X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment3.mdwn;h=e240b732b1405e176d696bbab3c5974a1d125f35;hp=f84d07f9fa2502b69ed535e172a48e85d8e03027;hb=0ff7150faf184995929760e39abf00eb65a9690b;hpb=bf78e84117aff1a2374786b5021dbbfeec567484

diff --git a/assignment3.mdwn b/assignment3.mdwn
index f84d07f9..e240b732 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.
 
@@ -29,14 +42,16 @@ Recall that version 1 style lists are constructed like this (see
 	; church numerals
 	let iszero = \n. n (\x. false) true in
 	let succ = \n s z. s (n s z) in
+	let add = \l r. l succ r 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
 	
 	; a fixed-point combinator for defining recursive functions
 	let Y = \f. (\h. f (h h)) (\h. f (h h)) in
 	let length = Y (\length l. isempty l 0 (succ (length (tail l)))) in
+	let fold = Y (\f l g z. isempty l z (g (head l)(f (tail l) g z))) in
 	
 	eq 2 2 yes no
 
@@ -117,7 +132,7 @@ 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