Re: Proposal: add indexed map and traverse to Data.List

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 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 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. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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