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

[Hooman Mehr via swift-evolution]

> On Feb 14, 2017, at 1:24 AM, Slava Pestov via swift-evolution 
>  wrote:
> 
>> 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.
> 

Why would we do that? They are called constraints, and we use “&” between them. 
They should move towards the more specific subclass, not the less restrained 
base class. So, I would say:

If the type contains two class-type constraints, it is only acceptable if one 
class constrain is a subclass of the other and the resulting constrain will be 
the subclass.

 ___
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

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

[David Hart via swift-evolution]

> On 14 Feb 2017, at 16:38, Matthew Johnson  wrote:
> 
> 
>> On Feb 14, 2017, at 5:28 AM, David Hart  wrote:
>> 
>> 
>> On 14 Feb 2017, at 10:24, Slava Pestov  wrote:
>> 
>>> 
 On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
  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/-subclass-existentials.md
 
 Anybody has counter arguments?
 
 Class and Subtype existentials
 Proposal: SE-
 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
 Base*
 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?
> 
> Did you mean to say if they *don’t* share a common base class?

Yes, sorry! Slava mentioned AnyObject when more than one class-type share a 
common base class. But the proposal states that as invalid so just want to 
understand what he meant.

>> 
>>> 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: typealia

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

[Matthew Johnson via swift-evolution]

> On Feb 14, 2017, at 5:28 AM, David Hart  wrote:
> 
> 
> On 14 Feb 2017, at 10:24, Slava Pestov  > wrote:
> 
>> 
>>> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
>>> mailto: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/-subclass-existentials.md
>>>  
>>> 
>>> 
>>> Anybody has counter arguments?
>>> 
>>> Class and Subtype existentials
>>> 
>>> Proposal: SE- 
>>> 
>>> 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
>>> Base*
>>> 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.

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

[Matthew Johnson via swift-evolution]

> On Feb 14, 2017, at 3:57 AM, Slava Pestov via swift-evolution 
>  wrote:
> 
> 
>> On Feb 14, 2017, at 1:30 AM, Adrian Zubarev > > 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 agree.

> 
> 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.

In my mind, allowing a protocol to “inherit” from a class would be the same as 
a supertype requirements.  I imagine the following to be roughly equivalent in 
meaning:

protocol Foo {}
func takesFoo(foo: F) {}


protocol Foo: MyClass {}
func takesFoo(foo: F) {}

All we’re doing is moving the supertype constraint to the protocol itself 
rather than requiring it to be repeated everywhere the protocol is used.  What 
conceptual difficulty is involved in allowing this?  Maybe I’m missing 
something, but I just don’t see it.

A second example that builds on the subtype existential syntax:

protocol Foo {}
typealias Bar = MyClass & Bar

func takesBar(bar: B) {}

A third example using the typealias in creating a new nominal protocol:

protocol Three: Bar {}

would be equivalent to 

protocol Three: MyClass, Foo {}

If we don’t allow this we are not preventing users from adding supertype 
constraints to a protocol.  We’re just requiring them to expose a protocol 
*without* the supertype requirement and use `&` (maybe with a typealias) 
everywhere they actually mention the protocol in their interface.  This seems a 
little bit fragile and boilerplate-y.  What is the downside of allowing a 
protocol that is intended to always have a supertype constraint from stating it 
directly in the declaration of the protocol?

> 
> Slava
> 
>> 
>> 
>> -- 
>> Adrian Zubarev
>> Sent with Airmail
>> 
>> Am 14. Februar 2017 um 10:25:43, Slava Pestov via swift-evolution 
>> (swift-evolution@swift.org ) schrieb:
>> 
>>> 
 On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
 mailto: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/-subclass-existentials.md
  
 
 
 Anybody has counter arguments?
 
 Class and Subtype existentials
 Proposal: SE- 
 
 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
 Base*
 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 

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

[David Hart via swift-evolution]

> On 14 Feb 2017, at 10:57, Slava Pestov via swift-evolution 
>  wrote:
> 
> 
>> On Feb 14, 2017, at 1:30 AM, Adrian Zubarev 
>>  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 
>> (swift-evolution@swift.org) schrieb:
>> 
>>> 
 On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
  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/-subclass-existentials.md
 
 Anybody has counter arguments?
 
 Class and Subtype existentials
 Proposal: SE-
 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
 Base*
 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 cla

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

[David Hart via swift-evolution]

> On 14 Feb 2017, at 10:24, Slava Pestov  wrote:
> 
> 
>> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
>>  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/-subclass-existentials.md
>> 
>> Anybody has counter arguments?
>> 
>> Class and Subtype existentials
>> Proposal: SE-
>> 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
>> Base*
>> 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 proposa

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

[Adrian Zubarev via swift-evolution]

If I recall correctly before Swift 3 something like protocol C : protocol 
{} was allowed, but it’s banned now.

