Can I extend Tuples in Swift?

Question

I'd like to write an extension for tuples of (e.g.) two value in Swift. For instance, I'd like to write this swap method:

let t = (1, "one")
let s = t.swap

such that s would be of type (String, Int) with value ("one", 1). (I know I can very easily implement a swap(t) function instead, but that's not what I'm interested in.)

Can I do this? I cannot seem to write the proper type name in the extension declaration.

Additionally, and I suppose the answer is the same, can I make a 2-tuple adopt a given protocol?

Answer 1

You cannot extend tuple types in Swift. According to Types, there are named types (which can be extended) and compound types. Tuples and functions are compound types.

See also (emphasis added):

Extensions
Extensions add new functionality to an existing class, structure, or enumeration type.

Answer 2

As the answer above states, you cannot extend tuples in Swift. However, rather than just give you a no, what you can do is box the tuple inside a class, struct or enum and extend that.

struct TupleStruct {
    var value: (Int, Int)
}
extension TupleStruct : Hashable {
    var hashValue: Int {
        return hash()
    }
    func hash() -> Int {
        var hash = 23
        hash = hash &* 31 &+ value.0
        return hash &* 31 &+ value.1
    }
}

func ==(lhs: TupleStruct, rhs: TupleStruct) -> Bool {
    return lhs.value == rhs.value
}

As a side note, in Swift 2.2, tuples with up to 6 members are now Equatable.

Answer 3

Details

• Xcode 11.2.1 (11B500), Swift 5.1

Solution

struct Tuple<T> {
    let original: T
    private let array: [Mirror.Child]
    init(_ value: T) {
        self.original = value
        array = Array(Mirror(reflecting: original).children)
    }

    func getAllValues() -> [Any] { array.compactMap { $0.value } }
    func swap() -> (Any?, Any?)? {
        if array.count == 2 { return (array[1].value, array[0].value) }
        return nil
    }
}

Usage

let x = (1, "one")
let tuple = Tuple(x)
print(x)                                        // (1, "one")
print(tuple.swap())                             // Optional((Optional("one"), Optional(1)))
if let value = tuple.swap() as? (String, Int) {
    print("\(value) | \(type(of: value))")      // ("one", 1) | (String, Int)
}