> On Mar 26, 2017, at 11:12, Karl Wagner via swift-evolution
> <[email protected]> wrote:
>
> I’d like to pitch the following two language changes. Both of them are
> technically possible today if you manually write thunks for the relevant
> protocol requirements, but it would be nice if we allowed them to be written
> directly:
>
> 1) Allow closures to satisfy function requirements in protocols
>
> protocol MyProtocol {
> func run(param: Int) -> String
> }
>
> struct MyStruct : MyProtocol {
> var run : (Int)->String // Satisfies requirement MyProtocol.run
> }
>
> Among other things, it would make writing type-erased wrappers in the style
> of AnyCollection much easier. The only obvious niggle is that the argument
> label wouldn’t be required when invoking the closure directly. The labels
> have no type-system significance, but it does make it subtly easier to write
> less generic code than you intend do. We could solve this by having
> code-completion favour protocol methods in this situation, or simply to
> require the label when invoking a closure which implements a known protocol
> requirement.
>
> 2) Allow functions with default parameters to satisfy function requirements
> in protocols
>
> protocol Sportsplayer {
> func goalsScored() -> Int
> }
>
> struct SoccerPlayer: Sportsplayer {
> struct GoalType : RawOptionSet {
> static let Shot = GoalType(0x1)
> static let Volley = GoalType(0x10)
> static let Header = GoalType(0x100)
> static let Any = GoalType(0x111)
> }
>
> // Default value .Any means this conforms to Sportsplayer
> func goalsScored(type: GoalType = .Any) {
> //...
> }
> }
>
> struct Footballer: Sportsplayer {
> struct GoalType : RawOptionSet {
> static let Touchdown = GoalType(0x1)
> static let FieldGoal = GoalType(0x10)
> static let Any = GoalType(0x11)
> }
>
> // Default value .Any means this conforms to Sportsplayer
> func goalsScored(type: GoalType = .Any) {
> //...
> }
> }
>
> I often find that I want to add some optional, implementation-specific
> parameter to a function which implements a protocol requirement. That’s
> currently not possible, and it’s a bit annoying.
IIRC, the issue with #2 is that protocols specify declaration-site details, but
default parameters are implemented at the call-site. At least I believe that
statement was accurate about a year(ish) ago... Dunno if anything has changed
since then.
If it can be made to work, though, I'd be in favor of it, and I think #1 as
well.
- Dave Sweeris
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