hw7: revisit indexJ

diff --git a/hw7/JoinList.hs b/hw7/JoinList.hs
index 7f13837e02d6bb79962d1fc654a6df9da70b5f4f..3dca2de3d50507e7295029d273ec9145747aa296 100644 (file)
--- a/hw7/JoinList.hs
+++ b/hw7/JoinList.hs
@@ -26,19 +26,10 @@ 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
+indexJ n (Single _ a) | n == 0 = Just a
+                      | otherwise = Nothing
+indexJ n (Append t l r) | n >= getSize (size t) = Nothing
+                        | n < 0 = Nothing
+                        | n < (getSize (size (tag l))) = indexJ n l
                         | otherwise = indexJ (n - getSize (size (tag l))) r
 
--- 2. drop
-dropJ :: (Sized b, Monoid b) =>
-         Int -> JoinList b a -> JoinList b a
-dropJ n x | n < 0 = x
-dropJ 0 x = x
-dropJ _ Empty = Empty
-dropJ _ (Single _ _) = Empty
-dropJ n x | (getSize (size (tag x))) < n = Empty
-dropJ n (Append _ l r) | n > (getSize (size (tag l))) =
-                           dropJ (n - getSize (size (tag l))) r
-                       | otherwise = let lft = dropJ n l in
-                                     Append ((tag lft) `mappend` (tag r)) lft r