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; Because case doesn't like quotes
(defconstant yes 'yes)
(defconstant no 'no)
(defconstant it 'it)
(defun random-elt (list)
"Choose a random element from the given list."
(if (null list)
nil
(elt list (random (length list)))))
(defun query-if (question &optional (pred (lambda (answer) t)))
"Ask until receive a proper answer."
(princ question)
(let ((answer (read)))
(if (funcall pred answer)
answer
(query-if question pred))))
(defun twenty-questions (db n)
"Guess what's in the user's mind and return the updated database."
(if (or (null db) (= n 0))
(let ((answer (query-if "What is it? ")))
(if (assoc answer db)
db
(cons (list answer) db)))
(let* ((guess (random-elt db))
(word (first guess))
(remain (remove guess db)))
(case (query-if (format nil "Is it a kind of ~a? " word)
(lambda (answer) (member answer (list yes no it))))
(yes (cons (cons word (twenty-questions (rest guess) (1- n))) remain))
(no (cons guess (twenty-questions remain (1- n))))
(it db)))))
(defun play (&optional (db nil))
"Play again and again."
(let ((n (query-if "How many questions can be asked? " #'integerp)))
(if (>= n 0)
(play (twenty-questions db n))
(print db))))
(play)
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