Untitled

{-
λ> toRList [13,81,80,79,78,77,76,19,40,41,42,43,44,45,48]
Cons 13 (Sequence 81 76 (Cons 19 (Sequence 40 45 (Cons 48 Nil))))
λ> fromRList (Cons 13 (Sequence 81 76 (Cons 19 (Sequence 40 45 (Cons 48 Nil)))))
[13,81,80,79,78,77,76,19,40,41,42,43,44,45,48]
-}

data RList = Cons Int RList
           | Sequence Int Int RList
           | Nil
           deriving (Show, Eq, Ord)

toRList :: [Int] -> RList
toRList = foldr f Nil
  where
    f :: Int -> RList -> RList
    f x list = case list of
        Nil -> Cons x Nil
        Cons y l ->
            if x `elem` [y-1, y+1] then Sequence x y l
            else Cons x list
        Sequence f t l ->
            if (x == f+1 && f > t) || (x == f-1 && f < t) then Sequence x t l
            else Cons x list

fromRList :: RList -> [Int]
fromRList list = case list of
    Nil -> []
    Cons x l -> x : fromRList l
    Sequence f t l ->
        (if f > t then reverse [t..f] else [f..t]) ++ fromRList l