SE-0360: Opaque result types with limited availability

Evolution Proposal Reviews
12

The example demonstrated by the proposal does not make it particularly clear, but this seems like a useful feature towards making APIs back-portable by 1st or 3rd parties. :tada:

For a trivial example, I could think of ways to make SwiftUI’s refreshable(action:) modifier available (albeit in a renamed form[1]) to older platform versions without resorting to SwiftUI._ConditionalContent the actual return type of the modifier:

extension View {
    // Note: no `@ViewBuilder` here!
    @available(iOS 14.0, *)
    func refreshable_(action: @escaping @Sendable () async -> Void) -> some View {
        if #available(iOS 15.0, *) {
            return self.refreshable(action: action)
        }
        // Something like https://github.com/siteline/SwiftUI-Introspect#scrollview
        ...
    }
}

(If the function interface to backport contains data types or protocols unavailable to older targeted platform versions, however, then I'm afraid something more is needed than this proposal alone.)

  • What is your evaluation of the proposal?

+1

  • Is the problem being addressed significant enough to warrant a change to Swift?

Yes.

  • Does this proposal fit well with the feel and direction of Swift?

It seems like a natural extension to the rules about #available (and #unavailable).

  • If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?

N/A

  • How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

Quick reading, but also gave it a quick try on the latest main snapshot toolchain using iOS Simulator 14.5 and 15.5.

  1. Completely shadowing the existing implementation would further need a way for the shadowing function’s body to refer back to the shadowed one in the SwiftUI module in a qualified form, say, return `SwiftUI.View`.refreshable(action: action). Something similar was discussed but mainly for module-level symbols, not members, in Pitch: Fully qualified name syntax. ↩︎

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