{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
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)


    tag :: Monoid m => JoinList m a -> m
    tag (Single m _) = m
    tag (Append m _ _) = m

    (+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
    (+++) elem1@(Single m1 _) elem2@(Single m2 _) =
        Append (mappend (tag elem1) (tag elem2)) elem1 elem2