WIP Add lots of logging to understand how the interpreter works

mceval/amb-natural-language.rkt

@@ -1,28 +1,22 @@
 (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))
 
-;; output of parse
+(define (log message level)
-'(sentence (noun-phrase (article the) (noun cat))
+  (define (output message level)
-           (verb eats))
+    (if (= level 0) message
+        (cons '_ (output message (- level 1)))))
+  (display (output message level))
+  (newline))
 
-(define (parse-sentence)
+(define (parse-word word-list level)
-  (list 'sentence
+  (log (list 'word-list word-list '*unparsed* *unparsed*) level)
-         (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* '())
@@ -33,43 +27,61 @@
     (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-noun-phrase)
-         (parse-verb-phrase)))
+         (parse-verb-phrase 1)))
 
-(define (parse-verb-phrase)
+(define (parse-noun-phrase level)
-  (define (maybe-extend verb-phrase)
-    (amb verb-phrase
-         (maybe-extend (list 'verb-phrase
-                             verb-phrase
-                             (parse-prepositional-phrase)))))
-  (maybe-extend (parse-word verbs)))
-
-(define (parse-simple-noun-phrase)
-  (list 'simple-noun-phrase
-        (parse-word articles)
-        (parse-word nouns)))
-
-(define (parse-noun-phrase)
   (define (maybe-extend noun-phrase)
+    (log '(maybe-extend-noun) level)
     (amb noun-phrase
          (maybe-extend (list 'noun-phrase
                              noun-phrase
-                             (parse-prepositional-phrase)))))
+                             (parse-prepositional-phrase level)))))
-  (maybe-extend (parse-simple-noun-phrase)))
+  (log '(parse-noun-phrase) level)
+  (maybe-extend (parse-simple-noun-phrase level)))
+
+;; Note maybe-extend is tail recursive
+(define (parse-verb-phrase level)
+  (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)))
+
+(define (parse-simple-noun-phrase level)
+  (log '(parse-simple-noun) level)
+  (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))))
+
+
+
+
+
+
+
+
 
 
 ;; Exercise 4.45: Output of (parse '(the professor lectures to the student in the class with the cat))
@@ -139,15 +151,51 @@
 
 (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.
+
+;; 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.

mceval/ambeval.rkt

@@ -209,6 +209,7 @@
             (begin
               (newline)
               (display ";;; Starting a new problem ")
+              (newline)
               (set! start-time (current-process-milliseconds))
               (ambeval input
                        env