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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
-- Exercise 2
indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a
indexJ _ Empty = Nothing
indexJ n (Single _ a)
| n == 0 = Just a
| otherwise = Nothing
indexJ n (Append m jl1 jl2)
| n < 0 || n >= m = Nothing
| n < tag1 = indexJ n jl1
| n >= tag1 = indexJ (n - tag1) jl2
where tag1 = tag jl1
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