X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=arithmetic.mdwn;h=fafecafdba3036c095fe1e50d26e5bf1da6395d9;hp=bc531ecddac814bf05bd763e06c24a01b7f04d24;hb=86a7982ef67a424a823da45ad03a839f64ec56a7;hpb=0b12f248cc9c6b7361a5e3b72d3096a264eec82f

diff --git a/arithmetic.mdwn b/arithmetic.mdwn
index bc531ecd..fafecafd 100644
--- a/arithmetic.mdwn
+++ b/arithmetic.mdwn
@@ -37,24 +37,31 @@ Here are a bunch of pre-tested operations for the untyped lambda calculus. In so
 	let make_list = \h t f z. f h (t f z)  in
 	let isempty = \lst. lst (\h sofar. false) true  in
 	let head = \lst. lst (\h sofar. h) junk  in
-	let tail = \lst. (\shift lst. lst shift (make_pair empty junk) get_2nd)
+	let tail_empty = empty  in
+	let tail = \lst. (\shift. lst shift (make_pair empty tail_empty) get_2nd)
 				; where shift is
 				(\h p. p (\t y. make_pair (make_list h t) t))  in
 	let length = \lst. lst (\h sofar. succ sofar) 0  in
 	let map = \f lst. lst (\h sofar. make_list (f h) sofar) empty  in
 	let filter = \f lst. lst (\h sofar. f h (make_list h sofar) sofar) empty  in ; or
 	let filter = \f lst. lst (\h. f h (make_list h) I) empty  in
-
-	; append list2 to list1 with: list1 make_list list2
 	let singleton = \x f z. f x z  in
-	let reverse = \lst. lst (\h sofar. sofar make_list (singleton h)) empty  in
-	let zip = \left right. left (\h sofar. sofar (\x y. isempty y
-										sofar
-										(make_pair (make_list (\u v. head y (u v) h) x) (tail y))
-									)
-									(make_pair empty (map right (\h u v. u v h)))
-								)
-						(\x y. reverse x)  in
+	; append [a;b;c] [x;y;z] ~~> [a;b;c;x;y;z]
+	let append = \left right. left make_list right  in
+	; very inefficient but correct reverse
+	let reverse = \lst. lst (\h sofar. append sofar (singleton h)) empty  in ; or
+	; more efficient reverse builds a left-fold instead
+	; (make_left_list a (make_left_list b (make_left_list c empty)) ~~> \f z. f c (f b (f a z))
+	let reverse = (\make_left_list lst. lst make_left_list empty) (\h t f z. t f (f h z))  in
+	; zip [a;b;c] [x; y; z] ~~> [(a,x);(b,y);(c,z)]
+	let zip = \left right. (\base build. reverse left build base (\x y. reverse x))
+                       ; where base is
+                       (make_pair empty (map (\h u. u h) right))
+                       ; and build is
+                       (\h sofar. sofar (\x y. isempty y
+                                               sofar
+                                               (make_pair (make_list (\u. head y (u h)) x)  (tail y))
+                       ))  in
 	let all = \f lst. lst (\h sofar. and sofar (f h)) true  in
 	let any = \f lst. lst (\h sofar. or sofar (f h)) false  in