Add exercise 4.47

mceval/amb-natural-language.rkt

@@ -139,15 +139,54 @@
 
 (sentence
  (simple-noun-phrase (article the) (noun professor))
- (verb-phrase (verb lectures)
+ (verb-phrase
-              (prep-phrase (prep to)
+  (verb lectures)
-                           (noun-phrase (simple-noun-phrase (article the) (noun student))
+  (prep-phrase (prep to)
-                                        (prep-phrase (prep in)
+               (noun-phrase (simple-noun-phrase (article the) (noun student))
-                                                     (noun-phrase
+                            (prep-phrase (prep in)
-                                                      (simple-noun-phrase (article the)
+                                         (noun-phrase
-                                                                          (noun class))
+                                          (simple-noun-phrase (article the)
-                                                      (prep-phrase (prep with)
+                                                              (noun class))
-                                                                   (simple-noun-phrase
+                                          (prep-phrase (prep with)
-                                                                    (article the)
+                                                       (simple-noun-phrase
-                                                                    (noun cat)))))))))
+                                                        (article the)
+                                                        (noun cat)))))))))
 ;; The professor lectures to the student, who is in the class with the cat
+
+
+;; Exercise 4.46
+
+;; If amb evaluated its arguments in any order other than
+;; left-to-right, then the recursive maybe-extend rules would never
+;; terminate, as the second argument is a recursive call to the
+;; procedure.  We rely on the recursion stopping at each decision
+;; point.
+;;
+;; Also, since parse has side-effects (namely, it removes a word from
+;; the input), it is important that we parse the different parts of
+;; the sentence in the correct order.
+
+;; Exercise 4.47
+
+(define (parse-verb-phrase level)
+  (log (list 'parse-verb-phrase level))
+  (amb (parse-word verbs)
+       (list 'verb-phrase
+             (parse-verb-phrase (+ level 1))
+             (parse-prepositional-phrase (+ level 1)))))
+
+;; In this case, if (parse-word verbs) fails, then we evaluate the 2nd
+;; argument, which calls parse-verb-phrase recursively, creating a new
+;; amb, which tries (parse-word verbs), which still fails.  In the
+;; orinal procedure, we evaluate (parse-word verbs) once and bind it
+;; to verb-phrase.  We then keep extending the phrase until it either
+;; succeeds or fails.  If it fails, the whole amb call fails.  The
+;; version here never fails because it keeps calling
+;; parse-verb-phrase, leading to an infinite recursion.  The only case
+;; where this procedure works is when we pass it a valid verb phrase
+;; and don't try again (ie trigger a fail).
+
+;; If we swap the order of the clauses in the amb, then we get an
+;; infinite loop, as the first clause immediately calls
+;; parse-verb-phrase recursively.  There is nothing to limit this
+;; recursion.