hw7: exercise 2, part 1

diff --git a/hw7/JoinList.hs b/hw7/JoinList.hs
index 4edc744bb9dab7df4805db88436a05c957ab4dbd..6ef51a89c42d9294a9c4e9684a7cd95cba988186 100644 (file)
--- a/hw7/JoinList.hs
+++ b/hw7/JoinList.hs
@@ -2,7 +2,7 @@
 module JoinList where
 
 import Data.Monoid
-
+import Sized
 
 data JoinList m a = Empty
                   | Single m a
@@ -20,3 +20,13 @@ Empty +++ x = x
 x +++ Empty = x
 alst1 +++ alst2 = Append ((tag alst1) `mappend` (tag alst2)) alst1 alst2
 
+-- exercise 2
+-- 1. index
+indexJ :: (Sized b, Monoid b) =>
+          Int -> JoinList b a -> Maybe a
+indexJ _ Empty = Nothing
+indexJ n _ | n < 0 = Nothing
+indexJ _ (Single _ a) = Just a
+indexJ n (Append _ l r) | n < (getSize (size (tag l))) = indexJ n l
+                        | otherwise = indexJ (n - getSize (size (tag l))) r
+