Move typelevel/algebra into cats repo.

[LukaJCB]

This was discussed during the cats meeting at Scala By The Bay this year and I think most of us thought it'd be a good idea.

[johnynek]

+1

[kailuowang]

+1 under a different root package or cats.algebra?

[johnynek]

maybe it should be cats algebra. It is a shame when we have two different artifact ids with the same class names inside.

[johnynek]

on second thought... I don't know why I said that. The artifact can be the same: org.typelevel:algebra so I don't see why we would have the artifact ID collision.

So in that case, it is best to just keep it as it is.

[denisrosset]

What is the timeframe for that move? I'd like to discuss in algebra some changes I am making in the Spire hierarchy, so that could impact what goes finally into cats.

[ctongfei]

@johnynek I prefer cats.algebra. Sometimes you have your own package called X and you have a package X.algebra and this sometimes causes path collision -- to refer to Typelevel Algebra you must do _root_.algebra. See use cases like this:
https://github.com/ctongfei/nexus/blob/master/algebra/src/main/scala/nexus/algebra/IsReal.scala#L12:15

So maybe cats.algebra._ is a better choice?

[benhutchison]

If algebra and cats merge, it’s an opportunity for aligning the syntax and type class definitions, see typelevel/algebra#199

[LukaJCB]

Would we want a separate module without the cats-core dependency, or a "real" merge with the dependency or possibly even same module?

[johnynek]

I think we want a separate module because projects like Spire and Algebird that don’t (generally) have higher kinder type classes.

We could put them in kernel, or make algebra depend only on kernel.

[benhutchison]

IIRC, Kernel was split out to enable the Algebra project to depend on Cats (and not the other way round). If that was the predominant reason (?), then I prefer condensing them into kernel than adding an intermediate node in the dependency tree.

[johnynek]

Kernel was split out of algebra. Cats used to depend on algebra but for some reason Cats wanted to break that dependency. As a compromise we moved the core abstractions into Kernel.

If we are to move all of algebra into kernel then we are back to the starting place of cats depending on all of Algebra but we have renamed it Kernel.

[LukaJCB]

What was the reason to split algebra into kernel? 🤔 Anyone of the old guard still have some info? @non maybe?

[johnynek]

There is a cats issue where it was discussed. The reason was cats wanted fewer dependencies I think and didn’t care about Rings.

[LukaJCB]

I've been trying to find an issue that accurately describes the actual problem with having the Ring and Lattice family in cats, and I can't really seem to find a good reason anywhere? Is it bloat? Jar size? Compatibility guarantees?