Is it possible to return early from an exhaustive `when`?

# announcements

[zak.taccardi]

Is it possible to return early from an exhaustive when?

[streetsofboston]

It should be. A return statement is an expression returning Nothing.

[streetsofboston]

val a: Int = when (value) {
    Hello -> 4
    There -> 6
    Bye -> return null
}
return "$a"

[FunkyMuse]

Can you share the use case?

[Nir]

it's not very hard to imagine use cases, e.g. you could have a when inside a loop that is used as part of the logic to iterate something, and perhaps one of the cases allows early exit

[Marc Knaup]

You could use break then. Except in Kotlin/JS, there you should definitely avoid break in when inside a loop 😅

[Nir]

not necessarily, it's two different situations... break is appropriate if you just need to get out of the loop. return is appropriate if you want to get out of the function.

[Nir]

I mean on the one hand I feel like I'm stating the obvious, on the other hand it apparently needs to be stated, so what can I say 🙂

[zak.taccardi]

sorry, yeah the idea is to break out of a when loop early

[zak.taccardi]

I know a return@label will work for functions, but idk how to label a when statement

[zak.taccardi]

this could be useful in a scenario where you have nested `when`/`if`s to reduce nesting

[zak.taccardi]

val exhaustive = when {
    exception != null -> {
        if (something) {
            // return out of when early - does not work
            return@when somethingElse 
        }
        // carry on my wayward son
        Result.raise(exception)
    }

[Nir]

@zak.taccardi You can always enclose the when in a run block

[Nir]

And return to the run

[Nir]

Usually though you can refactor so it becomes more elegant than that

[Nir]

If you show a code sample that has the essential elements from your example may be able to suggest something

[louiscad]

@Marc Knaup Why should break in when be avoided in Kotlin/JS?

[Marc Knaup]

@louiscad there's an open compiler bug. In KJS it breaks the when, not the loop.

[Marc Knaup]

At least in IR. No idea about legacy

[louiscad]

Can you link it?

[Marc Knaup]

https://youtrack.jetbrains.com/issue/KT-43943