Hi! Is it possible to declare a function type in Kotlin and have it implement a marker Interface?

functions cannot implement interfaces, but they can be cast to SAMs if the SAM is written in Java

*if the SAM is written in Java

Hm, seems this works

This will compile, but crash at runtime with

ClassCastException

.

Yes, it crashes... So the only options are declare this interface in Java or use object : ITest {...} ?

You can do something like this if you know the signature of the lambda:

interface MyMarker

inline fun MyMarker(crossinline body: () -> Unit): MyMarker = object : MyMarker, () -> Unit {
    override fun invoke() = body()
}

However as you can see it returns

MyMarker

, so you then don't have a way to invoke it. Or you could make it return

() -> Unit

, but then you can't assign it to

MyMarker

.

I'm trying to use JNA lib from Kotlin and define callbacks: a JNA callback is a SAM interface that extends a marker Callback (Java) interface. There is no problem declaring these interfaces in Kotlin, but I hoped it would be nice and easy to provide implementations of such callbacks using lambdas