Proposal: add indexed map and traverse to Data.List

[Elliot Cameron]

As long as the names are in Data.List I don't care as much. But these will
clash with the same names in lens which is very commonly imported
unqualified.

On Sat, Aug 17, 2019 at 3:59 PM chessai . <chessai1996 at gmail.com> wrote:

> 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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