Sorry, can you explain what you mean when you say you must have a setter? Why would you mutate the view and not the array itself (`foo[42] = .double(42)` as opposed to `foo.double[42] = 42`)?
On Fri, Nov 18, 2016 at 03:25 Adrian Zubarev via swift-evolution < [email protected]> wrote: > 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 ([email protected]) > 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 < > [email protected]> 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 > Sent with Airmail > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution >
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