Currently, I have an interface `Foo` as a base for...

# getting-started

[Matyáš Vítek]

Currently, I have an interface Foo as a base for all classes that can emit events, it has a function on that is used to register a handler for that event. The event has a generic type which specifies on which class can the event be emitted. I'd like to override the on function to accept events that can be emitted from Bar (and its subclasses) and the also keep the type of the target of handler the class that the on function is called on. Is there any way to implement this without adding a type argument to the Foo class itself?

interface Foo {
    fun on(signal: Event<in Foo>, handler: Foo.() -> Unit)
}

class Bar : Foo {
    override fun on(signal: Event<in Bar>, handler: Bar.() -> Unit) {
        // ...
    }
}

sealed interface Event<T : Foo> { /* ... */ }

data object SomeEvent : Event<Foo> { /* ... */ }

data object AnotherEvent : Event<Bar> { /* ... */ }

[Ruckus]

No, because you can always cast a Bar to a Foo, so it needs to accept anything Foo can.

[Ruckus]

Subclasses can only widen inputs and narrow outputs, not the other way around.

[Matyáš Vítek]

Oh, I kinda though that it would widen it, but ye, I get it now, thanks!