Throw What Don't Throw

So say you are trying out all this interesting new throw stuff in Swift 2. And say you’re running an early Beta in which many stdlib functions don’t handle throw closures yet. Or maybe you’re in the future and dealing with some other piece of code that you wish could handle a throw closure, but doesn’t. What do you do?

By now you may be asking “what the heck is a throw closure? Talk sense, man!” Let’s take a small step back and quickly introduce the new throws feature. That’s not the point of this article, though. You should go watch the WWDC videos. But basically, it’s like this. Say you have a function that might fail. In Swift 2, you can write it this way:

enum Error: ErrorType { case Negative }

let f: (Int) throws -> String = {
    guard $0 >= 0 else { throw Error.Negative }
    return "".join(Repeat(count: $0, repeatedValue: "X"))

First, the throws in the signature tells us that this function may throw. Functions may not throw errors unless they explicitly indicate that they can.

You might think of “throwing errors” as equivalent to “exceptions” in languages you’re familiar with, but it’s a little different. A throw is really just a fancy return. A throwing function can return either a type or an ErrorType. And “throws” is probably best thought of as somewhat opaque sugar around an Either type.

That tells us something very important:

A function that throws errors is a different type than one that doesn’t.

I don’t mean that it’s “some other kind of thing.” I mean like Int is a different type than String, and String is a different type than (Int) -> String, (Int) -> String is a different type than (Int) throws -> String. In fact, (Int) -> String is a subtype of (Int) throws -> String, which is pretty awesome and a little subtle, but we’ll get to that in another post.

So what does that mean? Let’s think of a simple case of map today (Swift 2 Beta 1):

print([1,2,3].map(f)) // Cannot invoke 'map' with an argument list of type '((Int) throws -> String)'

What’s going on here? Let’s look at the type signature:

func map<T>(@noescape transform: (Self.Generator.Element) -> T) -> [T]

So transform is of type (Element) -> T. We’re passing (Element) throws -> T. Remember I said that a non-throwing function is a subtype of a throwing function. So (Element) throws -> T is a supertype of what what this function wants. That’s like passing NSObject to something that wants UIView. You can’t do that.

So what do we do? Well for map, this is easy. We can just implement our own throwing version:

extension Array {
    func map<T>(@noescape transform: (Generator.Element) throws -> T) rethrows -> [T] {
        var result: [T] = []
        for x in self {
            result.append(try transform(x))
        return result

And now we can use it:

print(try [1,2,3].map(f))

Notice the use of try. This is pretty different than how try is used in other langauges, and another way that Swift’s error handling doesn’t quite match “exceptions.” Swift uses try to remind the programmer about functions that may throw errors. The compiler doesn’t need try. It doesn’t create scope, or mark control flow points, or anything like that. It’s not a function or a constructor. It’s just a keyword that Swift forces you to include so that you (and your coworkers) remember what’s going on. When you see try, you should think “hey, control could suddenly jump somewhere else from this point.” It reduces surprise when that happens, and conversely tells you where control can’t suddenly jump (i.e. everywhere without try). I think that’s pretty nice.

You may also notice both throws and rethrows in the method signature. I’ll get to that in a later blog post. Just trust me for now. This code would also work if you used throws in both places, but this is the better signature.

And one more “also notice.” Also notice that this is an overload of map. The closures have different types, so the compiler can pick the right one. Nice.

OK, that was a lot of setup, and you could probably figure out on your own how to rewrite map this way. And besides, by beta 2, I’m sure there will be a proper (re)throwing version of map. So why bother? For the next step.

I know how map is implemented. It’s really simple. But what if I didn’t know how it was implemented? How about some function that I’m not sure I could write correctly? How about a more obscure function that may not get throwing love quite so quickly? How about Array.withUnsafeBufferPointer? Ooohhh….

So here’s our signature:

func withUnsafeBufferPointer<R>(@noescape body: (UnsafeBufferPointer<T>) -> R) -> R

We want to accept a body that can throw, but we want to pass it to the existing method, which can’t accept a throwing closure. So what do we do? We go back to our old friend, Result. Here’s a super-simple Result implementation that can convert to and from throwing closures:

enum Result<T> {
    case Success(T)
    case Failure(ErrorType)

    func value() throws -> T {
        switch self {
        case .Success(let value): return value
        case .Failure(let err): throw err

    init(@noescape f: () throws -> T) {
        do    { self = .Success(try f()) }
        catch { self = .Failure(error) }

If you’re familiar with Result or Either, this should be pretty self-evident, but the key pieces are that result.value() will unwrap the result into either a value or a thrown error. And init will take a throwing closure and convert it into a Result. With that piece, here’s how we build our method:

extension Array {
    func withUnsafeBufferPointer<R>(@noescape body: (UnsafeBufferPointer<T>) throws -> R) throws -> R {
        return try self.withUnsafeBufferPointer { buf in
            return Result{ try body(buf) }

The closure body is of type (UnsafeBufferPointer<T>) throws -> R, which we can’t pass to withUnsafeBufferPointer. But our closure is of type (UnsafeBufferPointer<T>) -> Result<R>, which is just fine (no throws here, move along).

Let’s walk through the closure from the inside out.

  1. try body(buf). Execute our throwing closure using the buf provided to us by the default implementation.
  2. Result{...}. Capture it into a Result enum
  3. return Result{...}. Return the Result, not the underlying value.
  4. The whole function nows looks like return try result.value()
  5. This either returns the computed value (type R), or throws

This method is marked throws rather than rethrows because … reasons. (The final throw doesn’t come directly from body, but from value(). That’ll hopefully make more sense when I explain rethrows.)

And just for completeness (and because I needed it myself), we can do the same thing to withUnsafeMutableBufferPointer, but we need to give the compiler more type information because of the inout parameter:

extension Array {
    mutating func withUnsafeMutableBufferPointer<R>(@noescape body: (inout UnsafeMutableBufferPointer<T>) throws -> R) throws-> R {
        return try self.withUnsafeMutableBufferPointer { (inout buf: UnsafeMutableBufferPointer<T>) in
            return Result{try body(&buf)}}.value()

These particular implementations probably won’t be useful for long. I’m sure the stdlib team will quickly clean this up (and you probably shouldn’t be using withUnsafeBufferPointer very much anyway). But hopefully exploring how this works can give some insight into Swift’s new error handling system. Result isn’t dead; it still has interesting use cases like this one. But I expect those use cases to shrink, and I highly recommend exploring the new error handling and discover how to build great things with it.