Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Developer via swift-evolution]

That doesn't look like a variance issue to me, that's about the same 
"information" invariant I talked about before.  The former works because self 
resolves to an invariant type, the type of the implementing structure, which 
satisfies the requirement Self introduces.  The latter does not because Self 
indicates a level of specificity C cannot guarantee.  Self is magic, but it is 
also implemented as a generic parameter.  So think of it this way:

protocol Q {
  func bar() -> T { return Q() }
}

You wouldn't expect that to compile, would you?

~Robert Widmann

2015/12/28 12:49、Matthew Johnson  のメッセージ:

> 
>> On Dec 28, 2015, at 11:19 AM, Developer  wrote:
>> 
>> My understanding of Self is that it is a special generic parameter resolved 
>> by the type system to the type of the implementing structure.  That 
>> resolution must be invariant because the implementing structure (here, 
>> non-final classes) can choose to yank the protocol's invariants out from 
>> under you when it is subclassed.  Sure, retroactively, you can make things 
>> conform, but you also can't completely guarantee type safety with any kind 
>> of variance in Self in all cases. 
>> 
>> On the other hand, using the protocol itself in either position says that 
>> you only wish to restrict yourself to the protocol itself, not some specific 
>> implementation.  You are necessarily specifying an upper bound (here C) on 
>> the amount of "information" you can get out of the type, so it is possible 
>> to introduce variance because you will never violate the protocol's 
>> invariants by returning a subtype with a legal conformance.
>> 
>> Self doesn't mean two different things, your protocol declarations do!
> 
> My mind must be a little bit foggy this morning.  This works:
> 
> extension C: Q {
> func bar() -> Self { return self }
> }
> 
> What doesn’t work, regardless of whether C is final or not, is this:
> 
> extension C: Q {
> // Cannot convert return expression of type ‘C’ to return type ‘Self'
> func bar() -> Self { return C() }
> }
> 
> In order for classes to meet a protocol requirement with Self in the return 
> position you must specify Self (rather than the conforming type) as the 
> return type for the method.  Self in the return position of a method is 
> treated as covariant.
> 
> In order for classes to meet a protocol requirement with Self in parameter 
> position you must specify the type of the conforming class (you cannot 
> specify Self in an argument position).  Obviously the type of the conforming 
> class is invariant.
> 
> This is the sense in which Self in protocol declarations is inconsistent.  
> The requirements on conforming types are different - invariance for Self 
> parameters and covariance for Self return types.
> 
> IMO it would be much more clear if this distinction was explicit rather than 
> implicit based on the location of Self.  It would also be extremely useful in 
> some cases to be able to specify an invariant `ConformingSelf` return type.
> 
> 
>> 
>> ~Robert Widmann
>> 
>> 2015/12/28 11:49、Matthew Johnson via swift-evolution 
>>  のメッセージ:
>> 
>>> I have brought up the idea of a non-covarying Self a few times.  
>>> 
>>> I was surprised to realize that Self is actually non-covarying when used 
>>> for parameters in protocol declarations!
>>> 
>>> Here is an example demonstrating this:
>>> 
>>> protocol P {
>>>func foo(s: Self)
>>> }
>>> protocol Q {
>>>func bar() -> Self
>>> }
>>> 
>>> class C: P {
>>>// this works!  Self as an argument type in the protocol declaration 
>>> does not covary
>>>func foo(c: C) {}
>>> }
>>> 
>>> class D: C {}
>>> 
>>> extension C: Q {
>>>// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform 
>>> to protocol ‘Q'
>>>func bar() -> C { return self } 
>>> }
>>> 
>>> 
>>> It doesn’t make sense to allow a co-varying Self for parameters so I can 
>>> understand how the current state might have arisen.  At the same time, 
>>> using Self to mean two different things is inconsistent, confusing and it 
>>> doesn’t allow us to specify a non-covarying Self as a return type in 
>>> protocol requirements.  
>>> 
>>> As I have pointed out before, the ability to specify a non-covarying Self 
>>> as a return type would make it possible to design a protocol that can be 
>>> retroactively conformed to by non-final classes (such as those in Apple’s 
>>> frameworks).
>>> 
>>> I think it would be a very good idea to introduce a non-covarying Self 
>>> which would specify the type that adds conformance to the protocol and 
>>> require this Self to be used in places where covariance is not possible, 
>>> such as parameter types.  It would also be allowed elsewhere, such as 
>>> return types, making it easier to conform non-final classes when covariance 
>>> is not required by the protocol.
>>> 
>>> One possible name is `ConformingSelf`.  One thing I like about 

[swift-evolution] Self behaves inconsistently in protocol method signatures

[Matthew Johnson via swift-evolution]

I have brought up the idea of a non-covarying Self a few times.  

