #'(lambda (new-node) (cons new-node path))
             (remove-if #'(lambda (neighbor) (member neighbor path))
                        (get (first path) 'neighbors))))

im调用的函数看起来像这样的(hill-climb 's 'd)。

我得到的错误如下；

Error: attempt to call `CLOSERP' which is an undefined function.
[condition type: UNDEFINED-FUNCTION]

Answer 1

我通过编辑(defun straight-line-distance (node-1 node-2)函数修复了它。一定是出了什么差错。我重写了它，并使用了这个；

(defun straight-line-distance (node-1 node-2)
    (let ((coordinates-1 (get node-1 'coordinates))
                (coordinates-2 (get node-2 'coordinates)))
             (sqrt (+ (expt (- (first coordinates-1)
                               (first coordinates-2))
                             2)
                       (expt (- (second coordinates-1)
                                (second coordinates-2))
                             2)))))