From: Jim Pryor Date: Sat, 27 Nov 2010 04:45:19 +0000 (-0500) Subject: assign8 tweaks X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=94d8065f557f70e448841089c6860e208fed55f6 assign8 tweaks Signed-off-by: Jim Pryor --- diff --git a/assignment8.mdwn b/assignment8.mdwn index 7e358a8e..abc14d08 100644 --- a/assignment8.mdwn +++ b/assignment8.mdwn @@ -83,8 +83,13 @@ Some other Scheme details: * `#t` is true and `#f` is false + * `'(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)` `(list1)` `(list 1 2)` `(list 1 2 3)` are the same lists + * `'(1 2 3)` and `(cons 1 '(2 3))` are pairs that are also 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 @@ -97,7 +102,6 @@ (cond [(pair? tree) (helper (car tree) (lambda () (helper (cdr tree) tailk)))] - [(null? tree) (tailk)] [else (cons tree tailk)]))]) (helper tree (lambda () (list))))) @@ -112,8 +116,8 @@ (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)