From: Jim Pryor
Date: Mon, 4 Oct 2010 01:26:51 +0000 (-0400)
Subject: assign 4: hints for list_equal
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=1859a8c4b42139e3a557ec44dd8edd1e1acb71f9
assign 4: hints for list_equal
Signed-off-by: Jim Pryor
---
diff --git a/hints/assignment_4_hint_2.mdwn b/hints/assignment_4_hint_2.mdwn
index 78cb1bd4..5ee63f55 100644
--- a/hints/assignment_4_hint_2.mdwn
+++ b/hints/assignment_4_hint_2.mdwn
@@ -1,4 +1,51 @@
-Hints for list\_equal.
+Hints for `list_equal`.
+
+* If `left` is `[]`, what does `right` have to be for `left` and `right` to be equal? (Come on, it's not too hard, you can figure it out.)
+
+* Suppose on the other hand that `left` has head `left_hd` and tail `left_tl`.
+
+
+ - If `right` is then `[]`, are `left` and `right` equal?
+
- If `right` isn't `[]`, and its head isn't equal to `left_hd`, are `left` and `right` equal?
+
- If `right` isn't `[]` and its head *is* equal to `left_hd`, what else has to be the case for `left` and `right` to be equal?
+

+
+* Can you now write a recursive definition of the `list_equal` function?
+What's your base case?
+
+
+
+
diff --git a/lambda_library.mdwn b/lambda_library.mdwn
index 5623b9fa..6441d3b9 100644
--- a/lambda_library.mdwn
+++ b/lambda_library.mdwn
@@ -314,7 +314,7 @@ let list_equal =
; (might_for_all_i_know_still_be_equal?, tail_of_reversed_right)
; when left is empty, the lists are equal if right is empty
(make_pair
- (not (isempty right))
+ true ; for all we know so far, they might still be equal
(reverse right)
)
; when fold is finished, check sofar-pair