[swift-evolution] [Pitch] Named subscripts

[Adrian Zubarev]

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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