simplify the code a bit

[yorgey.git] / hw7 / JoinList.hs

diff --git a/hw7/JoinList.hs b/hw7/JoinList.hs
index 4edc744bb9dab7df4805db88436a05c957ab4dbd..916597e04fc56f1ae31483f5aa13c542976c8ee1 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 +20,45 @@ 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 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 n v@(Single _ _) | n == 0 = v
+                       | otherwise = Empty
+dropJ n (Append t l r) | n > getSize (size t) = Empty
+                       | n > leftSize =
+                           dropJ (n - leftSize) r
+                       | otherwise = let lft = dropJ n l in
+                                     Append ((tag lft) `mappend` (tag r)) lft r
+                       where leftSize = getSize . size . tag $ l
+
+-- 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 < leftSize = takeJ n l
+                         | otherwise = let rt = takeJ (n - leftSize) r in
+                                       Append ((tag l) `mappend` (tag rt)) l rt
+                         where leftSize = getSize . size . tag $ l
+