X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment_3_evaluator.mdwn;h=aba94f89cd0a21404f6e7fc51c119cd24e0baaba;hp=c1fac4ea943884fb5dd5107e30a0b7dcabbae591;hb=27ce0d45d4ab28840605ec2130f6ba4ecd9d6213;hpb=53dd78d21542f43fe9bc57b203010cdd5c77599b

diff --git a/assignment_3_evaluator.mdwn b/assignment_3_evaluator.mdwn
index c1fac4ea..aba94f89 100644
--- a/assignment_3_evaluator.mdwn
+++ b/assignment_3_evaluator.mdwn
@@ -1,46 +1,71 @@
 Here are the definitions pre-loaded for working on assignment 3:
 
-<textarea id="INPUT" style="border: 2px solid black; color: black; font-family:
-monospace; height: 3in; overflow: auto; padding: 0.5em; width: 100%;">
-	; booleans
-	let true = \x y. x in
-	let false = \x y. y in
-	let and = \l r. l (r true false) false in
-	let make_pair = \f s g. g f s in
-	let fst = true in
-	let snd = false in
-	let empty = make_pair true junk in
-	let isempty = \x. x fst in
-	let make_list = \h t. make_pair false (make_pair h t) in
-	let head = \l. isempty l err (l snd fst) in
-	let tail = \l. isempty l err (l snd snd) in
-	  
-	; a list of numbers to experiment on
-	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 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 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
-	  
-	; synonyms
-	let makePair = make_pair in
-	let nil = empty in
-	let isNil = isempty in
-	let makeList = make_list in
-	let isZero = iszero in
-	let mult = mul in
-	  
-	length (tail mylist)
+<textarea id="INPUT" style="border: 2px solid black; color: black; font-family: monospace; height: 3in; overflow: auto; padding: 0.5em; width: 100%;">
+; booleans
+let true = \x y. x in
+let false = \x y. y in
+let and = \l r. l (r true false) false in
+let or = \l r. l true r in
+;
+let make\_pair = \f s g. g f s in
+let get\_fst = true in
+let get\_snd = false in
+let empty = make\_pair true junk in
+let isempty = \x. x get\_fst in
+let make\_list = \h t. make\_pair false (make\_pair h t) in
+let head = \l. isempty l err (l get\_snd get\_fst) in
+let tail = \l. isempty l err (l get\_snd get\_snd) in
+;
+; a list of numbers to experiment on
+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
+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
+let fst = get\_fst in
+let snd = get\_snd in
+let nil = empty in
+let isNil = isempty in
+let makeList = make\_list in
+let isZero = iszero in
+let mult = mul in
+;
+let t1 = (make\_list 1 empty) in
+let t2 = (make\_list 2 empty) in
+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 (make\_list t3 empty)) in
+let tc = (make\_list t1 (make\_list t23 empty)) in
+;
+;sum-leaves t1 ; ~~> 1
+;sum-leaves t2 ; ~~> 2
+;sum-leaves t3 ; ~~> 3
+;sum-leaves t12 ; ~~> 3
+;sum-leaves t23 ; ~~> 5
+;sum-leaves ta ; ~~> 6
+;sum-leaves tb ; ~~> 6
+;sum-leaves tc ; ~~> 6 
+;
+; updated: added add, and fold for v1 lists; and defn of tb fixed
+; hint: 
+fold mylist add 0
 </textarea>
+
 <input id="PARSE" value="Normalize" type="button">
 <input id="ETA" type="checkbox">do eta-reductions too
 <noscript><p>You may not see it because you have JavaScript turned off. Uffff!</p></noscript>