[swift-evolution] [Draft] Introducing StaticSelf, an Invariant Self

[Patrick Smith]

I think the distinction between StaticSelf and Self will be very confusing to newcomers.

So the only reason why we must use StaticSelf instead of Self here is because we want NSURL to conform, and it isn’t final?

protocol StringCreatable {
    static func createWithString(s: String) -> StaticSelf
}

I find it a code smell that this would affect the protocol and not the class.

Why couldn’t you have this?

protocol StringCreatable {
    static func createWithString(s: String) -> Self
}

extension NSURL: StringCreatable {
 // can now conform conform because NSURL is fixed and matches the static
 // type of the conforming construct. Subclasses need not re-implement
 // NOTE: the return type can be declared as StaticSelf *or* as NSURL
 //       they are interchangeable
 static func createWithString(s: String) -> StaticSelf { 
     // ...
 }
}



> On 19 May 2016, at 3:37 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> As a wrap-up of the topic, I've updated our original draft with Nicola S's resolution.
> 
> https://gist.github.com/erica/995af96a0de2f2f3dc419935e8140927 <https://gist.github.com/erica/995af96a0de2f2f3dc419935e8140927>
> 
> -- E
> 
> 
>> On May 14, 2016, at 8:35 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On May 14, 2016, at 12:55 AM, Nicola Salmoria via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Matthew Johnson via swift-evolution <swift-evolution at ...> 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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>> 
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