enum Optional<T> {
    case None
    case Some(T)
}

sealed class Optional<out T> {
    object None : Optional<Nothing>()
    data class Some(val value: T) : Optional<T>()
}

Some

data class Some<out T>(val value: T): Optional<T>()

val object : MyClass? = ...

sealed class MessageResult<out T> {
    data class ok<out T>(val value: T): MessageResult<T>()
    data class failed(val error: MessageError): MessageResult<Nothing>()
}

I guess

Some

should be declared as

data class Some<out T>(val value: T): Optional<T>()

, otherwise it doesn’t compile?

Etiher

Scala

Arrow

Either

Try

Option