Is there a way to terminate a flow from within an intermediate operator lambda? For example, a flow...

# coroutines

[julian]

Is there a way to terminate a flow from within an intermediate operator lambda? For example, a flow emits 5 random numbers between 1-10. Inside map , if the number is over 5, you terminate the flow.

[octylFractal]

cancel()?

[octylFractal]

i.e. just cancel whatever job is in the coroutine context

[julian]

I tried cancel, but it looks like that operator isn't available on the receiver within an intermediate flow operator.

[julian]

Would there be a way to cancel the flow without canceling the context's job?

[octylFractal]

they're the same thing afaik

[octylFractal]

you can't do coroutineContext.cancel()?

[Adam Powell]

Or just throw CancellationException()

[julian]

Yup, that worked. Thanks @octylFractal. However, something a bit weird happens. For example:

val pf = flowOf(1,2,3).map { if (it == 3) coroutineContext.cancel() else it }
runBlocking {
    val l = pf.toList()
    println(l)
}

prints:

[1, 2, kotlin.Unit]

It's that last item kotlin.Unit that's surprising.

[octylFractal]

nothing returns void in kotlin, it always returns Unit, so you're returning Unit from cancel() there

[octylFractal]

in this case yes, perhaps throwing the CancellationException would be better

[julian]

K. I'll try that. Thanks!

[julian]

So throwing CancellationException works, but catch must also be used to be able to get the items emitted before the exception is thrown. Otherwise partial collection doesn't occur.

[julian]

Thanks @octylFractal @Adam Powell!

[Zach Klippenstein (he/him) [MOD]]

There's actually a PR open to introduce an operator that is for exactly this use case I think: https://github.com/Kotlin/kotlinx.coroutines/pull/2066

[julian]

Nice! Thanks @Zach Klippenstein (he/him) [MOD]! Maybe it'll be available soon.

[louiscad]

Throwing a CancellationException is not the right way as it's ambiguous with other cancellation causes. You can take a look at how the take intermediate operator is done, and that can help to do what you want. Or you can copy transformWhile that Zach linked, or wait for it to be merged (which might take a while).

[tseisel]

How about takeWhile ?