Proposal: add indexed map and traverse to Data.List

[Dmitriy Kovanikov]

I want to point out that there already exist Haskell package `ilist` that
provides indexed versions of each function for the list from `base`:

* http://hackage.haskell.org/package/ilist

This package comes with optimized implementations and custom fusion rules.
For example `mapWithIndex` is called `imap` and is implemented like this:

{- |/Subject to fusion./-}imap :: (Int -> a -> b) -> [a] -> [b]imap f
ls = go 0# ls  where    go i (x:xs) = f (I# i) x : go (i +# 1#) xs
go _ _ = []{-# NOINLINE [1] imap #-}

imapFB  :: (b -> t -> t) -> (Int -> a -> b) -> a -> (Int# -> t) ->
Int# -> timapFB c f = \x r k -> f (I# k) x `c` r (k +# 1#){-# INLINE
[0] imapFB #-}{-# RULES"imap"       [~1] forall f xs.    imap f xs =
build (\c n -> foldr (imapFB c f) (\_ -> n) xs 0#)"imapList"   [1]
forall f xs.    foldr (imapFB (:) f) (\_ -> []) xs 0# = imap f xs  #-}


I'm not trying to say that we shouldn't have `mapWithIndex` in `base`. But
the implementation for lists already exists and the inspiration about the
implementation can be taken from it.


On Sat, Aug 17, 2019 at 9:17 AM David Feuer <david.feuer at gmail.com> wrote:

> 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.
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