cpsTail :: [a] -> o -> ([a] -> o) -> o
cpsTail [] d = \f -> d
cpsTail (a:as) d = \f -> f as

cpsLoop :: (forall o. [a] -> o -> ([a] -> o) -> o) -> [a] -> [a]
cpsLoop f l =
  case f l l id of
    [] -> l
    list -> cpsLoop f list


safeTail :: [a] -> Maybe [a]
safeTail [] = Nothing
safeTail (x:xs) = Just xs

loop :: (a -> Maybe a) -> a -> a
loop f a = maybe a (loop f) (f a)