X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=assignment4.mdwn;h=8b8a19dc78c1b71ec20e64e6df79d8599f245755;hp=1eec95e1422d65266ef5349ae41484029db52ee7;hb=2dad468862d6fff48c8f499386dc571bd899bc28;hpb=a0f99722a5a10cb939258a6c7eba32a28f2944e8
diff --git a/assignment4.mdwn b/assignment4.mdwn
index 1eec95e1..8b8a19dc 100644
--- a/assignment4.mdwn
+++ b/assignment4.mdwn
@@ -32,12 +32,41 @@ How would you implement such a list comparison?
First, read this: [[Implementing trees]]
-- blah
+
- Write an implementation of leaf-labeled trees. You can do something v3-like, or use the Y combinator, as you prefer.
+
+ You'll need an operation `make_leaf` that turns a label into a new leaf. You'll need an operation `make_node` that takes two subtrees (perhaps leaves, perhaps other nodes) and joins them into a new tree. You'll need an operation `isleaf` that tells you whether a given tree is a leaf. And an operation `extract_label` that tells you what value is associated with a given leaf.
+
+
- The **fringe** of a leaf-labeled tree is the list of values at its leaves, ordered from left to right. For example, the fringe of this tree:
+
+ .
+ / \
+ . 3
+ / \
+ 1 2
+
+is [1;2;3]. And that is also the fringe of this tree:
+
+ .
+ / \
+ 1 .
+ / \
+ 2 3
+
+The two trees are different, but they have the same fringe. We're going to
+return later in the term to the problem of determining when two trees have the
+same fringe. For now, one straightforward way to determine this would be:
+enumerate the fringe of the first tree. That gives you a list. Enumerate the
+fringe of the second tree. That also gives you a list. Then compare the two
+lists to see if they're equal. (You just programmed this above.)
+
+Write the fringe-enumeration function. It should work on the implementation of
+trees you designed in the previous step.
(See [[hints/Assignment 4 hint 3]] if you need some hints.)
+
#Mutually-recursive functions#