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

[Matthew Johnson]

Sent from my iPad

> On May 18, 2016, at 9:57 PM, Patrick Smith <pgwsmith at gmail.com> wrote:
> 
> 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 { 
>      // ...
>  }
> }
> 

You can't do this because the Self return type in the protocol requirement specifically *requires* all subclasses to override the method and return an instance of the subclass type.  

Nevertheless, we have identified a workaround that is similar enough to StaticSelf that we have abandoned the proposal.  Please see The last couple posts in this thread if you're interested in the details.

> 
> 
>> 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
>> 
>> -- E
>> 
>> 
>>>> On May 14, 2016, at 8:35 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> 
>>>> On May 14, 2016, at 12:55 AM, Nicola Salmoria via swift-evolution <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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>> 
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