hw7: add takeJ

diff --git a/hw7/JoinList.hs b/hw7/JoinList.hs
index e498eedeee67aca1d79710a856558bce0918e7bb..7a353cded5f694c8e7fc606a1258bc8d8d86142a 100644 (file)
--- a/hw7/JoinList.hs
+++ b/hw7/JoinList.hs
@@ -47,3 +47,16 @@ dropJ n (Append t l r) | n > getSize (size t) = Empty
                        | otherwise = let lft = dropJ n l in
                                      Append ((tag lft) `mappend` (tag r)) lft r
 
+-- 3. take
+takeJ :: (Sized b, Monoid b) =>
+         Int -> JoinList b a -> JoinList b a
+takeJ 0 _ = Empty
+takeJ n _ | n < 0 = Empty
+takeJ _ Empty = Empty
+takeJ n v@(Single _ _) | n == 0 = v
+                       | otherwise = Empty
+takeJ n v@(Append t l r) | n > getSize (size t) = v
+                         | n < getSize (size (tag l)) = takeJ n l
+                         | otherwise = let rt = takeJ (n - getSize (size (tag l))) r in
+                                       Append ((tag l) `mappend` (tag rt)) l rt
+