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
--
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
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