Hello, I have a question about kotlin, not arrow, ...

# arrow

[zceejkr]

Hello, I have a question about kotlin, not arrow, but I think this might be a good place to ask anyways.
The question is, how do you build a simple sum-type in kotlin?
I have tried sealed classes, but I have hit a wall in this situation:
I wanted to do something like type S = A | B | C.
So i did like so:

sealed class S {}
data class A: S() {}
data class B: S() {}
data class C: S() {}

But then I have a Decoder<T> type, which is defined like so:

interface State<A, S> {
    val runState: (s: S) -> Pair<A, S>
}
typealias Decoder<A> = State<A, ByteArrayBuffer>

Then I created Decoder<A>, Decoder<B> and Decoder<C>.
And I wanted to use a when expression to create Decoder<S>. But the compiler complains that I am returning A instead of S in the Decoder<S> definition.
Any ideas what to do?