Add exercise 4.47

mceval/amb-natural-language.rkt

@@ -165,3 +165,28 @@
 ;; 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.