X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment_3_evaluator.mdwn;h=aba94f89cd0a21404f6e7fc51c119cd24e0baaba;hp=c4032ee37c7f1ec456a09f7ed5a88093f932bd64;hb=9597d0f4796382fd4b18e85f2c5eb37a5f60e715;hpb=d50ce29231fc7454463b0a1b7b898022db6b67d1

diff --git a/assignment_3_evaluator.mdwn b/assignment_3_evaluator.mdwn
index c4032ee3..aba94f89 100644
--- a/assignment_3_evaluator.mdwn
+++ b/assignment_3_evaluator.mdwn
@@ -22,6 +22,7 @@ let mylist = make\_list 1 (make\_list 2 (make\_list 3 empty)) in
 ; 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 = (\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
@@ -29,8 +30,8 @@ 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
 ;
 ; synonyms
 let makePair = make\_pair in
@@ -60,11 +61,9 @@ let tc = (make\_list t1 (make\_list t23 empty)) in
 ;sum-leaves tb ; ~~> 6
 ;sum-leaves tc ; ~~> 6 
 ;
-let add = \l r. l succ r in
-let fold = Y (\f g l z. isempty l z (g (head l)(f g (tail l) z))) in
-;
+; updated: added add, and fold for v1 lists; and defn of tb fixed
 ; hint: 
-fold add mylist 0
+fold mylist add 0
 </textarea>
 
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