[swift-evolution] Class and Subclass Existentials (Round 2)

Mon Feb 13 10:33:44 CST 2017

On class vs. AnyObject, probablt a dumb question but maybe worth
clarifying, we don't pretend to change it in a class declaration
right?

like

AnyObject A {
 var ....
}

apart from that looking forward for this!

On Sun, Feb 12, 2017 at 8:32 PM, David Hart via swift-evolution
<swift-evolution at 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
>
> Anybody has counter arguments?
>
> Class and Subtype existentials
>
> Proposal: SE-XXXX
> Authors: David Hart, Austin Zheng
> Review Manager: TBD
> Status: TBD
>
> 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.
>
> 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.
>
> 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:
>
> 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
>
> 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
>
> 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
>
> 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
>
> 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.
>
> 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.
>
> 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.
>
> Acknowledgements
>
> Thanks to Austin Zheng and Matthew Johnson 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 <matthew at anandabits.com> wrote:
>
>
> On Feb 9, 2017, at 2:44 PM, David Hart <david at hartbit.com> wrote:
>
>
> On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution
> <swift-evolution at swift.org> wrote:
>
>
>
> Sent from my iPad
>
> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution
> <swift-evolution at swift.org> wrote:
>
>
> On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution
> <swift-evolution at swift.org> wrote:
>
> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution
> <swift-evolution at 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
>
> 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
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-- 
Alejandro Martinez
http://alejandromp.com