Add exercise 4.47

mceval/amb-natural-language.rkt

@@ -1,22 +1,28 @@
 (define nouns '(noun student professor cat class))
+
 (define verbs '(verb studies lectures eats sleeps))
+
 (define articles '(article the a))
-(define prepositions '(prep for to in by with))
 
-(define (log message level)
-  (define (output message level)
-    (if (= level 0) message
-        (cons '_ (output message (- level 1)))))
-  (display (output message level))
-  (newline))
+;; output of parse
+'(sentence (noun-phrase (article the) (noun cat))
+           (verb eats))
 
-(define (parse-word word-list level)
-  (log (list 'word-list word-list '*unparsed* *unparsed*) level)
+(define (parse-sentence)
+  (list 'sentence
+         (parse-noun-phrase)
+         (parse-word verbs)))
+
+(define (parse-noun-phrase)
+  (list 'noun-phrase
+        (parse-word articles)
+        (parse-word nouns)))
+
+(define (parse-word word-list)
   (require (not (null? *unparsed*)))
   (require (memq (car *unparsed*) (cdr word-list)))
   (let ((found-word (car *unparsed*)))
     (set! *unparsed* (cdr *unparsed*))
-    (log (list (car word-list) found-word) level)
     (list (car word-list) found-word)))
 
 (define *unparsed* '())
@@ -27,61 +33,43 @@
     (require (null? *unparsed*))
     sent))
 
+
+;: (parse '(the cat eats))
+;; output of parse
+'(sentence (noun-phrase (article the) (noun cat)) (verb eats))
+
+(define prepositions '(prep for to in by with))
+
+(define (parse-prepositional-phrase)
+  (list 'prep-phrase
+        (parse-word prepositions)
+        (parse-noun-phrase)))
+
 (define (parse-sentence)
   (list 'sentence
-         ;;(parse-noun-phrase)
-         (parse-verb-phrase 1)))
+         (parse-noun-phrase)
+         (parse-verb-phrase)))
 
-(define (parse-noun-phrase level)
-  (define (maybe-extend noun-phrase)
-    (log '(maybe-extend-noun) level)
-    (amb noun-phrase
-         (maybe-extend (list 'noun-phrase
-                             noun-phrase
-                             (parse-prepositional-phrase level)))))
-  (log '(parse-noun-phrase) level)
-  (maybe-extend (parse-simple-noun-phrase level)))
-
-;; Note maybe-extend is tail recursive
-(define (parse-verb-phrase level)
+(define (parse-verb-phrase)
   (define (maybe-extend verb-phrase)
-    (log '(maybe-extend-verb-phrase) level)
     (amb verb-phrase
          (maybe-extend (list 'verb-phrase
                              verb-phrase
-                             (parse-prepositional-phrase level)))))
-  (log '(parse-verb-phrase) level)
-  (maybe-extend (parse-word verbs level)))
+                             (parse-prepositional-phrase)))))
+  (maybe-extend (parse-word verbs)))
 
-(define (parse-simple-noun-phrase level)
-  (log '(parse-simple-noun) level)
+(define (parse-simple-noun-phrase)
   (list 'simple-noun-phrase
-        (parse-word articles level)
-        (parse-word nouns level)))
-
-(define (parse-prepositional-phrase level)
-  (log '(parse-prep) level)
-  (list 'prep-phrase
-        (parse-word prepositions level)
-        (parse-noun-phrase level)))
-
-
-
-;; Note: not tail recursive
-(define (parse-verb-phrase level)
-  (log '(parse-verb-phrase) level)
-  (amb (parse-word verbs level)
-       (list 'verb-phrase
-             (parse-verb-phrase (+ level 1))
-             (parse-prepositional-phrase level))))
-
-
-
-
-
-
-
+        (parse-word articles)
+        (parse-word nouns)))
 
+(define (parse-noun-phrase)
+  (define (maybe-extend noun-phrase)
+    (amb noun-phrase
+         (maybe-extend (list 'noun-phrase
+                             noun-phrase
+                             (parse-prepositional-phrase)))))
+  (maybe-extend (parse-simple-noun-phrase)))
 
 
 ;; Exercise 4.45: Output of (parse '(the professor lectures to the student in the class with the cat))
@@ -193,7 +181,10 @@
 ;; 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.
+;; 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