lec07/JoinList.hs


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module JoinList where

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 m _ = m
tag Append m _ _ = m

(+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
(+++) jl1 jl2 = mappend jl1 jl2