Async code access to a variable in App fails in Dotty

[mkotsur]

Compiler version

3.2.0

Minimized code

import scala.concurrent.{Await, Future}
import scala.concurrent.duration._

object MapTest extends App {
  import concurrent.ExecutionContext.Implicits.global

  val test = "123"

  val dataF = Future { List("111", "222", "333", test) }

  Await.ready(dataF, 1.second)
}

Output

Exception in thread "main" java.lang.ExceptionInInitializerError
	at MapTest.main(MapTest.scala)
Caused by: java.util.concurrent.TimeoutException: Future timed out after [1 second]
	at scala.concurrent.impl.Promise$DefaultPromise.tryAwait0(Promise.scala:248)
	at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:255)
	at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:104)
	at scala.concurrent.Await$.$anonfun$ready$1(package.scala:174)
	at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:62)
	at scala.concurrent.Await$.ready(package.scala:124)
	at MapTest$.<clinit>(MapTest.scala:13)
	... 1 more
java.lang.NoClassDefFoundError: Could not initialize class MapTest$

Expectation

I'm aware of the intricacies of DelayedInit and App. However, since this works in Scala 2.13, I would expect this to also work in Scala 3 because the doc hasn't change: doc dotty, doc 2.13.3.

Moreover, looking at this statement:

It should be noted that this trait is implemented using the DelayedInit functionality, which means that fields of the object will not have been initialized before the main method has been executed.

it's not clear why wrapping code into a Future makes any difference. Isn't access to test on line 3 happening before the method has been executed? 🤔

object PrintTest extends App {
  val test = "123"
  println(test)
}

[SethTisue]

maybe scala/scala#10070 hasn't propagated to Scala 3 yet? it was included in Scala 2.13.9: https://www.scala-lang.org/api/2.13.9/scala/App.html (it's a bit peculiar that you linked to the 2.13.3 doc above; that's a quite old version)

[som-snytt]

2.13.3 is when -Xasync arrived. It was the second pandemic release. The pandemic is already quite old.

[som-snytt]

There are many Scala 2 tickets about deadlocking a class initializer (object). From pandemic era:

scala/bug#12278

There is a comment on that ticket about differences in dotty.

[odersky]

Yes, I think the dotty documentation of App is not up to date. I wonder where it came from? I assumed that it just copied the 2.13 docs?

[mkotsur]

Two small notes and two suggestions...

1. @SethTisue no real reason why I linked 2.13.3, not the latest one. Apologies if this confused you.

2. One more thing I noted: 3.1.2 has warning suppression @nowarn("cat=deprecation&origin=scala\\.DelayedInit") on the App trait, while 3.1.3 doesn't. I don't know if this is relevant.

While it would be very helpful if scala/scala#10070 changes were present in dotty's docs, I don't think it will necessarily prevent people from wasting time on writing and debugging the kind of code that I demonstrated. Even if you carefully read the whole thing (which not many people do tbh), you may still think that it's OK to use App for a quick sketch, and the only real risk is that it will be deprecated in the Future. Not that it will misbehave. Especially since the DelayedInit feature was dropped – it was said to be the cause of the funky initialization behaviour.

Is there a reason not to make the trait deprecated already?
Can we emphasize the recommendation to use @main by pulling it to the top of the text?

[SethTisue]

Yes, I think the dotty documentation of App is not up to date. I wonder where it came from? I assumed that it just copied the 2.13 docs?

That should be resolved in 3.2.1 by #16074