CODEWITHSUNDEEP
scalatry-catch
Eduardo
Eduardo

Reputation: 8412

Return Values from Catch

My application reads some configuration values from a file. If there are any errors in the process, I would like to set default values for those configuration parameters. What I am doing is:

val (param1, param2) = {
  try{
    val props = new java.util.Properties
    props.load(getClass.getResource("/myapp.properties").openStream)
    (
      props.getProperty("param1", "default1"),
      props.getProperty("param2", "default2")
    )
  }
  catch{
    case _ => ("default1", "default2")
  }
}

I know that what I am doing in the catch block won't work as expected if there is an exception. Is there a way to fix it? I'm using Scala 2.9

Upvotes: 2

Views: 1552

Answers (1)

Rex Kerr
Rex Kerr

Reputation: 167921

This will work, but it opens up the possibility of a run-time error since

val (a,b) = whatever

may be called as long as whatever is a supertype of Tuple2--in particular, it could be AnyRef or Any, which is indeed what the try/catch return value will be widened to if you have sufficiently mismatching types. You can make sure it's the right type by adding a type ascription:

val (a,b): (A, B) = whatever

and then the type checker will complain if the right-hand-side is not the correct type all the way through the try/catch.

For example:

val (a,b): (String, String) = try {
  ("perch", if (scala.util.Random.nextBoolean) throw new Exception else "halibut")
} catch {
  case e: Exception => ("salmon", "herring")
}

If you tried to add , "cod" after "herring" you'd get a compile-time error. This assignment, if entered repeatedly, will give you a=perch and b=halibut half the time and a=salmon and b=herring the other half.

Addendum: in 2.9 (or below, I presume, though I haven't checked), you have to put the type ascription on the try/catch statement like so:

val (a,b) = (try {
  ("perch", if (scala.util.Random.nextBoolean) throw new Exception else "halibut")
} catch {
  case e: Exception => ("salmon", "herring")
}): (String, String)

to get the type-checking of the catch.

Upvotes: 4

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