What’s New in Swift 2

At WWDC we found out the Swift team hasn’t been messing around sending crude watch drawings to each other like the rest of us — they’ve been hard at work on Swift 2.

We’ll have plenty of written and video Swift 2 tutorials for you soon, but in the meantime I wanted to highlight the most exciting changes so you can be prepared for the great migration to Swift 2 in the fall.

Error Handling

As Ray mentioned in his WWDC 2015 Initial Impressions post, error handling has been revamped in Swift 2. Instead of NSError objects and double pointers, we’re moving to a new system that looks similar to exception handling.

You may be familiar with code like this:

if drinkWithError(nil) {
  print("Could not drink beer! :[")
  return
}

Generally in Cocoa, you pass in a reference to an NSError object (an inout parameter in Swift) and then the method will assign the variable if there was a problem. However, the problem is that you can pass nil here to completely ignore the error; or, you can pass in the NSError but then never check it.

Swift 2 adds additional safety to your error checking. You use the throws keyword to specify which functions and methods could throw an error. Then you have the do, try, and catch keywords for when you call something that could throw:

// 1
enum DrinkError: ErrorType {
  case NoBeerRemainingError
}

// 2
func drinkWithError() throws {
  if beer.isAvailable() {
    // party!
  } else {
    // 3
    throw DrinkError.NoBeerRemainingError
  }
}

func tryToDrink() {
  // 4
  do {
    try drinkWithError()
  } catch {
    print("Could not drink beer! :[")
    return
  }
}

There are a few things to highlight here:

1. To create an error to throw, simply create an enum that derives from ErrorType.
2. You need to use the throws keyword to mark any function that can throw an error.
3. This throws an error, which will be caught in section 4.
4. Instead of try blocks, which might be familiar from other languages, you wrap any code that can throw an error in a do block. Then, you add the try keyword to each function call that could throw an error.

The new syntax is pretty lightweight and readable. Any API that currently uses NSError now uses this system, so we’ll be seeing a lot of it!

Binding

With Swift 1.2, we lost the “pyramid of doom” and gained the ability to test binding multiple optionals in one line:

if let pants = pants, frog = frog {
  // good stuff here!
}

That works just fine, but the issue for some people is that the “preferred” code path where all the optionals have some value is indented. That means you need to keep looking inside indented code blocks for the mainline part of your code, while the error conditions are outside.

If only there were some way to check that some of the optionals don’t have a value, and then exit early! That’s exactly what Swift 2 offers with its guard statement:

guard let pants = pants, frog = frog else {
  // sorry, no frog pants here :[
  return
}

// at this point, frog and pants are both unwrapped and bound!

Using guard means you can perform the optional binding (or any other operation, really) and provide a code block in the else to run if the condition fails. Then, you can continue on – in this case, the optionals frog and pants are now bound and are no longer optionals in scope.

This should make for much clearer code since guard lets you specify the state you’re actually expecting rather than checking for the error case.

Protocol Extensions

Object-oriented? Functional? There’s one more to add to the front of the line of what Swift is: a protocol-oriented programming language!

In Swift 1, protocols were like interfaces to specify a set of properties and methods that a class, struct, or enum would then conform to.

Now in Swift 2, you can extend protocols and add default implementations for properties and methods. You can already do this with classes and structs — adding new methods to String or Array, for example — but adding these to protocols now gives you a much wider reach.

extension CustomStringConvertible {
  var shoutyDescription: String {
    return "\(self.description.uppercaseString)!!!"
  }
}

let greetings = ["Hello", "Hi", "Yo yo yo"]

// prints ["Hello", "Hi", "Yo yo yo"]
print("\(greetings.description)")

// prints [HELLO, HI, YO YO YO]!!!
print("\(greetings.shoutyDescription)")

Note that the Printable protocol is now called CustomStringConvertible, which most Foundation objects conform to. With protocol extensions, you can extend wide swaths of the system with your own custom functionality. And rather than adding bits of custom code to many classes and structs and enums, you can write one general implementation and have it apply across a set of types.

The Swift team has been busy doing this already — if you’ve ever used map or filter in Swift, you may have thought they would do better as methods rather than as global functions. Thanks to the power of protocol extensions, there are a new set of methods on collection types such as map, filter, indexOf, and more!

let numbers = [1, 5, 6, 10, 16, 42, 45]

// Swift 1
find(filter(map(numbers, { $0 * 2}), { $0 % 3 == 0 }), 90)

// Swift 2
numbers.map { $0 * 2 }.filter { $0 % 3 == 0 }.indexOf(90) // returns 2

Thanks to protocol conformance, your Swift 2 code can be more concise and readable. With the Swift 1 version, you need to look at the calls inside out to understand what’s going on; in the Swift 2 version, the chain of functions is clear.

You’re also leveraging the power of protocol-oriented programming — check out the WWDC session on this topic and keep an eye out for future tutorials and articles here on the site!

Grab Bag

There were a ton of things announced throughout all the sessions, so I want to highlight a few more quick things:

• Objective-C generics – Apple has already started annotating all their Objective-C code so that types in Swift get the correct kind of optionality. That work continues with Objective-C generics, which will give Swift developers better type hinting. If you’re expecting a set of UITouch objects or an array of strings, that’s exactly what you’ll get rather than a collection of AnyObjects.
• Renamed syntax – println has left us after only a year; now it’s plain old print which now has a default second boolean argument set to true on whether to print a newline or not. With the do keyword focused on scope for error handling, do-while loops are now repeat-while. Similarly, there are many changes to protocol names such as Printable becoming CustomStringConvertible.
• Migrator – With all these small syntax changes, how are you going to get your codebase up to date? The Swift 1-to-2 migrator will come to the rescue and help bring things up to the latest standards and syntax changes. The migrator is even smart enough to update your code to use the new error handling and update your docblock comments to the new style of formatting!
• Open source! – The big news for the nerds is that Swift is going open source when Swift 2 is released in the fall. Likely this will lead to Swift being used for more than just iOS development, which makes it even more important to learn. Plus, it will be a great opportunity to get a look under the hood and even to contribute back and get your name in the Swift compiler commit history. ;]

Where To Go From Here?

This is just a sampling of the many things released; for the full details, check out the WWDC session videos and the updated Swift Programming Language book.

There will surely be more to come if anyone remembers the volume of changes between the first beta and the 1.0 release of Swift. We’ll be staying on top of all the updates here on the team, so keep an eye out for tutorials, book announcements, and videos as we dig into all the exciting changes to come.

What parts of Swift 2 are you most excited about? What would you like us to cover first? Let us know in the comments below!