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 this:

Events.handler(Handshake::class, object : EventHandler<Handshake> {
    override fun handle(event: Handshake) {
        println(event.sent)
    }
})

With a more convenient:

Events.handler(Handshake::class, EventHandler<Handshake> { println(it.sent) })

For some reason in reference to EventHandler :

More preferably however I'd like to use something even shorter like this: Events.handler(Handshake::class, { println(it.sent) })

Or use the advertised feature to use the method like this: Events.handler(Handshake::class) { println(it.sent) }

This is my Events object:

import java.util.*
import kotlin.reflect.KClass

object Events {

    private val map = HashMap<Class<*>, Set<EventHandler<*>>>()

    fun <T : Any> handler(eventType: KClass<T>, handler: EventHandler<T>) {
        handler(eventType.java, handler)
    }

    fun <T> handler(eventType: Class<T>, handler: EventHandler<T>) = handlers(eventType).add(handler)

    fun post(event: Any) = handlers(event.javaClass).forEach { it.handle(event) }

    operator fun plus(event: Any) = post(event)

    private fun <T> handlers(eventType: Class<T>): HashSet<EventHandler<T>> {
        var set = map[eventType]
        if (set == null) {
            set = HashSet<EventHandler<*>>()
            map.put(eventType, set)
        }
        return set as HashSet<EventHandler<T>>
    }

}

And my EventHandler interface:

@FunctionalInterface
interface EventHandler<T> {

    fun handle(event: T)

}

Answer 1

Assuming below that you really need EventHandler as a separate interface (eg for Java interop). If you don't, you can simply use a type alias (since Kotlin 1.1):

typealias EventHandler<T> = (T) -> Unit

In this case a simple lambda will work right away.

But if you don't want to use a type alias, the issue still stands. 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.

To support this syntax:

Events.handler(Handshake::class, EventHandler<Handshake> { println(it.sent) })

You can define a function named EventHandler :

fun <T> EventHandler(handler: (T) -> Unit): EventHandler<T> 
    = object : EventHandler<T> { 
        override fun handle(event: T) = handler(event) 
    }

To support this syntax:

Events.handler(Handshake::class, { println(it.sent) })

or this:

Events.handler(Handshake::class) { println(it.sent) }

You need to overload the handler function to take a function instead of EventHandler :

fun <T> Events.handler(eventType: Class<T>, handler: (T) -> Unit) = EventHandler(handler)

Answer 2

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<T> {
    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 ).