Thanks. This is helpful. I guess part of what I'm wondering is how much Kotlin interfaces compare to Scala traits in the sense explained here:

https://en.wikibooks.org/wiki/Scala/Traits

Or, in another example, how does use of Rust impl map onto Kotlin?

http://www.jonathanturner.org/2016/02/down-the-rabbit-hole-w...

I really don't mean this as a criticism of Kotlin, I'm just trying to map this all out. Kotlin seems nice and like something I could use, but I'm sure it has its boundaries also.