If I have: ```sealed interface Foo<R> object NoFoo : Foo<Nothing> data class SomeFoo<...

# getting-started

[dave08]

If I have:

sealed interface Foo<R>
object NoFoo : Foo<Nothing>
data class SomeFoo<R> : Foo<R>

// Is there any way this could work? (It doesn't... even though I don't use the R in NoFoo)
val baz: Foo<Baz> = NoFoo

[Rob Elliot]

Try using <out R>

[Rob Elliot]

sealed interface Foo<out R>
object NoFoo : Foo<Nothing>
data class SomeFoo<out R>(val r: R) : Foo<R>
interface Baz

val baz: Foo<Baz> = NoFoo

[dave08]

Thanks! I still have a bit of trouble with all this covariance etc... It seems in my case I had to use in with Any instead of Nothing...

[Rob Elliot]

Without know your real use case it's hard to comment... in Any sounds like a pretty useless type.

[dave08]

I don't really need it in that object... my use case is a bit complicated... I need that as a return type from a callback () -> Foo<SomeType> then, if I have a SomeFoo as a result of that callback, then I need it in something like setResult(someFoo.value)

[Rob Elliot]

Would this help?

sealed interface Foo<out R> {
  val value: R
}
object NoFoo : Foo<Nothing> {
  override val value: Nothing
    get() = error("unsupported")
}

data class SomeFoo<out R>(override val value: R) : Foo<R>

[Rob Elliot]

Slack is refusing to let me edit that...

val foo: Foo<String> = TODO()
if (foo is SomeFoo) {
  setResult(foo.value)
}

[Rob Elliot]

Alternatively, are you sure you aren't reinventing kotlin.Result?

[dave08]

I'm not... I have three possibilities 1) Don't change anything, 2) Set a new value (including setting to null), 3) Set an exception (that was caught). kotlin.Result only has the last two...

[dave08]

I wonder if having that <Any> there is so bad... compared to the alternatives that are much more verbose...?

[dave08]

I can't edit my posts either @Alina Dolgikh [JB]... it seems like someone restricted editing posts on this slack...

[dave08]

Or maybe they're just having problems now?