【Land of Lisp】一次练习：巫师文本冒险游戏

绪论

Common Lisp是一门多范式语言，支持多种编程模式，包括面向对象编程、函数式编程。但Common Lisp鼓励函数式编程，并且包含有许多函数式编程相关的功能。

《Land of Lisp》是一本寓教于乐的学习Lisp语法的书籍。这本书配以漫画插图来进行表达，并且将小游戏的制作作为演示和练习实例。本文就《Land of Lisp》第5、6章演示、练习给出相应的代码，并作一定的辅助的分析解释。

代码实现

基本框架

场景机制

;;;; 定义场景
(defparameter *nodes* '((living-room
                         (you are in the living-room.
                          a wizard is snoring loudly on the couch.))
                        (garden
                         (you are in a beautiful garden.
                          there is a well in front of you.))
                        (attic
                         (you are in the attic.
                          there is a giant welding torch in the corner.))))
(defparameter *edges* '((living-room (garden west door)
                         (attic upstairs ladder))
                        (garden (living-room east door))
                        (attic (living-room downstairs ladder))))
 
(defun describe-location (location nodes)
  "1. assoc将'(listA listB listC)中，listA/B/C的第一个元素作为关键字，获取listA/B/C
   2. (cadr list) => (car(cdr list)) 获取列表中第二个元素"
  (cadr (assoc location nodes)))
(defun describe-path (edge)
  `(there is a ,(caddr edge) going ,(cadr edge) from here.))
(defun describe-paths (location edges)
  "mapcar: (mapcar method list) 将method依次作用于list中各元素"
  (apply #'append (mapcar #'describe-path (cdr (assoc location edges)))))
 
;;;; 查看场景
(defparameter *location* 'living-room)
(defun look ()
  (append (describe-location *location* *nodes*)
          (describe-paths *location* *edges*)
          (describe-objects *location* *objects* *object-locations*)))
 
(defun walk (direction)
  "find的用法示范: (find 'y '((5 x) (3 y) (7 z)) :key #'cadr)
  => (3 Y)"
  (let ((next (find direction
                    (cdr (assoc *location* *edges*))
                    :key #'cadr)))
    (if next
        (progn (setf *location* (car next))
               (look))
        '(you cannot go that way.))))

物品机制

;;;; 定义物品
(defparameter *objects* '(whiskey bucket frog chain))
(defparameter *object-locations* '((whiskey living-room)
                                   (bucket living-room)
                                   (chain garden)
                                   (frog garden)))
(defun objects-at (loc objs obj-locs)
  "对于给定的loc地点，给定objs物品列表，检查是否处在该地点
  remove-if-not过滤掉列表中不符合条件的元素"
  (labels ((at-loc-p (obj)
             (eq (cadr (assoc obj obj-locs)) loc)))
    (remove-if-not #'at-loc-p objs)))
(defun describe-objects (loc objs obj-loc)
  (labels ((describe-obj (obj)
             `(you see a ,obj on the floor.)))
    (apply #'append (mapcar #'describe-obj (objects-at loc objs obj-loc)))))
 
;;;; 捡起物品
(defun pickup (object)
  "检查在当前loc地点下，输入的物品object是否存在
  若存在，则“将物品转移”到身体
  否则，输出禁止的提示语句"
  (cond ((member object
                 (objects-at *location* *objects* *object-locations*))
         (push (list object 'body) *object-locations*)
         `(you are now carrying the ,object))
        (t '(you cannot get that.))))
(defun inventory ()
  "将身体内的物品列出并输出为提示"
  (cons 'items- (objects-at 'body *objects* *object-locations*)))
 

用户界面

(defun game-repl ()
  (let ((cmd (game-read)))
    (unless (eq (car cmd) 'quit)
      (game-print (game-eval cmd))
      (game-repl))))

读取部分

(defun game-read ()
  "read-from-string: 把字符串转变为表达式/符号(去除双引号)"
  (let ((cmd (read-from-string
              (concatenate 'string "(" (read-line) ")"))))
    (flet ((quote-it (x)
             (list 'quote x)))
      (cons (car cmd) (mapcar #'quote-it (cdr cmd))))))

执行部分

(defparameter *allowed-commands* '(look walk pickup inventory))
(defun game-eval (sexp)
  (if (member (car sexp) *allowed-commands*)
      (eval sexp)
      '(i do not know that command.)))

显示部分

(defun tweak-text (lst caps lit)
  "给定字符列表，逐字扫描，调整大小写
  1. 如果为空格，将大小写设置保留到下一字符
  2. 如果为标点符号，设置下一字符为大写
  3. 如果出现双引号字符，开启lit(literal)不设置大小写，不扫描字符，不作处理
  4. 如果开启cap(capital)则将字母转换为为大写
  5. 否则，将字母转换为小写"
  (when lst
    (let ((item (car lst))
          (rest (cdr lst)))
      (cond ((eq item #\space) (cons item (tweak-text rest caps lit)))
            ((member item '(#\! #\? #\.)) (cons item (tweak-text rest t lit)))
            ((eq item #\") (tweak-text rest caps (not lit)))
            (lit (cons item (tweak-text rest nil lit)))
            (caps (cons (char-upcase item) (tweak-text rest nil lit)))
            (t (cons (char-downcase item) (tweak-text rest nil nil)))))))
 

(defun game-print (lst)
  "打印储存在lst中的符号。实现句首大写，引号中的部分保留大小写和符号
  1. prin1-to-string: 将lisp列表打印成字符串且不输出
  2. string-trim: (string-trim str1 str2)将字符串str2中，位于左侧或右侧的特定字符去除，直到遇到给定范围外的字符；特定字符由str1给定
  3. coerce: 
     - (coerce string 'list)将字符串string转换成字符列表
     - (coerce list 'string)将字符列表list转换为字符串
  4. tweak-text: 自定义函数，用于中字符列表中逐字检查，并中必要时作调整"
  (princ
   (coerce 'string (tweak-text
            (coerce (string-trim "() "
                                 (prin1-to-string lst)) 'list)
            t nil) 'string))
  (fresh-line))