meufoldr :: (a -> b -> b) -> b -> [a] -> b
meufoldr f base []     = base
meufoldr f base (x:xs) = f x (meufoldr f base xs)

meufoldl :: (b -> a -> b) -> b -> [a] -> b
meufoldl f base []     = base
meufoldl f base (x:xs) = meufoldl f (f base x) xs

-- "implementando" identidade de listas usando foldr e foldl

idr :: [a] -> [a]
idr = foldr (:) []

idl :: [a] -> [a]
idl = foldl (\xs x -> xs ++ [x]) []

naturais = [0,1..]

naturaisr = idr naturais
naturaisl = idl naturais