Hi. I have a data type data Line a = Line [a] [a] which groups elements into "ordered" (first list) and "other" (second list) ones. And i want a functions representing Line, which actually has two heads (one from 1st list and one from 2nd), as single-headed. In other words, i want to map a list over Line (preserving Line structure), i.e. implement a function with type f :: [a -> b] -> Line a I have done this using Traversable import Data.Monoid import qualified Data.Foldable as F import qualified Data.Traversable as T import Control.Applicative import Control.Monad.State instance Functor Line where fmap f (Line xs ys) = Line (map f xs) (map f ys) instance F.Foldable Line where foldMap f (Line xs ys) = (F.foldMap f xs) `mappend` (F.foldMap f ys) instance T.Traversable Line where traverse f (Line xs ys) = Line <$> (T.traverse f xs) <*> (T.traverse f ys) and my function (which actually used for adding separators (sp ++) into (Line String), and i don't want to add separator before the first element, regardless of whether first element is "ordered" or "other") looks like inlineSeps :: (a -> a) -> Line a -> Line a inlineSeps g = fst . flip runState (id : repeat g) . T.mapM f where f x = do (f : fs) <- get put fs return (f x) It works, but i'm not sure whether using state monad here is good? And whether this is good solution for such problem at all?