Neither of those has or intends to have HKT, and they'll never be an acceptable replacement for Scala without it. They're "improving faster" but only in that they're starting from further behind.
The reason why Kotlin is gaining marketshare is specifically because they aren’t chasing after things like HKTs. Kotlin is squarely going after the better Java market and not the advanced FP one.
I think Kotlin's design - adding lots of ad-hoc support for specific use cases but without the underlying general constructs that make them coherent - will come to bite them as and when the language needs to evolve over time. (Indeed to a certain extent it already has, as Java 8+ adopts Options which don't play nice with Kotlin's ?. etc.) I guess we'll see how things look in a few years.
? actually should play nicely with explicit Option types, they do in C# anyways.
I was excited about Kotlin at a first glance but came to feel this way as well, after looking into it a bit more.
We may never get HKT directly, but associated type constructors can give you similar powers and fit into the language more naturally, along with impl Trait in traits. We’ll see how it shakes out.
I thought I saw HKT planned for Rust, as in, being in the works.
Not directly, no. You’re probably thinking of ATC.
I thought this RFC was for HKT only: https://github.com/rust-lang/rfcs/issues/324 this is the one I have been paying some attention (not like I use HKT that often in Scala anyway)
That’s not an RFC; it’s just an issue where people talk about things. Actual RFCs are PRs to that repo that contain a design.