If I understood your opinion than you’re leaning towards:

protocol A {}
protocol B {}

class Super {}
class X : Super

typealias AB = A & B
typealias SuperA = Super & A

class Y : SuperA {}// Okay
class Z : X, SuperA {} // Okay because `X` is a subclass of `Super`
class ZZ : X {}

extension ZZ : SuperA {} // Okay because `ZZ` an descendant of `Super`
extension Y : AB {}  // Okay
extension Z : AB {}  // Okay

protocol P : SuperA {} // Banned, because it contains a concrete class type
protocol Q : AB {} // Okay
Plus as you mentioned allowing conforming to/inheriting from something like A & 
B directly.

One question that comes to my mind: would the latter not imply people of 
writing code like this class Xyz : SuperClass & P & Q instead of class Xyz : 
SuperClass, P, Q? Isn’t that the exact reason why it’s banned today?



-- 
Adrian Zubarev
Sent with Airmail

Am 14. Februar 2017 um 10:57:34, Slava Pestov (spes...@apple.com) schrieb:


On Feb 14, 2017, at 1:30 AM, Adrian Zubarev  
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 :-)

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 
(swift-evolution@swift.org) schrieb:


On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
 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/-subclass-existentials.md

Anybody has counter arguments?

Class and Subtype existentials
Proposal: SE-
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
Base*
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 t

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

[Slava Pestov via swift-evolution]

> On Feb 14, 2017, at 1:30 AM, Adrian Zubarev  
> 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 :-)

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 
> (swift-evolution@swift.org ) schrieb:
> 
>> 
>>> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
>>> mailto: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/-subclass-existentials.md
>>>  
>>> 
>>> 
>>> Anybody has counter arguments?
>>> 
>>> Class and Subtype existentials
>>> Proposal: SE- 
>>> 
>>> 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
>>> Base*
>>> 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
>>>  
>>> 

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

[Adrian Zubarev via swift-evolution]

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 
(swift-evolution@swift.org) schrieb:


On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
 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/-subclass-existentials.md

Anybody has counter arguments?

Class and Subtype existentials
Proposal: SE-
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
Base*
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 = 
AnyO

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

[Slava Pestov via swift-evolution]

> On Feb 12, 2017, at 12:32 PM, David Hart via swift-evolution 
>  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/-subclass-existentials.md
>  
> 
> 
> Anybody has counter arguments?
> 
> Class and Subtype existentials
> 
> Proposal: SE- 
> 
> 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
> Base*
> 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
> t

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

[Adrian Zubarev via swift-evolution]

No, you’re totally right there. The mentioned class keyword is the protocol 
constraint-keyword such as protocol X : >> class << { … }. Instead we would 
like to generalize AnyObject



-- 
Adrian Zubarev
Sent with Airmail

Am 13. Februar 2017 um 17:34:18, Alejandro Martinez via swift-evolution 
(swift-evolution@swift.org) schrieb:

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
 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/-subclass-existentials.md
>
> Anybody has counter arguments?
>
> Class and Subtype existentials
>
> Proposal: SE-
> 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
> Base*
>
> 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 

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

[Alejandro Martinez via swift-evolution]

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
 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/-subclass-existentials.md
>
> Anybody has counter arguments?
>
> Class and Subtype existentials
>
> Proposal: SE-
> 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
> Base*
>
> 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 O

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

[David Hart via swift-evolution]

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/-subclass-existentials.md

Anybody has counter arguments?

Class and Subtype existentials

Proposal: SE- 

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
Base*
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 AnyO

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

[Matthew Johnson via swift-evolution]

> On Feb 9, 2017, at 2:44 PM, David Hart  wrote:
> 
> 
> On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution 
> mailto:swift-evolution@swift.org>> wrote:
> 
>> 
>> 
>> Sent from my iPad
>> 
>> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution 
>> mailto:swift-evolution@swift.org>> wrote:
>> 
>>> 
 On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution 
 mailto:swift-evolution@swift.org>> wrote:
> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution 
> 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
>>  
>> 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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 swift-evolution@swift.org 
 https://lists.swift.org/mailman/listinfo/swift-evolution 
 
>>> ___
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>>> 
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Re: [swift-evolution] Class and Subclass Existentials (Round 2)

[David Hart via swift-evolution]

> On 9 Feb 2017, at 20:43, Matthew Johnson via swift-evolution 
>  wrote:
> 
> 
> 
> Sent from my iPad
> 
>> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution 
>>  wrote:
>> 
>> 
 On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution 
  wrote:
 On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution 
  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?

