Sorry, I didn't find a better place to ask this. Shouldn't a sealed class be able to implement a sea...

# general-advice

[juliocbcotta]

Sorry, I didn't find a better place to ask this.
Shouldn't a sealed class be able to implement a sealed interface?

sealed interface Container {
 interface Foo : Container
 interface Bar : Container
}

In some other package, this is what we can get today

sealed class BaseContainer {
   data class FooImpl(val something: Something) : Container.Foo
   object BarImpl: Container.Bar
}

With this I can't use BaseContainer instances as Container in an exhausting when without a explicit casting as BaseContainer can't implement the sealed class Container.

[CLOVIS]

You have to mark the sealed class itself as inheriting the Container interface

sealed class BaseContainer : Container { // <-
   data class FooImpl(val something: Something) : Container.Foo
   object BarImpl: Container.Bar
}

[juliocbcotta]

but we can't do that!

[juliocbcotta]

that is what I found odd, I would expect that to be allowed, but it isn't

[CLOVIS]

Ah, BaseContainer is in another module?

[juliocbcotta]

in another package... but it is the same as in another module

[juliocbcotta]

if we don't share the package, we can't do anything

[CLOVIS]

No, if it's in another package of the same module you should be able to inherit it, sealed is about modules

[juliocbcotta]

https://kotlinlang.org/docs/sealed-classes.html

No other subclasses may appear outside the module and package within which the sealed class is defined.

[CLOVIS]

Sorry, I wasn't aware it was limited to the package too. That's good to know.

[CLOVIS]

But well, then it makes sense that BaseContainer cannot be a Container .

[CLOVIS]

You can write

fun BaseContainer.asContainer(): Container = when (this) {
    is BaseContainer.FooImpl -> this
    is BaseContainer.BarImpl -> this
}

Of course, it's not as clean

[CLOVIS]

If I were to guess, the reason it's not automatic is that you could add another subclass to BaseContainer , that didn't inherit from Container , and that would break all code elsewhere that relied on Container

[Klitos Kyriacou]

I don't follow Ivan's example. this is a BaseContainer, and therefore it can't be a BaseContatiner.FooImpl or BarImpl because those two nested classes are not subclasses of BaseContainer.

[Klitos Kyriacou]

Anyway, it seems your original question has already been answered:

Shouldn't a sealed class be able to implement a sealed interface?

Yes, indeed it can. As long as it's in the same package.

[CLOVIS]

inside the when , this is smart-casted to BaseContainer.FooImpl (or BarImpl in the other branch), which is a valid value to return when a Container is expected

[Klitos Kyriacou]

It won't work: the compiler says Incompatible types: BaseContainer.FooImpl and BaseContainer. Which is understandable, as there is no relationship between those two types. It's just like you can't say if (myString is Int).
See playground

[CLOVIS]

Ah, I didn't notice FooImpl and BarImpl don't implement BaseContainer . In that case, there is no link whatsoever between them and BaseContainer .

[CLOVIS]

My version works if you add that inheritance relationship (playground)

[juliocbcotta]

thanks, that is better than what I was doing before.