unfoldl :: (a -> Maybe (a, b)) -> a -> [b]
unfoldl f a = go a []
  where
    go a l = case f a of
        Nothing     -> l
        Just (a, b) -> go a (b:l)

unfoldr :: (a -> Maybe (b, a)) -> a -> [b]
unfoldr f = go
  where
    go a = case f a of
        Nothing     -> []
        Just (b, a) -> b:go a

natl :: Int -> Maybe (Int, Int)
natl x
    | x <= 0    = Nothing
    | otherwise = Just (x-1, x)

testl :: Bool
testl = unfoldl natl 10 == [1,2,3,4,5,6,7,8,9,10]

natr :: Int -> Maybe (Int, Int)
natr = fmap (\(x,y) -> (y,x)) . natl

testr :: Bool
testr = unfoldr natr 10 == [10,9,8,7,6,5,4,3,2,1]