[swift-evolution] [Pitch] Named subscripts

Adrian Zubarev adrian.zubarev at devandartist.com
Fri Nov 18 03:25:44 CST 2016

Thank you guys for all your suggestions so far.

I understand the idea behind the generic subscript here, they are neat and highly needed, but even this approach won’t solve my issue of clarity here.

The Array I extend here has an Element of type Value which is an enum that wraps other types around (part of BSON).

I’d have to insert a huge pattern matching switch into that generic subscript and unwrap every possible type. Don’t get me wrong, this would work, because the result type is an optional, where I just can return nil if nothing matches.

But again I lose the clarity from the readers prospective, because I don’t know by reading code like array[at: 123] = someValue what kind of subscript I’m using here.

As already suggested, the view workaround would result in the exact the same syntax I look for, but it has it own downsides as I already mentioned (+ every time you’d need to instantiate a new view).

Adrian Zubarev
Sent with Airmail

Am 18. November 2016 um 09:55:00, Haravikk (swift-evolution at haravikk.me) schrieb:

Could this be addressed by allowing generic constraints on subscripts?
For example, with methods we can currently do:

struct Foo {
var values:[Any] = []

func get<T>(at:Int) -> T? {
return values.indices.contains(at) ? values[at] as? T : nil

func get<T>(at:Int, as theType:T.Type) -> T? {
return values.indices.contains(at) ? values[at] as? T : nil

mutating func set<T>(at:Int, to:T) {
if values.indices.contains(at) { values[at] = to }

let foo = Foo(values: [1.5, 2.5, 3.5, 1, 2, 3])
let a = foo.get(at: 0, as: Double.self)
let b:Double = foo.get(at: 1)!
let c:Int? = foo.get(at: 2)
let d = foo.get(at: 3, as: Double.self)
let e:Int = foo.get(at: 4)!
let f = foo.get(at: 5, as: Int.self)
i.e- the type is inferred from the call-site either with an explicit variable type, or by passing in the expected type as the second argument, which I think is a pretty neat way to do it.

If we could do the same with subscripts we could do something like:

struct Foo {
var values:[Any] = []

subscript<T>(_ at:Int) -> T? {
get { return values.indices.contains(at) ? values[at] as? T : nil }
set { if values.indices.contains(at) { values[at] = newValue } }

subscript<T>(_ at:Int, as theType:T.Type) -> T? {
return values.indices.contains(at) ? values[at] as? T : nil

let foo = Foo(values: [1.5, 2.5, 3.5, 1, 2, 3])
let a = foo[0, as: Double.self]
let b:Double = foo[1]!
let c:Int? = foo[2]
let d = foo[3, as: Double.self]
let e:Int = foo[4]!
let f = foo[5, as: Int.self]

Are generic constraints on subscripts part of the generics manifesto?

On 17 Nov 2016, at 20:14, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:

Dear Swift community,

while building a framework for BSON I had the following idea.

Here is a snippet of some code I do have in my module:

extension Array where Element == Document.Value {
    public func double(at index: Int) -> Double? {
        guard self.startIndex <= index && index < self.endIndex else { return nil }
        if case .double(let double) = self[index] {
            return double
        return nil
This function is used to query the array and check if the element at the given index is of a specific type. Now I would like also to implement a semi-schema setter.

The problem that I see, is the ugliness of the subscript I’d create.

Currently the code would read nicely let d = array.double(at: 42), but after change to a subscript the API would look odd array[doubleAt: 42] = 5.0.

Don’t get me wrong here, I also have methods with larger names like public func scopedJavaScript(at index: Int) -> …. You can easily imagine that such subscripts would look ugly array[scopedJavaScriptAt: 123] = ….

I propose to align the design of subscript with functions where one could optionally give subscript a name.

func name(label parameter: Type) -> ReturnType

subscript optionalName(label parameter: Type) -> ReturnType
This change would make my API nice and clean. array.scopedJavaScript[at: 213] = …

This also might be the opportunity to rethink the labeling rule on subscripts, but this shall not be the main focus of this pitch.

Adrian Zubarev
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