foldrとunfoldrは双対

並べてみるとわかりやすい

型の対応がキレイに取れている

foldr

code:hs

foldr f [] = f Nothing

foldr f (x:xs) = f (Just (x, foldr f xs))

unfoldr

code:hs

unfoldr f x = case f x of

Nothing -> []

Just (b, a) -> b : unfoldr f a