I suspect it's useful to visually signal that the call can fail, and make it obvious what's happening; after all, the "throws" annotation is at the target site, not on the caller end.

You need to prefix anything that throws with a "try":

    func mightThrow() throws {
      try mightAlsoThrow();
    }

Other than that, it looks exactly like C++ exceptions: Propagation happens along the chain of functions marked as "throws", until a "catch" handler intercepts it.

"try" is an expression, by the way, so you can do things like:

    let line = try file.readline()

There's also a "try!" statement that throws a runtime exception if the inner statement throws.