X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment8.mdwn;h=c954e955f249f5f3cf54c2f349ed2f710f962ea0;hp=7e358a8eb248f1f0a3f65720ccf8ddf2918a0427;hb=ad2f53317266fc3a57ba8dd05a0b90c55ef1d472;hpb=4174760988d0cdade3a7dfadf1ef56001ab7a7cf

diff --git a/assignment8.mdwn b/assignment8.mdwn
index 7e358a8e..c954e955 100644
--- a/assignment8.mdwn
+++ b/assignment8.mdwn
@@ -73,22 +73,34 @@
 	is equivalent to:
 
 		(if (test1 argument argument)
+			; then
 			result1
+			; else
 			(if (test2 argument argument)
+				; then
 				result2
+				; else
 				(if (test3 argument argument)
+					; then
 					result3
+					; else
 					result4)))
 
 	Some other Scheme details:
 
 	*	`#t` is true and `#f` is false
+	*	`(lambda () ...)` constructs a thunk
+	*	there is no difference in meaning between `[...]` and `(...)`; we just sometimes use the square brackets for clarity
+	*	`'(1 . 2)` and `(cons 1 2)` are pairs (the same pair)
 	*	`(list)` and `'()` both evaluate to the empty list
 	*	`(null? lst)` tests whether `lst` is the empty list
+	*	non-empty lists are implemented as pairs whose second member is a list
+	*	`'()` `'(1)` `'(1 2)` `'(1 2 3)` are all lists
+	*	`(list)` `(list 1)` `(list 1 2)` `(list 1 2 3)` are the same lists as the preceding
+	*	`'(1 2 3)` and `(cons 1 '(2 3))` are both pairs and lists (the same list)
 	*	`(pair? lst)` tests whether `lst` is a pair; if `lst` is a non-empty list, it will also pass this test; if `lst` fails this test, it may be because `lst` is the empty list, or because it's not a list or pair at all
 	*	`(car lst)` extracts the first member of a pair / head of a list
 	*	`(cdr lst)` extracts the second member of a pair / tail of a list
-	*	`(lambda () ...)` constructs a thunk
 
 	Here is the implementation:
 
@@ -97,7 +109,6 @@
 		                  (cond
 		                    [(pair? tree)
 		                      (helper (car tree) (lambda () (helper (cdr tree) tailk)))]
-		                    [(null? tree) (tailk)]
 		                    [else (cons tree tailk)]))])
 		    (helper tree (lambda () (list)))))
 		
@@ -112,8 +123,8 @@
 		(define (same-fringe? tree1 tree2)
 		  (stream-equal? (lazy-flatten tree1) (lazy-flatten tree2)))
 		
-		(define tree1 '(((1 2) (3 4)) (5 6)))
-		(define tree2 '(1 (((2 3) (4 5)) 6)))
+		(define tree1 '(((1 . 2) . (3 . 4)) . (5 . 6)))
+		(define tree2 '(1 . (((2 . 3) . (4 . 5)) . 6)))
 		
 		(same-fringe? tree1 tree2)