where clauses on contextually generic declarations

Paul Hudson      @twostraws

SE-0267 introduced the ability to attach a where clause to functions inside generic types and extensions.

For example, we could start with a simple Stack struct that let us push and pop values from a private array:

struct Stack<Element> {
    private var array = [Element]()

    mutating func push(_ obj: Element) {
        array.append(obj)
    }

    mutating func pop() -> Element? {
        array.popLast()
    }
}

Using SE-0267, we could add a new sorted() method to that stack, but only for times when the elements inside the stack conform to Comparable:

extension Stack {
    func sorted() -> [Element] where Element: Comparable {
        array.sorted()
    }
}

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Other changes in Swift 5.3…

• Multi-pattern catch clauses
• Multiple trailing closures
• Synthesized Comparable conformance for enums
• self is no longer required in many places
• Type-based program entry points
• Enum cases as protocol witnesses
• Refined didSet semantics
• A new Float16 type
• Swift Package Manager gains binary dependencies, resources, and more

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