Kotlin fun interfaces and the type system

fun-interface.kt

fun interface Action {
    fun run()
}

fun schedule(action: Action) = action.run()
fun scheduleFun(action: () -> Unit) = action.invoke()

fun f(): Unit = println("foo")

fun main() {
    // assignment
    val action: Action = Action { println("foo") }
    val action2: Action = { println("foo") } // Type mismatch. Required: Action Found: () → Unit
    val action3: Action = ::f // Type mismatch. Required: Action Found: KFunction0<Unit>
    val foo: () -> Unit = Action { println("foo") } // Type mismatch. Required: () → Unit Found: Action

    // passed as an argument
    schedule(Action { println("foo") })
    schedule { println("foo") }
    schedule(::f)
    schedule(Runnable { println("foo") }) // Type mismatch. Required: Action Found: Runnable
    scheduleFun(Action { println("foo") }) // Type mismatch. Required: () → Unit Found: Action
}