Why You Should Never Catch Throwable In Scala

Scala is a subtle beast and you should heed its warnings. Most Scala and Java programmers have heard that catching Throwable, a superclass of all exceptions, is evil and patterns like the following should be avoided:

try {
  aDangerousFunction()
} catch {
  case ex: Throwable => println(ex)
  // Or even worse
  case ex => println(ex)
}

This pattern is absurdly dangerous. Here’s why:

The Problem

In Java, catching all throwables can do nasty things like preventing the JVM from properly responding to a StackOverflowError or an OutOfMemoryError. Certainly not ideal, but not catastrophic. In Scala, it is much more heinous. Scala uses exceptions to return from nested closures. Consider code like the following:

def inlineMeAgain[T](f: => T): T = {
  f
}
 
def inlineme(f: => Int): Int = {
  try {
      inlineMeAgain {
        return f
      }
  } catch {
      case ex: Throwable => 5
  }
}
 
def doStuff {
    val res = inlineme {
      10
    }
    println("we got: " + res + ". should be 10")
}
doStuff

We use a return statement from within two nested closures. This seems like it may be a bit of an obscure edge case, but it’s certainly possible in practice. In order to handle this, the Scala compiler will throw a NonLocalReturnControl exception. Unfortunately, it is a Throwable, and you’ll catch it. Whoops. That code will print 5, not 10. Certainly not what was expected.

The Solution

While we can say “don’t catch Throwables” until we’re blue in the face, sometimes you really want to make sure that absolutely no exceptions get through. You could include the other exception types everywhere you want to catch Throwable, but that’s cumbersome and error prone. Fortunately, this is actually quite easy to handle, thanks to Scala’s focus on implementing much of the language without magic—the “catch” part of the try-catch is just some sugar over a partial function—we can define partial functions!

def safely[T](handler: PartialFunction[Throwable, T]): PartialFunction[Throwable, T] = {
  case ex: ControlThrowable => throw ex
  // case ex: OutOfMemoryError (Assorted other nasty exceptions you don't want to catch)
  
  //If it's an exception they handle, pass it on
  case ex: Throwable if handler.isDefinedAt(ex) => handler(ex)
  
  // If they didn't handle it, rethrow. This line isn't necessary, just for clarity
  case ex: Throwable => throw ex
}
 
// Usage:
/*
def doSomething: Unit = {
  try {
    somethingDangerous
  } catch safely {
    ex: Throwable => println("AHHH")
  }
}
*/

This defines a function “safely”, which takes a partial function and yields another partial function. Now, by simply using catch safely { /* catch block */ } we’re free to catch Throwables (or anything else) safely and restrict the list of all the evil exception types to one place in the code. Glorious.