Can I make a `Deferred<A>` `NonCancelable` b...

# coroutines

[simon.vergauwen]

Can I make a Deferred<A> NonCancelable by wrapping and awaiting it in a withContext block like below. And how does its scope get (re)wired correctly? Snippet seems to work although I don't quite understand it.

val def = thisWasPassedToMe()
val result = withContext(NonCancelable) {
  def.await()
}

[bdawg.io]

You will still get a CancellationException thrown if the backing Job for def gets cancelled

[bdawg.io]

NonCancellable will just make sure that your withContext itself doesn’t exit from being cancelled itself

[simon.vergauwen]

I'm not sure I 100% understand that last bit. Only what gets created within withContext is NonCancelable?

[simon.vergauwen]

Is there another way to make an existing Deferred NonCancelable?

[bdawg.io]

No, a Deferred is just a reference handle to the state of the coroutine

[simon.vergauwen]

Alright thanks. Guess I'll have to work around it.

[groostav]

@simon.vergauwen whats the parent coroutine factory? assumjing its either a launch or runBlocking you can use CoroutineStart.ATOMIC

[groostav]

eg

launch(CoroutineStart.ATOMIC){
  val def = thisWasPassedToMe()
  def.await()
}

[groostav]

also of course, finally blocks function through cancellation:

suspend fun doStuffWithThingPassedToMe(){
  try {
    val def = thisWasPassedToMe()
  }
  finally {
    //this will fire in all cercomstances (except abandonment!)
  }
}

[simon.vergauwen]

I need it for this function. https://github.com/arrow-kt/arrow/blob/master/modules/effects/arrow-effects-kotlinx-coroutines-data/src/main/kotlin/arrow/effects/coroutines/DeferredK.kt#L442

[simon.vergauwen]

It’s within a suspend function. However this Deferred is guaranteed to have never run.

[groostav]

sorry so your calling this library function or your looking for an implementation of release?

[groostav]

its funny I've read the scala red-book but I still dont understand arrow

[simon.vergauwen]

The point of this function is to acquire some resource. This resource is provided through a Deferred<A> that has never run and is lazily run.

[simon.vergauwen]

acquire is guaranteed not to be cancelled, and the release is guaranteed to be called. There is also a use function which you can use to consume the `acquire`d resource.

[simon.vergauwen]

In other words you can use this function to compose different lazily run Deferred to safely consume and release resources with a guarantee not to leak.

[simon.vergauwen]

We use this to provide a generic API for data types with similar behaviours like Observable, Flux, IO, …

[simon.vergauwen]

If you have any questions about Arrow feel free to swing by #arrow to ask questions and we’ll be glad to help!