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

[Matthew Johnson]

Sent from my iPad

> On May 19, 2016, at 3:14 AM, L. Mihalkovic <laurent.mihalkovic at gmail.com> wrote:
> 
> It strikes me that in a sense, the proposed StaticSelf is basically what  '_' does
> 
> By specifying StaticSelf we are trying to tell the compiler to ignore what is going on that that precise location to revert to something expressed 'earlier'. This is somehwat anologous to what we do when we use '_' in signatures: we escape the specifics of the current location, ony in that case there is no prior context to refer to, so the parameter winds up having no particular name.
> 
> If you try to read the examples with this interpretation: "nothing specific from this location" then you can see how you wind up 'inheriting' the previously expressed static type of self. So I would seriously consider using _ instead of adding StaticSelf.

This proposal is no longer being pursued.  Please have a look at the solution that has been identified if you're interested.

> 
> 
> Regards
> (From mobile)
> 
>> On May 18, 2016, at 7:37 PM, 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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