>> 
>>> 
>>> -Joe
>>> ___
>>> swift-evolution mailing list
>>> swift-evolution@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
>> ___
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Re: [swift-evolution] Class and Subclass Existentials (Round 2)

[Matthew Johnson via swift-evolution]

Sent from my iPad

> On Feb 9, 2017, at 1:30 PM, Hooman Mehr via swift-evolution 
>  wrote:
> 
> 
>>> On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution 
>>>  wrote:
>>> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution 
>>>  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.

> 
>> 
>> -Joe
>> ___
>> swift-evolution mailing list
>> swift-evolution@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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Re: [swift-evolution] Class and Subclass Existentials (Round 2)

[Hooman Mehr via swift-evolution]

> On Feb 9, 2017, at 10:47 AM, Joe Groff via swift-evolution 
>  wrote:
>> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution 
>> 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
>>>  
>>> 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.

> 
> -Joe
> ___
> swift-evolution mailing list
> swift-evolution@swift.org 
> https://lists.swift.org/mailman/listinfo/swift-evolution 
> 
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Re: [swift-evolution] Class and Subclass Existentials (Round 2)

[Joe Groff via swift-evolution]

> On Feb 9, 2017, at 4:26 AM, Step Christopher via swift-evolution 
>  wrote:
> 
> Looks good. Minor comments below:
> 
>> let t: P & C // Compiler error: subclass contraint must be in first position
> 
> Typo: contraint should be constraint
>>  
>> 
>> ...
>> typealias TA4 = D & P2
>> typealias TA5 = E & P2
>> 
>> typealias TA5 = TA1 & TA2
>> typealias TA5 = class & P1 & class & P2 // Expansion
>> typealias TA5 = class & P1 & P2 // Normalization
>> // TA5 is valid
> 
> 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.

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

[Step Christopher via swift-evolution]

Looks good. Minor comments below:

> let t: P & C // Compiler error: subclass contraint must be in first position

Typo: contraint should be constraint
> ...
> typealias TA4 = D & P2
> typealias TA5 = E & P2
> 
> typealias TA5 = TA1 & TA2
> typealias TA5 = class & P1 & class & P2 // Expansion
> typealias TA5 = class & P1 & P2 // Normalization
> // TA5 is valid

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

[Matthew Johnson via swift-evolution]

This looks really good, thank you for your work on this proposal David!  

I agree with Doug’s suggestion to pick #4 (typealias AnyObject = class) and 
move the other options to alternatives considered.  This seems like the best 
way to go given Swift’s source compatibility promise and will probably be the 
least controversial option as well.

> On Feb 6, 2017, at 3:18 PM, David Hart via swift-evolution 
>  wrote:
> 
> Hello mailing-list,
> 
> I rewrote the draft proposal concerning the class and subclass existentials. 
> Please let me know what you think, especially concerning the class and 
> AnyObject conundrum.
> 
> Regards,
> David.
> 
> https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/-subclass-existentials.md
>  
> 
> 
> Class and Subtype existentials
> 
> Proposal: SE- 
> 
> 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
> Base*
> 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 class keyword or of class type. The 
> equivalent to the above Objective-C types would look like this:
> 
> class & 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 class 
> keyword to enforce a class constraint:
> 
> protocol P {}
> struct S : P {}
> class C : P {}
> let t: P & class // Compiler error: class requirement must be in first 
> position
> let u: class & P = S() // Compiler error: S is not of class type
> let v: class & 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 contraint 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 cla

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

[Douglas Gregor via swift-evolution]

> On Feb 6, 2017, at 1:18 PM, David Hart  wrote:
> 
> Hello mailing-list,
> 
> I rewrote the draft proposal concerning the class and subclass existentials. 
> Please let me know what you think, especially concerning the class and 
> AnyObject conundrum.

Thanks for working on this! Some comments below.

