mapWithIndex :: (Int -> a -> b) -> [a] -> [b] mapWithIndex f = zipWith f [0..] traverseWithIndex :: Applicative f => (Int -> a -> f b) -> [a] -> f [b] traverseWithIndex f = sequenceA . mapWithIndex The real implementation of mapWithIndex (and therefore of traverseWithIndex) can be a "good consumer" for list fusion. mapWithIndex can be a "good producer" as well (which the naive implementation already accomplishes). Similar functions (with these or similar names) are already common in packages like vector, containers, unordered-containers, and primitive. A more general function would merge zipping with unfolding: zipWithUnfoldr :: (a -> b -> c) -> (s -> Maybe (b, s)) -> [a] -> s -> [c] zipWithUnfoldr f g as s = zipWith f as (unfoldr g s) But this doesn't seem like the friendliest or most obvious user interface, so I am not proposing to add it to base.