A modern programming language that makes developers happier.

kotlinlang

I was missing that functionality a lot from swifts enums, and though I'd heard the term 'sealed' classes I'd somehow thought it just meant 'final'...

Yes I think that's where I got the impression from.

Obviously a wrong one, but like many other times, I was glad to be wrong.  Sealed class is a very useful feature.

Especially since when you can inherit data class from it.

Fact it cannot be 'inner' and hence use the type parameters of the parent class is somewhat of a limitation