Why Scala's Try has no type parameter for exception type?

Question

I'm curious why scala.util.Try has no type parameter for the exception type like

abstract class Try[+E <: Throwable, +T] {
  recoverWith[U >: T](f: PartialFunction[E, Try[E, U]]): Try[E, U]
  ...
}

Would help with documentation, eg

def parseInt(s: String): Try[NumberFormatException, Int]

Still won't be able to express disjoint exception types like throws SecurityException, IllegalArgumentException , but at least one step in this direction.

Answer 1

This might be what you're looking for:

import scala.util.control.Exception._
import scala.util.{ Success, Failure }

def foo(x: Int): Int = x match {
  case 0 => 3
  case 1 => throw new NumberFormatException
  case _ => throw new NullPointerException
}

val Success(3) = catching(classOf[NumberFormatException]).withTry(foo(0))
val Failure(_: NumberFormatException) = catching(classOf[NumberFormatException]).withTry(foo(1))
// val neverReturns = catching(classOf[NumberFormatException]).withTry(foo(2))

See scala.util.control.Exception$

---

However, there's no way to specialize Try[T] to something like the hypothetical Try[ExcType, T] ; in order for that to work you'll need something like Either (but possibly something more sophisticated such as scalaz.\\/ , or, for more than 1 exception class, Shapeless' Coproduct ):

def bar(x: Int): Either[NumberFormatException, Int] = {
  catching(classOf[NumberFormatException]).withTry(foo(x)) match {
    case Success(x) => Right(x)
    case Failure(exc) => Left(exc.asInstanceOf[NumberFormatException])
  }
}

println(bar(0)) // Right(3)
println(bar(1)) // Left(java.lang.NumberFormatException)
// println(bar(2)) // throws NullPointerException

---

It should be possible to generalize that into a generic helper that works with any number of exception types. You'd definitely have to work with Shapeless' Coproduct and facilities for abstracting over arity in that case. Unfortunately, it's a non-trivial exercise and I don't have the time to implement that for you right now.