Proposal: add indexed map and traverse to Data.List

[chessai .]

I have wanted this for a while, and would prefer imap/itraverse as names.
Iirc this is how the similar functions in vector are named

On Sat, Aug 17, 2019, 10:47 AM Dmitriy Kovanikov <kovanikov at gmail.com>
wrote:

> 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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