X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment_3_evaluator.mdwn;h=aba94f89cd0a21404f6e7fc51c119cd24e0baaba;hp=f4f5fb929b3fa5b70d3fe27a0b9d77dde81a7170;hb=73b21c99d5a3f8184681a1fa1e0bb2b44ab4c34f;hpb=0a79dd27b99169648a1f03ec0b8847a1c5b822fc

diff --git a/assignment_3_evaluator.mdwn b/assignment_3_evaluator.mdwn
index f4f5fb92..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
@@ -48,7 +49,7 @@ let t3 = (make\_list 3 empty) in
 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
@@ -58,8 +59,11 @@ let tc = (make\_list t1 (make\_list t23 empty)) in
 ;sum-leaves t23 ; ~~> 5
 ;sum-leaves ta ; ~~> 6
 ;sum-leaves tb ; ~~> 6
+;sum-leaves tc ; ~~> 6 
 ;
-length (tail mylist)
+; updated: added add, and fold for v1 lists; and defn of tb fixed
+; hint: 
+fold mylist add 0
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