intervals :: (Eq a, Enum a) => [a] -> [(a, Maybe a)]
intervals []     = []
intervals (x:[]) = [(x, Nothing)]
intervals (x:xs) = (x, lastSuccessor x x xs) : intervals (dropSuccessors x xs)
  where
    lastSuccessor :: (Eq a, Enum a) => a -> a -> [a] -> Maybe a
    lastSuccessor l x []
        | l == x                  = Nothing
        | otherwise               = Just x
    lastSuccessor l x (y:ys)
        | succ x == y             = lastSuccessor l y ys
        | otherwise               = lastSuccessor l x []
    dropSuccessors :: (Eq a, Enum a) => a -> [a] -> [a]
    dropSuccessors x []           = []
    dropSuccessors x done@(y:ys)
        | succ x == y             = dropSuccessors y ys
        | otherwise               = done