IOApp.reportFailure is not invoked for a NonFatal exception

[kamilkloch]

object ReportFailureApp extends IOApp.Simple {

  override protected def reportFailure(err: Throwable): IO[Unit] = IO.println(s"Failure")

  override def run: IO[Unit] = IO(throw new Exception(""))
}

prints stack trace instead of "Failure".

[armanbilge]

This is the correct and expected behavior.

cats-effect/core/jvm/src/main/scala/cats/effect/IOApp.scala

Lines 235 to 243 in 25c948b

	/**
	* Configures the action to perform when unhandled errors are caught by the runtime. By
	* default, this simply delegates to [[cats.effect.std.Console!.printStackTrace]]. It is safe
	* to perform any `IO` action within this handler; it will not block the progress of the
	* runtime. With that said, some care should be taken to avoid raising unhandled errors as a
	* result of handling unhandled errors, since that will result in the obvious chaos.
	*/
	protected def reportFailure(err: Throwable): IO[Unit] =
	Console[IO].printStackTrace(err)

reportFailure is specifically for "unhandled" errors. An unhandled error is an error that is raised on a Fiber that nobody is joining.

For example:

import scala.concurrent.duration._

override def run: IO[Unit] = IO(throw new Exception("")).start *> IO.sleep(1.second)

In this case, the exception is raised on a Fiber with no listeners. So nobody would be notified about that error. Therefore it is unhandled, and it goes through the reportFailure mechanism.

---

An error raised in run is considered to be "handled" because it is being raised directly to your application's main. If you want to handle errors that are raised in run, use handleErrorWith or similar.