In parse-word, remove the restriction that the first item of *unparsed* is a member of word-list will succeed for any non-empty phrase. In order to return a valid word, just make found-word a random item from word-list.
(interpret '(define (parse-word word-list)
(require (not (null? *unparsed*)))
(let ((found-word (random-word (cdr word-list))))
(set! *unparsed* (cdr *unparsed*))
(list (car word-list) found-word))))
(interpret '(define (random-word word-list)
(list-ref word-list (random (length word-list)))))
Note the original program generates all possible sentence structures for the given input and so this version only generates a random example sentence for each of those structures. It doesn’t generate all possible sentences that have the same structure as the input.
;;; Amb-Eval input:
(parse '(the professor lectures to the cat in the class))
;;; Starting a new problem
;;; Amb-Eval value:
(sentence
(simple-noun-phrase (article a)
(noun professor))
(verb-phrase (verb-phrase (verb eats)
(prep-phrase (prep by)
(simple-noun-phrase (article the)
(noun cat))))
(prep-phrase (prep to)
(simple-noun-phrase (article a)
(noun class)))))
;;; Amb-Eval input:
try-again
;;; Amb-Eval value:
(sentence
(simple-noun-phrase (article a)
(noun professor))
(verb-phrase (verb eats)
(prep-phrase (prep by)
(noun-phrase (simple-noun-phrase (article the)
(noun cat))
(prep-phrase (prep in)
(simple-noun-phrase (article the)
(noun professor)))))))
;;; Amb-Eval input:
try-again
;;; Amb-Eval value:
(sentence
(noun-phrase (simple-noun-phrase (article a)
(noun professor))
(prep-phrase (prep in)
(simple-noun-phrase (article a)
(noun professor))))
(verb-phrase (verb sleeps)
(prep-phrase (prep with)
(simple-noun-phrase (article a)
(noun cat)))))
;;; Amb-Eval input:
try-again
;;; Amb-Eval value:
(sentence
(noun-phrase
(noun-phrase (simple-noun-phrase (article a)
(noun professor))
(prep-phrase (prep in)
(simple-noun-phrase (article a)
(noun professor))))
(prep-phrase (prep for)
(simple-noun-phrase (article a)
(noun class))))
(verb sleeps))
;;; Amb-Eval input:
try-again
;;; Amb-Eval value:
(sentence
(noun-phrase
(simple-noun-phrase (article a)
(noun professor))
(prep-phrase (prep in)
(noun-phrase (simple-noun-phrase (article a)
(noun professor))
(prep-phrase (prep to)
(simple-noun-phrase (article the)
(noun class))))))
(verb sleeps))
;;; Amb-Eval input:
try-again
;;; There are no more values of
(parse '(the professor lectures to the cat in the class))
Update:
Having looked at the Exercise 4.50, I think there is another answer, which is probably what the authors were looking for:
(interpret '(define (remove-item item items)
(cond ((null? items) null)
((equal? item (car items)) (cdr items))
(else (cons (car items) (remove-item item (cdr items)))))))
(interpret '(define (random-element-of items)
(require (not (null? items)))
(let ((item (list-ref items (random (length items)))))
(amb item (random-element-of (remove-item item items))))))
(interpret '(define (parse-word word-list)
(require (not (null? *unparsed*)))
(let ((found-word (random-element-of (cdr word-list))))
(set! *unparsed* (cdr *unparsed*))
(list (car word-list) found-word))))
This gives a more comprehensive result than last time as all possible sentences that match all possible sentence structures derived from the given sentence.
SICP exercises 