Read Statement Being Skipped Over, Unbound Variable - lisp

Question

I'm still working on my number guessing game in Common Lisp, and I've reached a standstill. When the following code is invoked:
;;;; 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)))
I get the error
* (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)
Why is it skipping over the read statement in read-guess? I'm pretty sure the variable is unbound because it's not letting me enter anything in.

Answer 1

Check the syntax of LET.
Example:
(let ((variable (compute-some-value)))
(do-something-with variable)
(do-some-more))