GNU Common Lisp程序输入流错误求助：对象内输入流终止

错误原因 #

这个READ错误是因为代码中存在未闭合的括号，Lisp读取器解析到文件末尾时，仍有未匹配的开括号（来自defmacro里的progn），导致无法完成S表达式解析。

具体修复步骤 #

1. 闭合未完成的括号 #

原defmacro mac中的progn语句没有闭合，需要在dfs函数定义结束后添加一个闭合括号，完成progn和defmacro的定义：原代码中(defun dfs ...))))之后直接接了(let ...)，补一个)即可让progn的所有内容被正确包裹。

2. 修正状态合法性检查的错误 #

原valid-state-p函数存在三个问题：

• 传教士数量判断错误：(< m 3)会排除初始状态的3个传教士，改为(<= m 3)
• 船的状态符号不统一：代码里用I但调用时传的是l，统一改为'(l r)
• 遗漏核心规则：传教士与食人魔问题中，当某一岸传教士数量大于0时，不能少于食人魔数量，否则状态非法

修正后的valid-state-p：

(defun valid-state-p (state)
  (let ((m (car state)) (c (cadr state)) (b (caddr state))
        (m-left (- 3 m)) (c-left (- 3 c)))
    (and (>= m 0) (<= m 3)
         (>= c 0) (<= c 3)
         (member b '(l r))
         (or (= m 0) (>= m c))
         (or (= m-left 0) (>= m-left c-left)))))

3. 补充右岸的合法移动逻辑 #

原legal-moves函数中，船在右岸（r）时没有任何移动逻辑，导致无法返回左岸，补充对应移动选项：

(when (equal b 'r)
  (when (and (<= m 1) (<= c 3))
    (push (move state 2 0) moves))
  (when (and (<= m 2) (<= c 3))
    (push (move state 1 0) moves))
  (when (and (<= m 3) (<= c 1))
    (push (move state 0 2) moves))
  (when (and (<= m 3) (<= c 2))
    (push (move state 0 1) moves))
  (when (and (<= m 2) (<= c 2))
    (push (move state 1 1) moves)))

4. 修正DFS函数的条件分支问题 #

原代码中dfs函数的if分支数量错误，改为unless更简洁正确，避免逻辑执行异常。

修复后的完整代码 #

(defmacro mac (start end)
  `(progn
     (defparameter *start* ',start)
     (defparameter *end* ',end)
     (defparameter *visited* '())

     (defun valid-state-p (state)
       (let ((m (car state)) (c (cadr state)) (b (caddr state))
             (m-left (- 3 m)) (c-left (- 3 c)))
         (and (>= m 0) (<= m 3)
              (>= c 0) (<= c 3)
              (member b '(l r))
              (or (= m 0) (>= m c))
              (or (= m-left 0) (>= m-left c-left)))))

     (defun goal-state-p (state)
       (equal state *end*))

     (defun move (state dm dc)
       (if (equal (caddr state) 'l)
           (list (- (car state) dm) (- (cadr state) dc) 'r)
           (list (+ (car state) dm) (+ (cadr state) dc) 'l)))

     (defun legal-moves (state)
       (let* ((m (car state)) (c (cadr state)) (b (caddr state)) (moves '()))
         (when (equal b 'l)
           (when (and (>= m 2) (>= c 0))
             (push (move state 2 0) moves))
           (when (and (>= m 1) (>= c 0))
             (push (move state 1 0) moves))
           (when (and (>= m 0) (>= c 2))
             (push (move state 0 2) moves))
           (when (and (>= m 0) (>= c 1))
             (push (move state 0 1) moves))
           (when (and (>= m 1) (>= c 1))
             (push (move state 1 1) moves)))
         (when (equal b 'r)
           (when (and (<= m 1) (<= c 3))
             (push (move state 2 0) moves))
           (when (and (<= m 2) (<= c 3))
             (push (move state 1 0) moves))
           (when (and (<= m 3) (<= c 1))
             (push (move state 0 2) moves))
           (when (and (<= m 3) (<= c 2))
             (push (move state 0 1) moves))
           (when (and (<= m 2) (<= c 2))
             (push (move state 1 1) moves)))
         (remove-if-not #'valid-state-p moves)))

     (defun dfs (state path)
       (if (goal-state-p state)
           (reverse path)
           (unless (member state *visited*)
             (setf *visited* (cons state *visited*))
             (dolist (next-state (legal-moves state))
               (let ((result (dfs next-state (cons next-state path))))
                 (when result
                   (return result)))))))))

(let ((solution (dfs *start* '())))
  (if solution
      (dolist (step solution)
        (format t "~a ~a ~a~%" (car step) (cadr step) (caddr step)))
      (format t "nil")))
(mac '(3 3 l) '(0 0 r))

内容的提问来源于stack exchange，提问作者Droopity Droop