Kotlin extension on interface

Question

Let's say I have an interface Base and that we implement that interface in class Base1 : Base.

I would expect an extension function in the form

fun ArrayList<Base>.myFun()

to also work on arrayListOf(Base1(), Base1()).myFun(), but it doesn't. It requires the list to be of type Base instead of Base1.

Isn't this really strange? Or am I just missing something?

And, what are my options to write a function available on all subclasses of an interface?

Thanks!

Answer 1

You need to allow extension function to accept child implementation

interface Base
class Base1: Base

fun ArrayList<out Base>.myFun() = println(toString())

fun main(args: Array<String>) {
    arrayListOf(Base1(), Base1()).myFun()

}