I was surprised to realize that Self is actually non-covarying when used for 
parameters in protocol declarations!

Here is an example demonstrating this:

protocol P {
func foo(s: Self)
}
protocol Q {
func bar() -> Self
}

class C: P {
// this works!  Self as an argument type in the protocol declaration does 
not covary
func foo(c: C) {}
}

class D: C {}

extension C: Q {
// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform to 
protocol ‘Q'
func bar() -> C { return self } 
}


It doesn’t make sense to allow a co-varying Self for parameters so I can 
understand how the current state might have arisen.  At the same time, using 
Self to mean two different things is inconsistent, confusing and it doesn’t 
allow us to specify a non-covarying Self as a return type in protocol 
requirements.  

As I have pointed out before, the ability to specify a non-covarying Self as a 
return type would make it possible to design a protocol that can be 
retroactively conformed to by non-final classes (such as those in Apple’s 
frameworks).

I think it would be a very good idea to introduce a non-covarying Self which 
would specify the type that adds conformance to the protocol and require this 
Self to be used in places where covariance is not possible, such as parameter 
types.  It would also be allowed elsewhere, such as return types, making it 
easier to conform non-final classes when covariance is not required by the 
protocol.

One possible name is `ConformingSelf`.  One thing I like about this name is 
that it makes it very clear that it is the type that introduces protocol 
conformance.

I’m interested in hearing thoughts on this.

Matthew
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Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Matthew Johnson via swift-evolution]

> On Dec 28, 2015, at 11:19 AM, Developer  wrote:
> 
> My understanding of Self is that it is a special generic parameter resolved 
> by the type system to the type of the implementing structure.  That 
> resolution must be invariant because the implementing structure (here, 
> non-final classes) can choose to yank the protocol's invariants out from 
> under you when it is subclassed.  Sure, retroactively, you can make things 
> conform, but you also can't completely guarantee type safety with any kind of 
> variance in Self in all cases. 
> 
> On the other hand, using the protocol itself in either position says that you 
> only wish to restrict yourself to the protocol itself, not some specific 
> implementation.  You are necessarily specifying an upper bound (here C) on 
> the amount of "information" you can get out of the type, so it is possible to 
> introduce variance because you will never violate the protocol's invariants 
> by returning a subtype with a legal conformance.
> 
> Self doesn't mean two different things, your protocol declarations do!

My mind must be a little bit foggy this morning.  This works:

extension C: Q {
func bar() -> Self { return self }
}

What doesn’t work, regardless of whether C is final or not, is this:

extension C: Q {
// Cannot convert return expression of type ‘C’ to return type ‘Self'
func bar() -> Self { return C() }
}

In order for classes to meet a protocol requirement with Self in the return 
position you must specify Self (rather than the conforming type) as the return 
type for the method.  Self in the return position of a method is treated as 
covariant.

In order for classes to meet a protocol requirement with Self in parameter 
position you must specify the type of the conforming class (you cannot specify 
Self in an argument position).  Obviously the type of the conforming class is 
invariant.

This is the sense in which Self in protocol declarations is inconsistent.  The 
requirements on conforming types are different - invariance for Self parameters 
and covariance for Self return types.

IMO it would be much more clear if this distinction was explicit rather than 
implicit based on the location of Self.  It would also be extremely useful in 
some cases to be able to specify an invariant `ConformingSelf` return type.


> 
> ~Robert Widmann
> 
> 2015/12/28 11:49、Matthew Johnson via swift-evolution 
> > のメッセージ:
> 
>> I have brought up the idea of a non-covarying Self a few times.  
>> 
>> I was surprised to realize that Self is actually non-covarying when used for 
>> parameters in protocol declarations!
>> 
>> Here is an example demonstrating this:
>> 
>> protocol P {
>>func foo(s: Self)
>> }
>> protocol Q {
>>func bar() -> Self
>> }
>> 
>> class C: P {
>>// this works!  Self as an argument type in the protocol declaration does 
>> not covary
>>func foo(c: C) {}
>> }
>> 
>> class D: C {}
>> 
>> extension C: Q {
>>// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform to 
>> protocol ‘Q'
>>func bar() -> C { return self } 
>> }
>> 
>> 
>> It doesn’t make sense to allow a co-varying Self for parameters so I can 
>> understand how the current state might have arisen.  At the same time, using 
>> Self to mean two different things is inconsistent, confusing and it doesn’t 
>> allow us to specify a non-covarying Self as a return type in protocol 
>> requirements.  
>> 
>> As I have pointed out before, the ability to specify a non-covarying Self as 
>> a return type would make it possible to design a protocol that can be 
>> retroactively conformed to by non-final classes (such as those in Apple’s 
>> frameworks).
>> 
>> I think it would be a very good idea to introduce a non-covarying Self which 
>> would specify the type that adds conformance to the protocol and require 
>> this Self to be used in places where covariance is not possible, such as 
>> parameter types.  It would also be allowed elsewhere, such as return types, 
>> making it easier to conform non-final classes when covariance is not 
>> required by the protocol.
>> 
>> One possible name is `ConformingSelf`.  One thing I like about this name is 
>> that it makes it very clear that it is the type that introduces protocol 
>> conformance.
>> 
>> I’m interested in hearing thoughts on this.
>> 
>> Matthew
>> ___
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>> https://lists.swift.org/mailman/listinfo/swift-evolution 
>> 

