X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment8.mdwn;h=f09556f075c06c75c835b94cbc2cb7ff1fedf6f1;hp=a66c26ae54b9b4e1dc5a86428ed7d5eabeed6c27;hb=fff7086c5dec8448c3a5369f3df88b50ffd06e6b;hpb=57e4fb87e24913780f5ed25f975986775adc068a;ds=sidebyside

diff --git a/assignment8.mdwn b/assignment8.mdwn
index a66c26ae..f09556f0 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
@@ -47,7 +45,9 @@
 						| 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
 			;;
@@ -64,7 +64,7 @@
 			;;
 
 
-2.	Here's another implementation of the same-fringe function, in Scheme. It's taken from <http://c2.com/cgi/wiki?SameFringeProblem>. It uses thunks to delay the evaluation of code that computes the tail of a list of a tree's fringe. It also involves passing continuations as arguments. Your assignment is to fill in the blanks in the code, **and also to supply comments to the code,** to explain what every significant piece is doing. Don't forget to supply the comments, this is an important part of the assignment.
+2.	Here's another implementation of the same-fringe function, in Scheme. It's taken from <http://c2.com/cgi/wiki?SameFringeProblem>. It uses thunks to delay the evaluation of code that computes the tail of a list of a tree's fringe. It also involves passing "the rest of the enumeration of the fringe" as a thunk argument (`tail-thunk` below). Your assignment is to fill in the blanks in the code, **and also to supply comments to the code,** to explain what every significant piece is doing. Don't forget to supply the comments, this is an important part of the assignment.
 
 	This code uses Scheme's `cond` construct. That works like this;
 
@@ -109,11 +109,11 @@
 	Here is the implementation:
 
 		(define (lazy-flatten tree)
-		  (letrec ([helper (lambda (tree tailk)
+		  (letrec ([helper (lambda (tree tail-thunk)
 		                  (cond
 		                    [(pair? tree)
-		                      (helper (car tree) (lambda () (helper _____ tailk)))]
-		                    [else (cons tree tailk)]))])
+		                      (helper (car tree) (lambda () (helper _____ tail-thunk)))]
+		                    [else (cons tree tail-thunk)]))])
 		    (helper tree (lambda () _____))))
 		
 		(define (stream-equal? stream1 stream2)