X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment3.mdwn;h=8a4efcc0d34dd9de201013699e3156f7de26cc2c;hp=df06d526a99c58495f34260629d8977bbc1f9e91;hb=29f15e8e027b34b893874598824bdbff45096c78;hpb=6a60dbeb81ccc4949724a6db13a264a0b322a22d

diff --git a/assignment3.mdwn b/assignment3.mdwn
index df06d526..8a4efcc0 100644
--- a/assignment3.mdwn
+++ b/assignment3.mdwn
@@ -1,6 +1,19 @@
 Assignment 3
 ------------
 
+Erratum corrected 11PM Sun 3 Oct: the following line
+
+	let tb = (make_list t12 (make_list t3 empty)) in
+
+originally read 
+
+	let tb = (make_list t12 t3) in
+
+This has been corrected below, and in the preloaded evaluator for 
+working on assignment 3, available here: [[assignment 3 evaluator]].
+
+<hr>
+
 Once again, the lambda evaluator will make working through this
 assignment much faster and more secure.
 
@@ -30,7 +43,7 @@ Recall that version 1 style lists are constructed like this (see
 	let iszero = \n. n (\x. false) true in
 	let succ = \n s z. s (n s z) in
 	let mul = \m n s. m (n s) in
-	let pred = \n. iszero n 0 (length (tail (n (\p. make_list junk p) empty))) in
+	let pred = (\shift n. n shift (make\_pair 0 0) get\_snd) (\p. p (\x y. make\_pair (succ x) x)) in
 	let leq = \m n. iszero(n pred m) in
 	let eq = \m n. and (leq m n)(leq n m) in
 	
@@ -107,7 +120,7 @@ be your base case for your recursive functions that operate on these
 trees.
 
 <OL start=6>
-<LI>Write a function that sums the number of leaves in a tree.
+<LI>Write a function that sums the values at the leaves in a tree.
 
 Expected behavior:
 
@@ -117,7 +130,7 @@ Expected behavior:
 	let t12 = (make_list t1 (make_list t2 empty)) in
 	let t23 = (make_list t2 (make_list t3 empty)) in
 	let ta = (make_list t1 t23) in
-	let tb = (make_list t12 t3) in
+	let tb = (make_list t12 (make_list t3 empty)) in
 	let tc = (make_list t1 (make_list t23 empty)) in
 
 	sum-leaves t1 ~~> 1