Hi folks, can someone please tell me why the follo...

# announcements

[Andy Gibel]

Hi folks, can someone please tell me why the following isn't possible?

object MoonWidgetInput
object MoonWidgetOutput
object MoonWidgetData

abstract class BaseVehicle<INPUT, OUTPUT>

abstract class BaseWidget<INPUT, DATA, OUTPUT>() {
    abstract var vehicle : BaseVehicle<INPUT, OUTPUT>
}

class MoonRover : BaseVehicle<MoonWidgetInput, MoonWidgetOutput>()

class MoonWidget : 
    BaseWidget<MoonWidgetInput, MoonWidgetData, MoonWidgetOutput>(){
    override var vehicle = MoonRover()
}

[Andy Gibel]

The vehicle override in MoonWidget is telling me that MoonRover is not a type of overriden BaseVehicle<MoonWidgetInput, MoonWidgetOutput>

[ephemient]

generic types are invariant by default

[ephemient]

I presume you intended

abstract class BaseVehicle<in INPUT, out OUTPUT>
abstract class BaseWidget<in INPUT, DATA, out OUTPUT>

or something along those lines

[ephemient]

invariant: <Base> and <Derived> are unrelated. example: neither MutableList<Base> nor MutableList<Derived>` can be used in the place of the other

[ephemient]

covariant: <Derived> can be used in place of <Base>. example: List<Derived> can be used in place of List<Base>

[ephemient]

contravariant: <Base> can be used in place of <Derived>. example: (Base) -> Unit can be used in place of (Derived) -> Unit

[Andy Gibel]

thanks! Hmm ok I see the problem but not so clearly the fix. Since my base is the parameterized BaseWidget<X,Y,Z> and derrived is a class extending that, it's THAT relationship that'd have to be covariant right?

[ephemient]

if you only declare BaseVehicle<in INPUT, out OUTPUT> it should work

[Andy Gibel]

I seem to get the same error with this change. As a workaround I could have an abstract setter function instead but I think it'd be cleaner with a property

[Andy Gibel]

variance always confuses the hell out of me for some reason 😕

[ephemient]

works with an explicit type

override var vehicle: BaseVehicle<MoonWidgetInput, MoonWidgetOutput> = MoonRover()

[ephemient]

var has to be invariant and without the type it's inferring it from MoonRover

[Andy Gibel]

Got it! Thanks so much for the detail on explanation. Appreciate it