[Pitch] Named subscripts

Evolution Pitches
6

Yup :grinning:

···

Sent from my moss-covered three-handled family gradunza

On Nov 18, 2016, at 12:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

I believe Dave had a design more like this in mind:

struct _View<T> {
    let array: Array<Any>
    subscript(index: Int) -> T? {
        guard index >= 0 && index < array.count else { return nil }
        return array[index] as? T
    }
}

extension Array {
    var double: _View<Double> {
        return _View(array: self)
    }
}

On Fri, Nov 18, 2016 at 2:00 AM, Adrian Zubarev via swift-evolution <swift-evolution@swift.org> wrote:
Hi Dave,

Thank you for your answer. I have to admit this is a ‘workaround’ but it will make everything even worse.

From:

public func scopedJavaScript(at index: Int) -> (javaScript: String, scope: Document)?
To:

public subscript(at index: Int) -> (javaScript: String, scope: Document)?

public var scopedJavaScript: Array {
    get { return self }
    set { /* implementation artifact */ }
}
Now I could write code like array.scopedJavaScript.scopedJavaScript.scopedJavaScript and so one, which makes no sense any more.

Where we could simply allow:

public subscript scopedJavaScript(at index: Int) -> (javaScript: String, scope: Document)?
This would ensure that the user can only write something like:

array.scopedJavaScript[at: 42] // get the value
array.scopedJavaScript[at: 42] = (…, …) // set the value
Is there anything that speaks against optionally named subscripts?
Technical reasons?
Swiftiness?

--
Adrian Zubarev
Sent with Airmail

Am 17. November 2016 um 23:33:44, Dave Abrahams via swift-evolution (swift-evolution@swift.org) schrieb:

on Thu Nov 17 2016, Adrian Zubarev <swift-evolution@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] = …

You do that by giving your Array a scopedJavaScript property, and
making that indexable.

--
-Dave

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