- Doug

> Regards,
> David.
> 
> https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/-subclass-existentials.md
>  
> 
> 
> Class and Subtype existentials
> 
> Proposal: SE- 
> 
> 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
> Base*
> 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 class keyword or of class type. The 
> equivalent to the above Objective-C types would look like this:
> 
> class & 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 class 
> keyword to enforce a class constraint:
> 
> protocol P {}
> struct S : P {}
> class C : P {}
> let t: P & class // Compiler error: class requirement must be in first 
> position
> let u: class & P = S() // Compiler error: S is not of class type
> let v: class & P = C() // Compiles successfully
Is

let w: class = C()

intended to work? (I believe the answer is “yes”)
> 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 contraint 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
Okay.
> 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

Technically, if one class is a subclass of the other, we can take the subclass. 
We actually already do this in generic constraints:

class C { }
class D: C { }

func f(_: T) where T: C, T: D { }  // same as just "where T: D”

We could be more strict with existentials, but it seems unnecessary.
>  
> class
>  and AnyObject
> 
> 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

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

[Adrian Zubarev via swift-evolution]

Looks really promising to me. :)

Aside the class existentials, I still miss generalized struct and enum 
constraints (this is out of scope of this proposal, and also not meant to be a 
request). Sometimes I really want to tell someone who’s using my code base “no 
you’re not allowed to create a class type here, because the way you’re doing it 
is ugly, as if Objective-C, Java and C# had a child called quasimodo”. By 
default such constraints would not enforce value semantics, but at least 
they’ll do some good to some codebase.

Speaking of existentials again, struct & ProtocolFoo & ProtocolBar would be the 
same as if we had protocol FooBar : struct, ProtocolFoo, ProtocolBar {}. Such 
an existential/protocol will reject any class instance.

Alternatively there could exist AnyValue which would cover any value type 
including tuples. (As a sidenote: IMHO AnyValue should not conform to something 
like ValueSemantics by default, because there might be some point where you 
don’t want value semantics to apply.)



-- 
Adrian Zubarev
Sent with Airmail

Am 6. Februar 2017 um 22:18:19, David Hart via swift-evolution 
(swift-evolution@swift.org) schrieb:

Hello mailing-list,

I rewrote the draft proposal concerning the class and subclass existentials. 
Please let me know what you think, especially concerning the class and 
AnyObject conundrum.

Regards,
David.

https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/-subclass-existentials.md

Class and Subtype existentials
Proposal: SE-
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
Base*
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 class keyword or of class type. The equivalent to 
the above Objective-C types would look like this:

class & 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 class 
keyword to enforce a class constraint:

protocol P {}
struct S : P {}
class C : P {}
let t: P & class // Compiler error: class requirement must be in first position
let u: class & P = S() // Compiler error: S is not of class type
let v: class & 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 contraint 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 = class & P1
typealias TA2 = class & P2
typealias TA3 = C & P2
typealias TA4 = D & P2
typealias TA5 = E & P2

typealias TA5 = TA1 & TA2
typealias TA5 = class & P1 & class & P2 // Expansion
typealias TA5 = class & P1 & P2 // Normalization
// TA5 is valid

typealias TA6 = TA1 & TA3
typealias TA6 = class & P1 & C & P2 // Expansion
typealias TA6 = C & P1 & P2 // Normalization (class < C)
// TA6 is valid

typealias TA7 = TA3 & TA4
typealias TA7 = C & P2 & D & P2 // Expansion
typealias TA7 = D & P2 // Normalization (C < D)
// TA7 is valid

typealias TA8 = TA4 & TA5
typealias TA8 = D & P2 & E & P2 // Expansion
typealias TA8 = D & E & P2 // Normalization
// 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 

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

[David Hart via swift-evolution]

Hello mailing-list,

I rewrote the draft proposal concerning the class and subclass existentials. 
Please let me know what you think, especially concerning the class and 
AnyObject conundrum.

Regards,
David.

https://github.com/hartbit/swift-evolution/blob/e6411d8a9e7924bbd8a48fc292bf08d58a8d1199/proposals/-subclass-existentials.md
 


Class and Subtype existentials

Proposal: SE- 

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
Base*
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 class keyword or of class type. The equivalent to 
the above Objective-C types would look like this:

class & 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 class keyword 
to enforce a class constraint:

protocol P {}
struct S : P {}
class C : P {}
let t: P & class // Compiler error: class requirement must be in first position
let u: class & P = S() // Compiler error: S is not of class type
let v: class & 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 contraint 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 = class & P1
typealias TA2 = class & P2
typealias TA3 = C & P2
typealias TA4 = D & P2
typealias TA5 = E & P2

typealias TA5 = TA1 & TA2
typealias TA5 = class & P1 & class & P2 // Expansion
typealias TA5 = class & P1 & P2 // Normalization
// TA5 is valid

typealias TA6 = TA1 & TA3
typealias TA6 = class & P1 & C & P2 // Expansion
typealias TA6 = C & P1 & P2 // Normalization (class < C)
// TA6 is valid

typealias TA7 = TA3 & TA4
typealias TA7 = C & P2 & D & P2 //