hw7: exercise 1

diff --git a/hw7/JoinList.hs b/hw7/JoinList.hs
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+{-# OPTIONS_GHC -Wall #-}
+module JoinList where
+
+import Data.Monoid
+
+
+data JoinList m a = Empty
+                  | Single m a
+                  | Append m (JoinList m a) (JoinList m a)
+                    deriving (Eq, Show)
+
+-- exercise 1
+tag :: Monoid m => JoinList m a -> m
+tag (Empty) = mempty
+tag (Single t _) = t
+tag (Append t _ _) = t
+
+(+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
+Empty +++ x = x
+x +++ Empty = x
+alst1 +++ alst2 = Append ((tag alst1) `mappend` (tag alst2)) alst1 alst2
+