As a wrap-up of the topic, I've updated our original draft with Nicola S's resolution.
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On May 14, 2016, at 8:35 AM, Matthew Johnson via swift-evolution <swift-evolution@swift.org> wrote:
On May 14, 2016, at 12:55 AM, Nicola Salmoria via swift-evolution <swift-evolution@swift.org> wrote:
Matthew Johnson via swift-evolution <swift-evolution@...> writes:
I agree it’s a bit tricky. But that’s better than not possible at all.
You just need a typealias and a same type constraint to make this work as
expected / desired:protocol Makable {
typealias RootMakable = StaticSelf
static func make(value: Int) -> StaticSelf
}func makeWithZero<T: Makable where T == T.RootMakable>(x: Int) -> T {
return T.make(value: 0) // works now
}Now that we have a typealias we can refer to the binding of StaticSelf and
constrain it as necessary for whatever purpose we have in mind. In some
cases that will be a same type constraint so that our code works properly
with class clusters. I don’t have concrete examples of other use cases but
can imagine use cases constraining the typealias to a protocol, for example.You can do that today:
protocol Makable {
associatedtype MadeType
static func make(value: Int) -> MadeType
}func makeWithZero<T: Makable where T == T.MadeType>(x: Int) -> T {
return T.make(value: 0)
}You can't currently constrain MadeType to be the same as the conforming
type, but, does it matter? What kind of extra guarantees would that give,
since you need to add the extra constraint anyway in generic code?Wow, this is pretty cool. Thank you very much for pointing this out Nicola!
I haven’t seen this approach to solving the problem. Given the amount of discussion this problem has received I am surprised nobody has shared this solution yet. I just checked in Xcode 7.3 and it works there. It isn’t dependent on any pre-release features.
Instead of using StaticSelf under the current proposal:
protocol StringInitializable {
static func initializeWith(string: String) -> StaticSelf
}We just add an associatedtype defaulted to Self:
protocol StringInitializable {
associatedtype Initialized = Self // where Self: Initialized
static func initializeWith(string: String) -> Initialized
}extension NSURL: StringInitializable {
static func initializeWith(string: String) -> NSURL {
return NSURL()
}
}func makeWith<T: StringInitializable where T == T.Initialized>(string: String) -> T {
return T.initializeWith(string: string)
}There are two minor downsides to this approach:
1. You can’t copy and paste the method signature.
2. You can theoretically conform a type completely unrelated to `Initialized` to the protocol, thus violating the semantics.I think we can live with these downsides. Maybe the `Self: Initialized` will be possible someday. That would be pretty close to StaticSelf. The only difference would be that subclasses still have flexibility to override with their own type.
Now that a reasonable way to do this with existing language features has been identified I will withdraw this proposal. If this approach doesn’t address use cases others have in mind for StaticSelf please speak up!
Doug, if you’re reading this, does the `where Self: Initialized` (i.e. arbitrary subclass constraints) fall into the scope of your “completing generics” manifesto? This is a concrete use case that would utilize subclass constraints.
-Matthew
Nicola
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