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

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[Leonid]

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

[Czar]

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

[diesieben07]

*if the SAM is written in Java

[Leonid]

Hm, seems this works

[Leonid]

interface ITest : java.util.RandomAccess {
    fun callback()
}
val tmp = { println() } as ITest

[Leonid]

ITest is declared in Kotlin

[diesieben07]

This will compile, but crash at runtime with ClassCastException.

[Leonid]

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

[diesieben07]

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.

[diesieben07]

I am questioning what you are trying to achieve in the first place, to be honest.

[Leonid]

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