Add indexed fold functions

[elliotdickison]

I was missing the index parameter available in JavaScript's reduce and reduceRight. These changes add indexedFoldl and indexedFoldr to handle use cases where that parameter is helpful/necessary. These functions are identical to List.foldl and List.foldr except that the index of the current element being processed is passed to the reducer function along with the element itself.

[jvoigtlaender]

You forgot to export the functions from the module.

I would swap the order of the first and second arguments to the step function.

For both functions, simpler implementations seem possible by just combining "indexedMap (,)" and foldl/foldr.

[elliotdickison]

Woah thanks for the lightning fast response. Added the exports and switched the args (good call).

I did consider indexedMap + the fold functions, but went with the current implementation to avoid taking an extra pass over the list. The particular use-case that I first wrote these for involved fairly large lists, and I figured that the (albeit very slight) performance gain was worth the slightly more complex implementation under the hood.

[jvoigtlaender]

For indexedFoldr you do have two traversals in the currrent version as well, due to the List.length call. But I do get that it may still be more efficient than going via indexedMap.

[elliotdickison]

Interesting, I didn't know that (makes sense). I can't think of a way around it w/o dropping down to JS though (and even then it would be 2 traversals at best instead of the current 3, looking at the implementation of List.foldr). Thoughts?

[jvoigtlaender]

Let's hear what the maintainer of this package thinks. @abadi199

[abadi199]

I think the current implementation is fine. If anyone can think of a better implementation, just make a new pull request. I'll merge this and publish the package.

[abadi199]

Published! http://package.elm-lang.org/packages/elm-community/list-extra/3.1.0/ Thanks @elliotdickison for your contribution.