How to handle different cases of Failure from Try

Question

I want to handle different cases of Failure (returned as Try).

example code

main(args(0)) match {
  case Success(result) => result.foreach(println)
  case Failure(ex) => ex match {
    case FileNotFoundException =>
      System.err.println(ex.getMessage)
    case StatsException =>
      System.err.println(ex.getMessage)
    case _ => {
      ex.printStackTrace()
    }
      System.exit(1)
  }
}

If its a StatsException or FileNotFoundException just print the message, for all other exceptions print a stack trace.

However ex is only ever a Throwable, and so case StatsException is a fruitless type test (a value of type Throwable cannot also be a StatsException.type according to IntelliJ)

Worse I get compile errors: java.io.FileNotFoundException is not a value

What's the best way to handle different cases of Failure in an idiomatic way?

I assume `StatsException` inherits from `Throwable` so `ex` can very well be an instance of `StatsException`. — Gregor Raýman, May 15 '15 at 08:51

score 3 · Accepted Answer · edited May 23 '17 at 12:30

3

I would simplify your own answer:

main(args(0)) map (_ foreach println) recover {
  case ex@(_: FileNotFoundException | _: StatsException) =>
    System.err.println(ex.getMessage)
    System.exit(1)
  case ex =>
    ex.printStackTrace()
    System.exit(1)
}

In the first case clause we are matching two alternative. scala forbids declarations like case ex: FileNotFoundException | ex: StatsException, but we could assign whole match result via @ symbol. See this question for more details

edited May 23 '17 at 12:30

Community

1
1

answered May 15 '15 at 09:52

Odomontois

15,918
2
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Thanks - what does the @ in case ex@ mean? – tekumara May 15 '15 at 10:02

tekumara · Answer 2 · 2015-05-15T09:13:34.987

2

This seems to work:

main(args(0)) match {
  case Success(result) => result.foreach(println)
  case Failure(ex: FileNotFoundException) =>
    System.err.println(ex.getMessage)
    System.exit(1)
  case Failure(ex: StatsException) =>
    System.err.println(ex.getMessage)
    System.exit(1)
  case Failure(ex) =>
    ex.printStackTrace()
    System.exit(1)
}

Would be nice to have the System.exit(1) call shared by all Failure cases.

edited May 15 '15 at 09:13

answered May 15 '15 at 09:00

tekumara

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2

Well, you can write `case Failure(ex) => ex match { case ex: StatsException => ...; case ex: FileNotFoundException => ...; }; System.exit(1)`, almost like you tried initially. – Vladimir Matveev May 15 '15 at 09:33

How to handle different cases of Failure from Try

2 Answers2