Scala Supports Non-Local Returns

Stephen Haberman - 07 Jan 2010

Writing some Scala code today, I found myself using non-local returns without even thinking about it. After realizing what I had done, I dug a little deeper to see what was really going on.

Take this completely made up, nonsensical example:

    object Foo {
      def main(args: Array[String]) {
        foo(List(1, 2, 3))
      }

      def foo(l: List[Int]): Int = {
        l.foreach { (i) =>
          println(i)
          return 5
        }
        return 10
      }
    }

This code will print "1" and then exit.

Perhaps this is obvious, that the "return 5" applies to the "foo" method, so the values 2 and 3 in "l" will not have a chance to be printed.

However, think about what is going on under the covers--Scala is passing the foreach method an anonymous inner class with a "void apply(int i)" method. And inside of that "apply" method is the code between the "{ (i) => ... }".

So, how does code inside of the "apply" method cause its caller to perform an early exit, without the "foreach" even knowing about it?

Exceptions.

Here is the decompiled version of "print":

    public int print(List l) {
      Object localObject = new Object();
      int exceptionResult1 = 0;
      try {
        l.foreach(new AbstractFunction1() {
          public static final long serialVersionUID = 0L;
          public final Nothing. apply(int i) {
            Predef..MODULE$.println(BoxesRunTime.boxToInteger(i));
            // here is the "return"--it puts "5" into an exception
            throw new NonLocalReturnException(
              this.nonLocalReturnKey1$1,
              BoxesRunTime.boxToInteger(5));
          }
        });
        return 10;
      } catch (NonLocalReturnException localNonLocalReturnException)  {
        if (localNonLocalReturnException.key() == localObject) {
          // get "5" back out of the exception
          return BoxesRunTime.unboxToInt(localNonLocalReturnException.value());
        }
        throw localNonLocalReturnException;
      }
    }

I think the decompiler got a little confused with "nonLocalReturnKey", but you can see the basic idea is that any early return inside of a closure is converted into an exception that is then caught outside of the closure where a proper return call can be done.

I personally think this is handy, once you know what is going on. But from what I've picked up, any closures that make it into Java 7 will not support non-local returns and instead disallow the return keyword inside of closures. Which, I guess, at this point any Java closures are better than no closures at all.

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