X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment8.mdwn;h=f09556f075c06c75c835b94cbc2cb7ff1fedf6f1;hp=5a2780d8459e95e45fa07ea0de4ce26c26a88a76;hb=53965b8b9083936486589d19e5846d76f2945a24;hpb=1c203e3ca6a3b29830af36ea6f6b3a86be93675f

diff --git a/assignment8.mdwn b/assignment8.mdwn
index 5a2780d8..f09556f0 100644
--- a/assignment8.mdwn
+++ b/assignment8.mdwn
@@ -29,16 +29,13 @@
 	 
 
 		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
@@ -48,7 +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
 			;;
@@ -65,7 +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 (`tailk`s) 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;
 
@@ -110,11 +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)