Split {Functor,Foldable,Traversable}WithIndex to a separate package

[arybczak]

These can be made a standalone package easily.

Then:

• unordered-containers and vector can supply instances instead of optics-extra
• people can use it without pulling in optics as the classes are useful on their own
• lens can switch to using these classes so that we don't duplicate them (I saw claims that they need to have ifolded, itraversed and imapped as class member for performance, but I doubt that's the case considering that we don't do that and have the same performance in benchmarks as lens).

Thoughts?

[phadej]

I'd like to have that package, especially if we get lens use it. Otherwise I think it would be useless.

If lens would also use map-with-index (because indexed-functors is something different, given indexed-profunctors), then there might be enough "pressure" to reverse the dependency so unordered-containers and vector would depend on it, not the other way around. This is justified as it is more likely (though highly unlikely) that FunctorWithIndex would be in base than that the HashMap would be there.

So, I think we have to convince Edward (about the performance) first before making the package. I also think that ifolded in lens don't need to be a member, but it's just a hunch.

[adamgundry]

I think this might be useful to packages other than lens, and if someone wants it then splitting it out makes sense, but I agree the motivation is much more compelling if lens itself can use it. (I wonder if any other lens libraries might benefit from it? My first thought was microlens but that doesn't seem to have indexed optics.)

[ocharles]

Is this perhaps a resurrection of Edward's https://hackage.haskell.org/package/keys-3.12.3 library?

[arybczak]

@ocharles Right, it looks like it would be a subset of it. Most of the dependencies of this package would have to be reversed though.

[phadej]

This is done now.