For instance, Kotlin is using the same collections library as Java. So there are no adapters or conversions needed. Scala has its own collection library i

For instance, Kotlin is using the same collections library as Java. So there are no adapters or conversions needed. Scala has its own collection library instead. Years ago Scala tried the same thing Kotlin currently tries to sell as the hot new stuff. That approach was abandoned altogether, because. it. just. doesn't. work. It's a death by thousand cuts. You can keep sprinkling compiler magic on top of it, but it will be an ongoing game of whack-a-mole. See all the typesystem unsoundness and how they had to abandon (non-)null annotations for existing code because it just didn't scale/work. The whole concept of having mutable and immutable collections within the same API hierarchy that Kotlin tries to sell is also considered to be one of the remaining design issues of Scala collections. Those were terrible times. So happy that Scala doesn't do that anymore!