Comment by BoorishBears

Maybe I'm missing something but why does this not work with Java?

Is it the fact you would put your xs.map call in a mapper or something?

Because I know for sure you can do it in Kotlin: https://github.com/michaelbull/kotlin-result

But I don't see reified generics as a requirement for what you describe, and that's the main Kotlin-only feature I see being used