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Swift 4 is Officially Available: What's New

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Swift’s latest major release contains many changes and updates to the language and the standard library, most notably new String features, extended collections, archival and serialization, and more.

In Swift 4 strings conform to the Collection protocol, which makes them directly iterable and provides all the facilities you are used to with collection and sequences, for example:

for c in myString {
myString.filter { c in
  return boolCheck(c)
let l = myString.count
let myString2 = myString.dropFirst()

Additionally, string slices are now instances of type Substring, which conforms to StringProtocol and can thus be used identically to type String. This change helps improve performance of slicing, since a Substring will not copy the string slice. The copy operation can be deferred to the time the Substring is converted into a String to be consumed by some API.

Other new string features are support for Unicode 9 and multi-line literals.

Swift 4 also improves the way you can create, use, and manage collection types, such as Dictionary and Set.

First of all, you can now create a dictionary from a sequence of tuples as well as specify how you handle duplicates, if any, both when creating a dictionary or merging two dictionaries:

let items = ["ItemA", "ItemB", "ItemC", "ItemA"]
let prices = [14.40, 41.63, 3.71, 15.63]
let catalog1 = Dictionary(uniqueKeysWithValues: zip(items, prices))
let catalog2 = Dictionary(prices, uniquingKeysWith: { (l, r) in
    l })
let catalog3 = Dictionary(prices, uniquingKeysWith: { (l, r) in
    l + r })
let merged = catalog.merge(catalog3) { (l, r) in r }

Dictionary and Set now can be filtered into another object of the original type, instead of into an Array. Furthermore, dictionaries support a new mapValues method:

let catalog4 = catalog.filter { $0.value < 15.0 }
let catalog5 = catalog.mapValues { $0 * 1.2 }

Another useful addition to dictionaries is the possibility of specifying a default value when accessing its elements, which makes the subscript operator return a non-opt type:

let price1 : Float = catalog['none', default: 0.0]
let price2 : Float? = catalog['none']

All Collection types support generic subscripts in Swift 4. This means you could define the following JSON struct and not cast the result of indexing into the dictionary:

struct JSON {

    init(dictionary: [String:Any]) {
    subscript<T>(key: String) -> T? {

let json = ...
let result: String? = json['item']

Another welcome enhancement to the language is support for archival and serialization, which previously required dealing with NSObject and NSCoding, which was not an option for struct and enum types. Instead, Swift 4 adds serialization to all types through the Codable protocol. Ole Begemann provided a nice introduction to coding and decoding in Swift 4. For example, this is how you can define a Codable type:

struct Card: Codable, Equatable {
    enum Suit: String, Codable {
        case clubs, spades, hearts, diamonds

    enum Rank: Int, Codable {
        case two = 2, three, four, five, six, seven, eight, nine, ten, jack, queen, king, ace

    var suit: Suit
    var rank: Rank

    static func ==(lhs: Card, rhs: Card) -> Bool {
        return lhs.suit == rhs.suit && lhs.rank == rhs.rank
let hand = [Card(suit: .clubs, rank: .ace), Card(suit: .hearts, rank: .queen)]

As a last note, Swift 4 has two language modes, selected through the -swift-version compiler option. In Swift 3.2 mode, the compiler will accept the majority of sources compatible with Swift 3.x compilers. In this mode, most Swift 4 language features will be available, but updates to previously existing APIs will not. In Swift 4.0 mode, you will get access to all Swift 4 features, at the expense of some source changes, which as usual can be handled using Xcode’s migration assistant.

There is much more to Swift 4. Do not miss Swift maintainer Ted Kremenek announcement and Ole Begemann’s interactive Playground demoing all new features.

Swift 4 is included in Xcode 9 and can be manually installed in Xcode 8.3.

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