In a coroutine, I’m using the `coroutineScope {}` , calling several async methods, and collecting th...

# coroutines

[spierce7]

In a coroutine, I’m using the coroutineScope {} , calling several async methods, and collecting the deferreds into a MutableList<Deferred<Any>>. Before the coroutine is over, I call deferreds.awaitAll() on the list. This isn’t actually necessary if I don’t care about the results, right? A parent suspend function won’t complete until all it’s children coroutines have completed, right?

[andylamax]

Yes, but now if you don't care about the results, you need to consider using using launch instead of async

[spierce7]

So I don’t care about the actual results, but I do care if an exception is thrown. This is being done in a server request (ktor), and if there is an error doing any of the operations, I want it to bubble to the request and return an exception

[spierce7]

I think launch would ignore the errors as well.

[Stylianos Gakis]

Would it though? Can you try making a repro in Kotlin playground to show when that would happen maybe?

[spierce7]

I’m pretty sure the parent coroutine will succeed, even if the child coroutine fails.

[spierce7]

This is just my understanding of the documentation from when I read it 5 years ago

[spierce7]

the only way to get an exception from a launch is through an uncaught exception handler (I think)

[julian]

I’m pretty sure the parent coroutine will succeed, even if the child coroutine fails.

Depends what sort of Job is in the parent coroutine's context.

[julian]

launch would ignore the errors

It doesn't. But unlike coroutineScope, exceptions can't be caught by wrapping in try-catch. And depending on the Job in the launch-ed coroutine parent's context, failure may not propagate up the job hierarchy.

[spierce7]

Depends what sort of Job is in the parent coroutine’s context.

What are the different types of Job for this?

[julian]

A SupervisorJob will not fail or propagate failure up the job hierarchy when its children fail. Whereas, a regular Job will.

[spierce7]

So I assumed the SupervisorJob would get the exception and then not fail, but you are saying it doesn’t even propogate?

[julian]

By "propagate" I mean failing the jobs higher in the tree. Is that what you mean?

[julian]

If you're also asking whether a CoroutineExceptionHandler at the root of the tree will respond to an exception thrown within a coroutine whose parent job is a SupervisorJob - it will. Here's code showing that using supervisorScope, and another using SupervisorJob directly.

[gildor]

So I don’t care about the actual results, but I do care if an exception is thrown

Because coroutineScope will rethrow exception, you are fine. If you want to catch it, just wrap coroutineScope with try/catch as any other suspend function

And you almost never want supervisorScope on practice

[Stylianos Gakis]

When doing this, one needs to be careful to always re-throw CancellationException though right? (In case the try-catch you’re referring to catches wide enough to catch a CancellationException)