Generalised unfoldr

unfoldr.hs

-- See http://www.staff.city.ac.uk/~ross/papers/FingerTree.html
class Monoid m => Measured a m where
    measure :: a -> m

instance Measured a [a] where
    measure a = [a]

-- Generalised to any resulting monoid from `unfoldr :: (b -> Maybe (a, b)) -> b -> [a]`
unfoldr :: Measured a m => (b -> Maybe (a, b)) -> b -> m
unfoldr f b = case f b of
    Nothing -> mempty
    Just (a, b) -> measure a <> unfoldr f b

-- This definition remains unchanged compared to the one based on the standard `unfoldr`
iterate :: (a -> a) -> a -> [a]
iterate f = unfoldr (\a -> Just (a, f a))