Exercise 4.48: add parsing complex sentences

mceval/amb-natural-language.rkt

@@ -6,15 +6,12 @@
 
 (define adjectives '(adjective happy lazy))
 
+(define connectives '(connective and but or))
+
 ;; output of parse
 '(sentence (noun-phrase (article the) (noun cat))
            (verb eats))
 
-(define (parse-sentence)
-  (list 'sentence
-         (parse-noun-phrase)
-         (parse-word verbs)))
-
 (define (parse-noun-phrase)
   (list 'noun-phrase
         (parse-word articles)
@@ -48,11 +45,20 @@
         (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