:thread: hey guys, I’m facing an issue with Kotlin type system and generic class types. In particula...

# server

[Marcus Brito]

🧵 hey guys, I’m facing an issue with Kotlin type system and generic class types. In particular, I’m working with Akka Typed and its ReceiveBuilder#onMessage signature.

[Marcus Brito]

The problem is that the behavior itself is generic:

private class MyBehavior<T> : AbstractBehavior<Message<T>>

[Marcus Brito]

the receive builder then has a method with the signature:

[Marcus Brito]

onMessage(type: Class<T>, handler: Function<T, Behavior<T>>)

[Marcus Brito]

type is supposed to be a subtype of T, but with the generics argument in there I can’t get it to compile without an explicit cast

[Marcus Brito]

is that the only way, or is there a better solution?

[Marcus Brito]

the real code:

[Emil Kantis]

does this work? fun <T, Y: Class<T>> onMessage(type: Y, handler: Function<T, Behavior<T>>)

[Marcus Brito]

without the cast I’ll get a type mismatch

[Marcus Brito]

@Emil Kantis not really; the problem is that ::class will always get the star projection, not the generic variant I need.

[Marcus Brito]

in Java I can do .onMessage(SubType.class, this::handler), where handler is private Behavior<...> handler(SubType<T> msg)

[Marcus Brito]

I get an unchecked generics warning, but it does work; apparently in Kotlin the best I can do is creating that var with the class object

[Marcus Brito]

I tried defining an extension method with a reified type param, but it’s also quite verbose — the compiler doesn’t infer the right types from the handler alone.

[Marcus Brito]

that is:

inline fun <T, reified U : T> ReceiveBuilder<T>.onMessage(noinline handler: (U) -> Behavior<T>): ReceiveBuilder<T> {
    return onMessage(U::class.java, handler)
}

[Marcus Brito]

and then I can do:

return newReceiveBuilder()
            .onMessage<MessageProcessorDriverMessage<T>, MessageProcessorDriverMessageReceived<T>> { onMessage(it) }

[Marcus Brito]

wait, that does work well if I use a method reference!

[Marcus Brito]

after defining…

inline fun <T, reified U : T> ReceiveBuilder<T>.addHandler(noinline handler: (U) -> Behavior<T>): ReceiveBuilder<T> {
    return onMessage(U::class.java, handler)
}

[Marcus Brito]

I can do

return new ReceiveBuilder().addHandler(::onMessage)

[Emil Kantis]

great work @Marcus Brito! 🙂