Kotlin: Use a lambda in place of a functional interface?

Question

In Java we can do this Events.handler(Handshake.class, hs -> out.println(hs));

In Kotlin however I am trying to replicate the behavior to replace thi

Answer 1

A lot of nice things has happened to Kotlin since @andrey-breslav posted the answer. He mentions:

It is that Kotlin only does SAM-conversion for functions defined in Java. Since Events.handler is defined in Kotlin, SAM-conversions do not apply to it.

Well, that's no longer the case for Kotlin 1.4+. It can use SAM-conversion for Kotlin functions if you mark an interface as a "functional" interface:

// notice the "fun" keyword
fun interface EventHandler {
    fun handle(event: T)
}

You can read the YouTrack ticket here: https://youtrack.jetbrains.com/issue/KT-7770. There's also an explanation why Kotlin needs a marker for such interfaces unlike Java (@FunctionalInterface is only informational and has no effect on the compiler).