> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution > <swift-evolution@swift.org> wrote: > > Hi Matthew, > > Your arguments made sense to me. I modified the proposal to choose strategy > number 3: deprecating and removing class over several versions to favour > AnyObject. Mind having another proof read? > > https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md > > <https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md> > > Anybody has counter arguments? > > Class and Subtype existentials > > Proposal: SE-XXXX > <https://github.com/hartbit/swift-evolution/blob/subclass-existentials/proposals/XXXX-subclass-existentials.md> > Authors: David Hart <http://github.com/hartbit/>, Austin Zheng > <http://github.com/austinzheng> > Review Manager: TBD > Status: TBD > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#introduction>Introduction > > This proposal brings more expressive power to the type system by allowing > Swift to represent existentials of classes and subtypes which conform to > protocols. > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#motivation>Motivation > > Currently, the only existentials which can be represented in Swift are > conformances to a set of protocols, using the &protocol composition syntax: > > Protocol1 & Protocol2 > On the other hand, Objective-C is capable of expressing existentials of > classes and subclasses conforming to protocols with the following syntax: > > id<Protocol1, Protocol2> > Base<Protocol>* > We propose to provide similar expressive power to Swift, which will also > improve the bridging of those types from Objective-C. > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#proposed-solution>Proposed > solution > > The proposal keeps the existing & syntax but allows the first element, and > only the first, to be either the AnyObjectkeyword or of class type. The > equivalent to the above Objective-C types would look like this: > > AnyObject & Protocol1 & Protocol2 > Base & Protocol > As in Objective-C, the first line is an existential of classes which conform > to Protocol1 and Protocol2, and the second line is an existential of subtypes > of Base which conform to Protocol. > > Here are the new proposed rules for what is valid in a existential > conjunction syntax: > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#1-the-first-element-in-the-protocol-composition-syntax-can-be-the-anyobject-keyword-to-enforce-a-class-constraint>1. > The first element in the protocol composition syntax can be the AnyObject > keyword to enforce a class constraint: > > protocol P {} > struct S : P {} > class C : P {} > let t: P & AnyObject // Compiler error: AnyObject requirement must be in > first position > let u: AnyObject & P = S() // Compiler error: S is not of class type > let v: AnyObject & P = C() // Compiles successfully > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#2-the-first-element-in-the-protocol-composition-syntax-can-be-a-class-type-to-enforce-the-existential-to-be-a-subtype-of-the-class>2. > The first element in the protocol composition syntax can be a class type to > enforce the existential to be a subtype of the class: > > protocol P {} > struct S {} > class C {} > class D : P {} > class E : C, P {} > let t: P & C // Compiler error: subclass constraint must be in first position > let u: S & P // Compiler error: S is not of class type > let v: C & P = D() // Compiler error: D is not a subtype of C > let w: C & P = E() // Compiles successfully > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#3-when-a-protocol-composition-type-contains-a-typealias-the-validity-of-the-type-is-determined-using-the-following-steps>3. > When a protocol composition type contains a typealias, the validity of the > type is determined using the following steps: > > Expand the typealias > Normalize the type by removing duplicate constraints and replacing less > specific constraints by more specific constraints (a class constraint is less > specific than a class type constraint, which is less specific than a > constraint of a subclass of that class). > Check that the type does not contain two class-type constraints
You could generalize this and instead say that if the type contains two class-type constraints, the resulting existential type is the common base class of the two classes, or AnyObject if they do not share a common base class. Also, I’d like to see some discussion about class-constrained existentials appearing in the inheritance clause of a protocol. IMHO, we should ban this: typealias MyType = SomeClass & SomeProtocol protocol SomeOtherProtocol : MyType {} Slava > class C {} > class D : C {} > class E {} > protocol P1 {} > protocol P2 {} > typealias TA1 = AnyObject & P1 > typealias TA2 = AnyObject & P2 > typealias TA3 = C & P2 > typealias TA4 = D & P2 > typealias TA5 = E & P2 > > typealias TA5 = TA1 & TA2 > // Expansion: typealias TA5 = AnyObject & P1 & AnyObject & P2 > // Normalization: typealias TA5 = AnyObject & P1 & P2 > // TA5 is valid > > typealias TA6 = TA1 & TA3 > // Expansion: typealias TA6 = AnyObject & P1 & C & P2 > // Normalization (AnyObject < C): typealias TA6 = C & P1 & P2 > // TA6 is valid > > typealias TA7 = TA3 & TA4 > // Expansion: typealias TA7 = C & P2 & D & P2 > // Normalization (C < D): typealias TA7 = D & P2 > // TA7 is valid > > typealias TA8 = TA4 & TA5 > // Expansion: typealias TA8 = D & P2 & E & P2 > // Normalization: typealias TA8 = D & E & P2 > // TA8 is invalid because the D and E constraints are incompatible > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#class-and-anyobject>class > and AnyObject > > This proposal merges the concepts of class and AnyObject, which now have the > same meaning: they represent an existential for classes. To get rid of the > duplication, we suggest only keeping AnyObject around. To reduce > source-breakage to a minimum, class could be redefined as typealias class = > AnyObject and give a deprecation warning on class for the