Proposal: Add mapWithIndex to Data.List

[Henning Thielemann]

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