How to handle unhandled exception throw in monix onErrorHandle

Question

I am using monix tasks and i am trying to catch a Throwable and then convert to a custom error. I have removed/changed the code to be simple and relevant. This is the code (question follows after the code snippet):

import io.netty.handler.codec.http.HttpRequest
import monix.reactive.Observable
import io.netty.buffer.ByteBuf
import monix.eval.Task
import com.mypackage.Response


private[this] def handler(
      request: HttpRequest,
      body: Observable[ByteBuf]
  ): Task[Response] = {

    val localPackage = for {
      failfast <- Task.eval(1 / 0)
    } yield failfast

    // Failure case.
    localPackage.onErrorRecoverWith {
        case ex: ArithmeticException =>
          print(s"LOG HERE^^^^^^^^^^^^^^^")
          return Task.now(
            Response(HttpResponseStatus.BAD_REQUEST,
                     None,
                     None)
          )
    }.runAsync

    // Success case.
    localPackage.map { x => 
       x match {
        case Right(cool) =>
          Response(
            HttpResponseStatus.OK,
            None,
            cool
          )
        case Left(doesntmatter) => ???
      }
  }
}

I am able to see the print statement but the expected Task.now(Response(... is not being returned. Instead the method that calls the handler method is throwing an error. How do i make it return the Task[Response] ?

The success case works, the failure case does not.

Edit #1 : Fix errors in scala code.

Edit #2 This is how I fixed it.

    // Success case.
    localPackage.map { x => 
       x match {
        case Right(cool) =>
          Response(
            HttpResponseStatus.OK,
            None,
            cool
          )
        case Left(doesntmatter) => ???
      }
  }.onErrorRecoverWith {
        case ex: ArithmeticException =>
          print(s"LOG HERE^^^^^^^^^^^^^^^")
          return Task.now(
            Response(HttpResponseStatus.BAD_REQUEST,
                     None,
                     None)
          )
    }

I was thinking in terms of future and forgot the lazy eval nature of task. Also I understood how the CancellableFuture value was being discarded in the failure task.

Answer 1

Several problems with your sample.

For one this code isn't valid Scala:

val localPackage = for {
  failfast <- 1 / 0
} yield failfast

I guess you meant Task.eval(1 / 0) .

Also onErrorHandle does not have a Task as a return type, you were probably thinking of onErrorHandleWith . And it's a pretty bad idea to give it a partial function (ie a function that can throw exceptions due to matching errors) — if you want to match on that error, then better alternatives are onErrorRecover and onErrorRecoverWith , which take partial functions as arguments.

So here's a sample:

import monix.eval._
import monix.execution.Scheduler.Implicits.global

val task = Task.eval(1 / 0).onErrorRecoverWith {
  case _: ArithmeticException => Task.now(Int.MinValue)
}

task.runAsync.foreach(println)
//=> -2147483648

Hope this helps.