data Stream a = a :& Stream a

unfoldStream :: (b -> (a, b)) -> b -> Stream a
unfoldStream f = \b -> case f b of
  (a, b’) -> a :& unfoldStream f b’

iterate :: (a -> a) -> a -> Stream a
iterate f = unfoldStream $ \a -> (a, f a)

repeat :: a -> Stream a
repeat = unfoldStream $ \a -> (a, a)

zip :: Stream a -> Stream b -> Stream (a, b)
zip = curry $ unfoldStream $ \(a :& as, b :& bs) -> ((a, b), (as, bs))