On Sat, 31 Jan 2015, David Feuer wrote:
Vectors offer `imap` and sequences offer `mapWithIndex`. I think lists deserve to have something like this too. The obvious implementation, using `zipWith`, is unlikely to be the most efficient (because foldr/build fusion doesn't handle zips so wonderfully).
To a first approximation:
mapWithIndex :: (Int -> a -> b) -> [a] -> [b] mapWithIndex f xs = build $ \c n -> let go x cont !i = f i x `c` cont (i+1) in foldr go (`seq` n) 0
It can also be written using Traversable. I have once written the following more general function for this purpose: zipWithTraversable :: (Traversable f) => (a -> b -> c) -> Stream a -> f b -> f c zipWithTraversable f as0 = snd . mapAccumL (\(Stream.Cons a as) b -> (as, f a b)) as0