Hello! I have a number of classes that might or might not be resolved to some external data source. ...

# codereview

[jwass]

Hello! I have a number of classes that might or might not be resolved to some external data source. So that just means wrapping in a container. A combination of optional / decorator pattern.

sealed class Resolvable<T>
class Resolved<T>(val value: T, val pk: Long) : Resolvable<T>()
class Unresolved<T>(val value: T) : Resolvable<T>() {
    fun resolved(pk: Long) = Resolved(value, pk)
}

Now I want to take a Collection<Resolvable<ItemIdentifier>> and filter only unresolved ones, into a Collection<Unresolved<ItemIdentifier>>.
This works:

val x : Collection<Unresolved<ItemIdentifier>> = itemIdentifiers.mapNotNull { when(it) {is Unresolved -> it else -> null }}

But I wonder if there's a more idiomatic way to do it? (type annotation on x just for clarity).

[Orhan Tozan]

val x: Collection<Unresolved<ItemIdentifier>> = itemIdentifiers.filterIsInstance<Unresolved<ItemIdentifier>>()

[jwass]

!!!! Thank you! Don't know why I didn't see that.

[Orhan Tozan]

Because with an if else, the compiler thinks there is a chance that there would be an else. When basically assures the compiler those are the only 2 cases

[jwass]

So the same sealed class guarantees just don't apply to if?

[dave08]

I think if you put else error("Invalid case") after that if , it should work.

[dave08]

But it's probably not as nice...

[jwass]

Thank you. Not as nice and also not as correct I think. I think there's a clear nudge to use when.

[Tobias Suchalla]

BTW, there might be an even easier option (as seen here).

sealed class Resolvable<T> {
    abstract val value: T
}

data class Resolved<T>(
    override val value: T, 
    val pk: Long
) : Resolvable<T>()

data class Unresolved<T>(
    override val value: T
) : Resolvable<T>()

That way there is always a value on a Resolvable<T>, no need to unwrap.

[jwass]

Thank you! Sorry I didn’t get the notification about this. I did try this approach actually. It works… but in the end it was too verbose to use cleanly.