oh, I just realised the code above is fine, the re...

# announcements

[David Smith]

oh, I just realised the code above is fine, the reason is that my code uses a function, not just the value

sealed class Parent() {
    data class Child(val value: String) : Parent()
}
data class Other(val value: (Parent) -> String)
val child = Child("something")
val f: (Child) -> String = { child -> child.value}
val x = Other(f as (Parent) -> String)

[diesieben07]

Yes, the function expects a Child. You are casting it to something that expects a Parent, so someone could pass in an Other to your casted function - which would make that casted function fail.

[David Smith]

I don’t quite understand, in what situation could this fail?

[David Smith]

any function who’s input is a subtype of Parent wouldn’t fail

[diesieben07]

If Parent had more than one child:
x.value(Parent.Child2())
Now the function fails, because it assumes it gets a Child, not a Child2

[David Smith]

aahhh

[David Smith]

got it thanks

[diesieben07]

Look up covariant vs. contravariant vs. invariant

[David Smith]

I understand it, I just didn’t see it

[David Smith]

it’s obvious now, thanks!