高階型別約束

可以使用 where 子句為泛型指定幾種型別約束：

func doSomething<T where T: Comparable, T: Hashable>(first: T, second: T) {
    // Access hashable function
    guard first.hashValue == second.hashValue else {
        return
    }
    // Access comparable function
    if first == second {
        print("\(first) and \(second) are equal.")
    }
}

在引數列表後面寫 where 子句也是有效的：

func doSomething<T>(first: T, second: T) where T: Comparable, T: Hashable {
    // Access hashable function
    guard first.hashValue == second.hashValue else {
        return
    }
    // Access comparable function
    if first == second {
        print("\(first) and \(second) are equal.")
    }
}

擴充套件可以限制為滿足條件的型別。該功能僅適用於滿足型別條件的例項：

// "Element" is the generics type defined by "Array". For this example, we
// want to add a function that requires that "Element" can be compared, that
// is: it needs to adhere to the Equatable protocol.
public extension Array where Element: Equatable {
    /// Removes the given object from the array.
    mutating func remove(_ element: Element) {
        // We could also use "self.index(of: element)" here, as "index(of:_)"
        // is also defined in an extension with "where Element: Equatable".
        // For the sake of this example, explicitly make use of the Equatable.
        if let index = self.index(where: { $0 == element }) {
            self.remove(at: index)
        } else {
            fatalError("Removal error, no such element:\"\(element)\" in array.\n")
        }
    }
}