X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment8.mdwn;h=256b2ce95a021045763f4b378541a7d5b17020dc;hp=495ef2f7218b10182b4e955b8d606dae4dea9c70;hb=911d868126d0b91047b362cb909cdfeb503cd16b;hpb=b0ae43400b808fc96a9762d25868d6517e904a48

diff --git a/assignment8.mdwn b/assignment8.mdwn
index 495ef2f7..256b2ce9 100644
--- a/assignment8.mdwn
+++ b/assignment8.mdwn
@@ -2,9 +2,10 @@
 
 		type 'a tree = Leaf of 'a | Node of ('a tree * 'a tree)
 
-		type 'a starred_tree = Root | Starring_Left of 'a starred_pair | Starring_Right of 'a starred_pair
-		and 'a starred_pair = { parent : 'a starred_tree; sibling: 'a tree }
-		and 'a zipper = { tree : 'a starred_tree; filler: 'a tree };;
+		type 'a starred_level = Root | Starring_Left of 'a starred_nonroot | Starring_Right of 'a starred_nonroot
+		and 'a starred_nonroot = { parent : 'a starred_level; sibling: 'a tree };;
+
+		type 'a zipper = { level : 'a starred_level; filler: 'a tree };;
 
 		let rec move_botleft (z : 'a zipper) : 'a zipper =
 			(* returns z if the targetted node in z has no children *)
@@ -22,22 +23,19 @@
 
 
 		let new_zipper (t : 'a tree) : 'a zipper =
-			{tree = Root; filler = t}
+			{level = Root; filler = t}
 			;;
 
 	 
 
 		let make_fringe_enumerator (t: 'a tree) =
-			(* create a zipper targetting the root of t *)
-			let zstart = new_zipper t
-			in let zbotleft = move_botleft zstart
+			(* create a zipper targetting the botleft of t *)
+			let zbotleft = move_botleft (new_zipper t)
 			(* create a refcell initially pointing to zbotleft *)
 			in let zcell = ref (Some zbotleft)
 			(* construct the next_leaf function *)
 			in let next_leaf () : 'a option =
 				match !zcell with
-				| None -> (* we've finished enumerating the fringe *)
-					None
 				| Some z -> (
 					(* extract label of currently-targetted leaf *)
 					let Leaf current = z.filler
@@ -47,6 +45,8 @@
 						| Some z' -> Some (move_botleft z')
 					(* return saved label *)
 					in Some current
+				| None -> (* we've finished enumerating the fringe *)
+					None
 				)
 			(* return the next_leaf function *)
 			in next_leaf