> On 14 Feb 2017, at 10:24, Slava Pestov <[email protected]> wrote:
>
>
>> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution
>> <[email protected]> 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
>
> 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.
But if they share a common base class, the existential is invalid. Did I
misunderstand your generalization?
> 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 {}
Yep, I'll make that clear. It should be disallowed IMHO. Thanks!
> 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
>> 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 <[email protected]> wrote:
>>>>
>>>>
>>>> On Feb 9, 2017, at 2:44 PM, David Hart <[email protected]> wrote:
>>>>
>>>>
>>>> On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution
>>>> <[email protected]> wrote:
>>>>
>>>>>
>>>>>
>>>>> Sent from my iPad
>>>>>
>>>>>> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution
>>>>>> <[email protected]> wrote:
>>>>>>
>>>>>>
>>>>>>>> On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution
>>>>>>>> <[email protected]> wrote:
>>>>>>>> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution
>>>>>>>> <[email protected]> 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
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> [email protected]
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>> _______________________________________________
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>>
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