First cut of parsing complex sentences

mceval/amb-natural-language.rkt

@@ -4,6 +4,10 @@
 
 (define articles '(article the a))
 
+(define adjectives '(adjective happy lazy))
+
+(define connectives '(and but or))
+
 ;; output of parse
 '(sentence (noun-phrase (article the) (noun cat))
            (verb eats))
@@ -40,16 +44,26 @@
 
 (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
+(define (parse-simple-sentence)
+  (list 'simple-sentence
          (parse-noun-phrase)
          (parse-verb-phrase)))
 
+(define (parse-sentence)
+  (define (maybe-extend sentence)
+    (amb sentence
+         (maybe-extend (list 'compound-sentence
+                             sentence
+                             (parse-word connectives)
+                             (parse-simple-sentence)))))
+  (maybe-extend (parse-simple-sentence)))
+
 (define (parse-verb-phrase)
   (define (maybe-extend verb-phrase)
     (amb verb-phrase
@@ -58,10 +72,11 @@
                              (parse-prepositional-phrase)))))
   (maybe-extend (parse-word verbs)))
 
+;; Extend this to allow an adjective between the article and noun.
 (define (parse-simple-noun-phrase)
   (list 'simple-noun-phrase
         (parse-word articles)
-        (parse-word nouns)))
+        (parse-adjective-phrase)))
 
 (define (parse-noun-phrase)
   (define (maybe-extend noun-phrase)
@@ -71,6 +86,14 @@
                              (parse-prepositional-phrase)))))
   (maybe-extend (parse-simple-noun-phrase)))
 
+;; An adjective phrase is any number of adjectives followed by a
+;; simple noun.
+(define (parse-adjective-phrase)
+  (amb (list (parse-word nouns))
+       (cons
+         (parse-word adjectives)
+         (parse-adjective-phrase))))
+
 
 ;; Exercise 4.45: Output of (parse '(the professor lectures to the student in the class with the cat))