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

[Erica Sadun]

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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