閱讀聲明被忽略，無約束變量

Question

我仍在Common Lisp中進行數字猜測遊戲，並且我已經停滯不前。當下面的代碼被調用：

;;;; number-game.lisp 
;;;; 
;;;; Andrew Levenson 
;;;; 10/25/2010 
;;;; 
;;;; Simple number guessing game. User has 
;;;; five guesses to determine a number between 
;;;; one and one hundred, inclusive (1-100). 

;;; Set global variable for the target number: 
(defparameter *target* nil) 

;;; Set the iterator so we may check the number of guesses 
(defparameter *number-of-guesses* 0) 

;;; Welcome the user 
(defun welcome-user() 
(format t "Welcome to the number guessing game!~%")) 

;;; Prompt for a guess 
(defun prompt-for-guess() 
(format t "Please enter your guess (1-100): ") 
(finish-output nil) ; nil directs finish-output to standard IO 
(check-guess 'read-guess)) 

;;; Read in a guess 
(defun read-guess() 
(let (guess (read))) 
(if (numberp guess) ; If true, return guess. Else, call prompt-for-guess 
    (progn 
    (setq *number-of-guesses* (+ *number-of-guesses* 1)) 
    guess) 
    (prompt-for-guess))) 

;;; Check if the guess is higher than, lower than, or equal to, the target 
(defun check-guess (fn) 
(setq guess (funcall fn)) 
(if (equal guess *target*) 
    (equal-to) 
    (if (> guess *target*) 
    (greater-than (guess)) 
    (if (< guess *target*) 
    (less-than (guess)))))) 

;;; If the guess is equal to the target, the game is over 
(defun equal-to() 
(format t "Congratulations! You have guessed the target number, ~a!~%" *target*) 
(y-or-n-p "Play again? [y/n] ")) 

;;; If the guess is greater than the target, inform the player. 
(defun greater-than (guess) 
(format t "Sorry, ~a is greater than the target.~%" guess) 
(if (< *number-of-guesses* 6) 
    (prompt-for-guess) 
    (game-over))) 

;;; If the guess is less than the target, inform the player. 
(defun less-than (guess) 
(format t "Sorry, ~a is less than the target.~%" guess) 
(if (< *number-of-guesses* 6) 
    (prompt-for-guess) 
    (game-over))) 

;;; If the player has run out of guesses, give them the option 
;;; of playing the game again. 
(defun game-over() 
(y-or-n-p "You have run out of guesses. Play again? [y/n] ")) 


;;; Play the game 
(defun play() 
;; If it's their first time playing this session, 
;; make sure to greet the user. 
(unless (> *number-of-guesses* 0) 
    (welcome-user)) 
;; Reset their remaining guesses 
(setq *number-of-guesses* 0) 
;; Set the target value 
(setq *target* 
    ;; Random can return float values, 
    ;; so we must round the result to get 
    ;; an integer value. 
    (round 
    ;; Add one to the result, because 
    ;; (random 100) yields a number between 
    ;; 0 and 99, whereas we want a number 
    ;; from 1 to 100 inclusive. 
    (+ (random 100) 1))) 
(if (equal (prompt-for-guess) "y") 
    (play) 
    (quit))) 

我得到的錯誤

* (play) 
Welcome to the number guessing game! 
Please enter your guess (1-100): 
debugger invoked on a UNBOUND-VARIABLE in thread #<THREAD 
                "initial thread" RUNNING 
                {AA14959}>: 
    The variable GUESS is unbound. 

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL. 

restarts (invokable by number or by possibly-abbreviated name): 
    0: [ABORT] Exit debugger, returning to top level. 

(READ-GUESS) 

爲什麼跳過讀聲明read-guess？我很確定變量沒有被綁定，因爲它不讓我輸入任何東西。

Answer 1

檢查LET的語法。

實施例：

(let ((variable (compute-some-value))) 
    (do-something-with variable) 
    (do-some-more))