> On 14 Feb 2017, at 10:57, Slava Pestov via swift-evolution
> <[email protected]> wrote:
>
>
>> On Feb 14, 2017, at 1:30 AM, Adrian Zubarev
>> <[email protected]> wrote:
>>
>> Why can’t we completely ban this?
>>
>> protocol A {}
>> protocol B {}
>> typealias AB = A & B
>>
>> protocol C : AB {} // Allowed, but could be also banned
>>
>> protocol D : A & B {} // Error
>>
>
> I didn’t even know the last one there was banned. /me hangs head in shame.
>
> I think either both should be supported, or neither one should be supported.
> I’m leaning toward the former :-)
I'm all for supporting it, but then what about:
class C {}
protocol P {}
class D : C & P {}
If that's the case, we have a duplicate syntax for inheritance clauses, and we
might as well keep only one. Hint, hint to Doug :)
> However note that unlike protocols that inherit from classes, this does not
> create any conceptual difficulties in the language; it’s merely a syntactic
> quirk. I’m more concerned about banning protocols that inherit from
> typealiases that contain classes.
>
> Slava
>
>>
>>
>> --
>> Adrian Zubarev
>> Sent with Airmail
>>
>> Am 14. Februar 2017 um 10:25:43, Slava Pestov via swift-evolution
>> ([email protected]) schrieb:
>>
>>>
>>>> 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.
>>>
>>> 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
>>>> 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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