I loved Ruby: an OO lang with some FP on top where it makes sense. I found Kotlin to be very similar to Ruby: primary on OO lang with FP on top where it makes s

I loved Ruby: an OO lang with some FP on top where it makes sense. I found Kotlin to be very similar to Ruby: primary on OO lang with FP on top where it makes sense, but now with compile-time types (slightly but fundamentally better than Java's) built in. The main thing I miss in Kotlin is proper, first-class sum types. There is a trick with using inheritance to achieve it, but not good enough for me. They've become a fundamental tool for me (same level as record (data class) and collections). I get how that would make Java-interop much harder, and that that's likely why they've been left out.