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Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Joe Groff via swift-evolution]

> On Dec 28, 2015, at 8:49 AM, Matthew Johnson via swift-evolution 
>  wrote:
> 
> I have brought up the idea of a non-covarying Self a few times.  
> 
> I was surprised to realize that Self is actually non-covarying when used for 
> parameters in protocol declarations!
> 
> Here is an example demonstrating this:
> 
> protocol P {
>func foo(s: Self)
> }
> protocol Q {
>func bar() -> Self
> }
> 
> class C: P {
>// this works!  Self as an argument type in the protocol declaration does 
> not covary
>func foo(c: C) {}
> }
> 
> class D: C {}
> 
> extension C: Q {
>// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform to 
> protocol ‘Q'
>func bar() -> C { return self } 
> }
> 
> 
> It doesn’t make sense to allow a co-varying Self for parameters so I can 
> understand how the current state might have arisen.  At the same time, using 
> Self to mean two different things is inconsistent, confusing and it doesn’t 
> allow us to specify a non-covarying Self as a return type in protocol 
> requirements.  
> 
> As I have pointed out before, the ability to specify a non-covarying Self as 
> a return type would make it possible to design a protocol that can be 
> retroactively conformed to by non-final classes (such as those in Apple’s 
> frameworks).
> 
> I think it would be a very good idea to introduce a non-covarying Self which 
> would specify the type that adds conformance to the protocol and require this 
> Self to be used in places where covariance is not possible, such as parameter 
> types.  It would also be allowed elsewhere, such as return types, making it 
> easier to conform non-final classes when covariance is not required by the 
> protocol.
> 
> One possible name is `ConformingSelf`.  One thing I like about this name is 
> that it makes it very clear that it is the type that introduces protocol 
> conformance.
> 
> I’m interested in hearing thoughts on this.

I proposed this a while back — see "controlling protocol conformance 
inheritance" from a while back. The current rules preserve inheritability of 
the protocol conformance in all cases. `Self` in argument positions maps to the 
root class of the conformance, since a method taking `Base` can also take any 
`Derived` class and thereby satisfy the conformance for `Derived`. In return 
positions, the derived type must be produced. I think there's value in 
controlling this behavior, but the control belongs on the conformer's side, not 
the protocol's. For some class hierarchies, the subclasses are intended to be 
API themselves in order to extend behavior. Every UIView subclass is 
interesting independently, for example, and ought to satisfy `NSCoding` and 
other requirements independently. In other class hierarchies, the base class is 
intended to be the common API, and subclasses are just implementation details. 
NSString, NSURL, etc. exemplify this—for most purposes the common `NSCoding` 
implementation is sufficient for all NSStrings. Likewise, you probably want 
NSURL to conform to `StringLiteralConvertible` but don't particularly care what 
subclass you get out of the deal. I had proposed the idea of modifying a 
class's conformance declaration to allow it control whether the conformance is 
inherited:

extension NSString: static NSCoding { } // NSCoding conformance statically 
applies to only NSString, Self == NSString
extension UIView: required NSCoding { } // NSCoding conformance is required of 
all subclasses, Self <= UIView

-Joe
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Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Developer via swift-evolution]

My understanding of Self is that it is a special generic parameter resolved by 
the type system to the type of the implementing structure.  That resolution 
must be invariant because the implementing structure (here, non-final classes) 
can choose to yank the protocol's invariants out from under you when it is 
subclassed.  Sure, retroactively, you can make things conform, but you also 
can't completely guarantee type safety with any kind of variance in Self in all 
cases. 

On the other hand, using the protocol itself in either position says that you 
only wish to restrict yourself to the protocol itself, not some specific 
implementation.  You are necessarily specifying an upper bound (here C) on the 
amount of "information" you can get out of the type, so it is possible to 
introduce variance because you will never violate the protocol's invariants by 
returning a subtype with a legal conformance.

