thread hey guys I m facing an issue with Kotlin type system

Question

thread 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 · Answer

The problem is that the behavior itself is generic `private class MyBehavior lt T gt AbstractBehavior lt Message lt T gt gt `

Marcus Brito · Answer

the receive builder then has a method with the signature

Marcus Brito · Answer

`onMessage type Class lt T gt handler Function lt T Behavior lt T gt gt `

Marcus Brito · Answer

`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 · Answer

is that the only way or is there a better solution

Marcus Brito · Answer

the real code

Emil Kantis · Answer

does this work `fun lt T Y Class lt T gt gt onMessage type Y handler Function lt T Behavior lt T gt gt `

Marcus Brito · Answer

without the cast I ll get a type mismatch

Marcus Brito · Answer

< Emil Kantis> not really the problem is that ` class` will always get the star projection not the generic variant I need

Marcus Brito · Answer

in Java I can do ` onMessage SubType class this handler ` where handler is `private Behavior lt gt handler SubType lt T gt msg `

Marcus Brito · Answer

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 · Answer

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 · Answer

that is ```inline fun lt T reified U T gt ReceiveBuilder lt T gt onMessage noinline handler U gt Behavior lt T gt ReceiveBuilder lt T gt return onMessage U class java handler ```

Marcus Brito · Answer

and then I can do ``` return newReceiveBuilder onMessage lt MessageProcessorDriverMessage lt T gt MessageProcessorDriverMessageReceived lt T gt gt onMessage it ```

Marcus Brito · Answer

wait that does work well if I use a method reference

Marcus Brito · Answer

after defining ```inline fun lt T reified U T gt ReceiveBuilder lt T gt addHandler noinline handler U gt Behavior lt T gt ReceiveBuilder lt T gt return onMessage U class java handler ```

Marcus Brito · Answer

I can do ```return new ReceiveBuilder addHandler onMessage ```

Emil Kantis · Answer

great work < Marcus Brito> slightly smiling face