first version of > Swift this proposal is implemented in. Later, class could be removed in a > subsequent version of Swift. > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#source-compatibility>Source > compatibility > > This change will not break Swift 3 compability mode because Objective-C types > will continue to be imported as before. But in Swift 4 mode, all types > bridged from Objective-C which use the equivalent Objective-C existential > syntax could break code which does not meet the new protocol requirements. > For example, the following Objective-C code: > > @interface MyViewController > - (void)setup:(nonnull > UIViewController<UITableViewDataSource,UITableViewDelegate>*)tableViewController; > @end > is imported into Swift-3 mode as: > > class MyViewController { > func setup(tableViewController: UIViewController) {} > } > which allows calling the function with an invalid parameter: > > let myViewController: MyViewController() > myViewController.setup(UIViewController()) > The previous code continues to compile but still crashs if the Objective-C > code calls a method of UITableViewDataSource or UITableViewDelegate. But if > this proposal is accepted and implemented as-is, the Objective-C code will be > imported in Swift 4 mode as: > > class MyViewController { > func setup(tableViewController: UIViewController & UITableViewDataSource > & UITableViewDelegate) {} > } > That would then cause the Swift code run in version 4 mode to fail to compile > with an error which states that UIViewController does not conform to the > UITableViewDataSource and UITableViewDelegate protocols. > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#alternatives-considered>Alternatives > considered > > An alternative solution to the class/AnyObject duplication was to keep both, > redefine AnyObject as typealias AnyObject = class and favor the latter when > used as a type name. > > > <https://github.com/hartbit/swift-evolution/tree/subclass-existentials#acknowledgements>Acknowledgements > > Thanks to Austin Zheng <http://github.com/austinzheng> and Matthew Johnson > <https://github.com/anandabits> who brought a lot of attention to > existentials in this mailing-list and from whom most of the ideas in the > proposal come from. > >> On 9 Feb 2017, at 21:50, Matthew Johnson <matt...@anandabits.com >> <mailto:matt...@anandabits.com>> wrote: >> >> >>> On Feb 9, 2017, at 2:44 PM, David Hart <da...@hartbit.com >>> <mailto:da...@hartbit.com>> wrote: >>> >>> >>> On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution >>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>> >>>> >>>> >>>> Sent from my iPad >>>> >>>> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution >>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>>> >>>>> >>>>>> On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution >>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>>>>>> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution >>>>>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>>>>>> Looks good. Minor comments below: >>>>>>> The typealias 'T5' is repeated as both an initial composition, and as a >>>>>>> demonstration of combining typealiases. >>>>>>> >>>>>>>> This proposal merges the concepts of class and AnyObject, which now >>>>>>>> have the same meaning: they represent an existential for classes. They >>>>>>>> are four solutions to this dilemna: >>>>>>>> Do nothing. >>>>>>>> Replace all uses of AnyObject by class, breaking source compatibility. >>>>>>>> Replace all uses of class by AnyObject, breaking source compatibility. >>>>>>>> Redefine AnyObject as typealias AnyObject = class. >>>>>>> I agree with other comments on recommending 4 here, and covering the >>>>>>> others as alternatives >>>>>>>> >>>>>>>> <https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/XXXX-subclass-existentials.md#source-compatibility>I >>>>>>>> agree that we need the typealias for compatibility. I think it's >>>>>>>> still worth discussing whether the `AnyObject` typealias should *only* >>>>>>>> be there for compatibility; it could be deprecated or obsoleted in >>>>>>>> Swift 4 or future language versions. >>>>> >>>>> I think it might be worth keeping to provide a more sensible >>>>> capitalization alternative than lower case “class” when used as a type >>>>> name: >>>>> >>>>> var obj: class // this looks weird because of capitalization. >>>>> >>>>> var obj: AnyObject // this looks better. >>>> >>>> I agree that it looks better and would choose AnyObject if source >>>> compatibility weren't an issue. One option that wasn't listed was to drop >>>> 'class' but use a multi-release deprecation strategy and a fix-it to >>>> facilitate a smooth transition. If the community is willing to adopt this >>>> approach it would be my first choice. >>> >>> You mean option 3? >> >> Pretty much, but option 3 does not make it clear that it won’t break source >> immediately in Swift 4. I think it becomes much more reasonable if Swift >> 3.1 code still compiles in Swift 4 mode, but with a deprecation warning. >> >> The reason I prefer `AnyObject` to `class` is because I think it’s ugly to >> have `class` as the name of an existential type. Type names are uppercase >> in Swift. It is also used to compose with protocols which also use >> uppercase names in Swift. Because it appears in contexts which use an >> uppercase convention it makes sense for this to have an uppercase name. >> `AnyObject` seems like the obvious choice if we’re going to go in that >> direction. >> >>> >>>>> >>>>>> >>>>>> -Joe >>>>>> _______________________________________________ >>>>>> swift-evolution mailing list >>>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >>>>> _______________________________________________ >>>>> swift-evolution mailing list >>>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>> <https://lists.swift.org/mailman/listinfo/swift-evolution> > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution