Hello I have a number of classes that might or might not be

Question

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 lt T gt class Resolved lt T gt val value T val pk Long Resolvable lt T gt class Unresolved lt T gt val value T Resolvable lt T gt fun resolved pk Long = Resolved value pk ``` Now I want to take a `Collection lt Resolvable lt ItemIdentifier gt gt ` and filter only unresolved ones into a `Collection lt Unresolved lt ItemIdentifier gt gt ` This works ```val x Collection lt Unresolved lt ItemIdentifier gt gt = itemIdentifiers mapNotNull when it is Unresolved gt it else gt null ``` But I wonder if there s a more idiomatic way to do it type annotation on `x` just for clarity

Orhan Tozan · Answer

```val x Collection lt Unresolved lt ItemIdentifier gt gt = itemIdentifiers filterIsInstance lt Unresolved lt ItemIdentifier gt gt ```

jwass · Answer

Thank you Don t know why I didn t see that

jwass · Answer

I ve got another question related to the above example I have a sealed class and both implementations contain the same field So I want a function on the abstract type to retrieve it ```sealed class Resolvable lt T gt There s a value in either case fun unwrap T = when this is Resolved gt this value is Unresolved gt this value fun unwrap2 T = if this is Resolved lt T gt this value else if this is Unresolved lt T gt this value data class Resolved lt T gt val value T val pk Long Resolvable lt T gt data class Unresolved lt T gt val value T Resolvable lt T gt ``` My compiler is happy with the `unwrap ` when I write it with a `when` But with an `if` there s a problem with the `else` branch and it thinks that the return type is `Unit` not `T` Any ideas why the `when` version is OK but the `if` version is not

Orhan Tozan · Answer

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 · Answer

So the same sealed class guarantees just don t apply to `if`

dave08 · Answer

I think if you put `else error Invalid case ` after that `if` it should work

dave08 · Answer

But it s probably not as nice

jwass · Answer

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

Tobias Suchalla · Answer

BTW there might be an even easier option as seen <https kotlinlang slack com archives C0B8MA7FA p1634683101076500 thread ts=1634679026 074400 amp cid=C0B8MA7FA|here> ```sealed class Resolvable lt T gt abstract val value T data class Resolved lt T gt override val value T val pk Long Resolvable lt T gt data class Unresolved lt T gt override val value T Resolvable lt T gt ``` That way there is always a `value` on a `Resolvable lt T gt ` no need to unwrap

jwass · Answer

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