Self doesn't mean two different things, your protocol declarations do!

~Robert Widmann

2015/12/28 11:49、Matthew Johnson via swift-evolution 
 のメッセージ:

> I have brought up the idea of a non-covarying Self a few times.  
> 
> I was surprised to realize that Self is actually non-covarying when used for 
> parameters in protocol declarations!
> 
> Here is an example demonstrating this:
> 
> protocol P {
>func foo(s: Self)
> }
> protocol Q {
>func bar() -> Self
> }
> 
> class C: P {
>// this works!  Self as an argument type in the protocol declaration does 
> not covary
>func foo(c: C) {}
> }
> 
> class D: C {}
> 
> extension C: Q {
>// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform to 
> protocol ‘Q'
>func bar() -> C { return self } 
> }
> 
> 
> It doesn’t make sense to allow a co-varying Self for parameters so I can 
> understand how the current state might have arisen.  At the same time, using 
> Self to mean two different things is inconsistent, confusing and it doesn’t 
> allow us to specify a non-covarying Self as a return type in protocol 
> requirements.  
> 
> As I have pointed out before, the ability to specify a non-covarying Self as 
> a return type would make it possible to design a protocol that can be 
> retroactively conformed to by non-final classes (such as those in Apple’s 
> frameworks).
> 
> I think it would be a very good idea to introduce a non-covarying Self which 
> would specify the type that adds conformance to the protocol and require this 
> Self to be used in places where covariance is not possible, such as parameter 
> types.  It would also be allowed elsewhere, such as return types, making it 
> easier to conform non-final classes when covariance is not required by the 
> protocol.
> 
> One possible name is `ConformingSelf`.  One thing I like about this name is 
> that it makes it very clear that it is the type that introduces protocol 
> conformance.
> 
> I’m interested in hearing thoughts on this.
> 
> Matthew
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Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Matthew Johnson via swift-evolution]

> On Dec 28, 2015, at 12:04 PM, Developer  wrote:
> 
> That doesn't look like a variance issue to me, that's about the same 
> "information" invariant I talked about before.  The former works because self 
> resolves to an invariant type, the type of the implementing structure, which 
> satisfies the requirement Self introduces.  The latter does not because Self 
> indicates a level of specificity C cannot guarantee.  Self is magic, but it 
> is also implemented as a generic parameter.  So think of it this way:
> 
> protocol Q {
>   func bar() -> T { return Q() }
> }
> 
> You wouldn't expect that to compile, would you?

It actually does work for structs and for final classes because Self becomes 
invariant when used in a return type position for them.

protocol Q {
func bar() -> Self
}

final class C: Q {
func bar() -> C { return C() }
}


> 
> ~Robert Widmann
> 
> 2015/12/28 12:49、Matthew Johnson  > のメッセージ:
> 
>> 
>>> On Dec 28, 2015, at 11:19 AM, Developer >> > wrote:
>>> 
>>> My understanding of Self is that it is a special generic parameter resolved 
>>> by the type system to the type of the implementing structure.  That 
>>> resolution must be invariant because the implementing structure (here, 
>>> non-final classes) can choose to yank the protocol's invariants out from 
>>> under you when it is subclassed.  Sure, retroactively, you can make things 
>>> conform, but you also can't completely guarantee type safety with any kind 
>>> of variance in Self in all cases. 
>>> 
>>> On the other hand, using the protocol itself in either position says that 
>>> you only wish to restrict yourself to the protocol itself, not some 
>>> specific implementation.  You are necessarily specifying an upper bound 
>>> (here C) on the amount of "information" you can get out of the type, so it 
>>> is possible to introduce variance because you will never violate the 
>>> protocol's invariants by returning a subtype with a legal conformance.
>>> 
>>> Self doesn't mean two different things, your protocol declarations do!
>> 
>> My mind must be a little bit foggy this morning.  This works:
>> 
>> extension C: Q {
>> func bar() -> Self { return self }
>> }
>> 
>> What doesn’t work, regardless of whether C is final or not, is this:
>> 
>> extension C: Q {
>> // Cannot convert return expression of type ‘C’ to return type ‘Self'
>> func bar() -> Self { return C() }
>> }
>> 
>> In order for classes to meet a protocol requirement with Self in the return 
>> position you must specify Self (rather than the conforming type) as the 
>> return type for the method.  Self in the return position of a method is 
>> treated as covariant.
>> 
>> In order for classes to meet a protocol requirement with Self in parameter 
>> position you must specify the type of the conforming class (you cannot 
>> specify Self in an argument position).  Obviously the type of the conforming 
>> class is invariant.
>> 
>> This is the sense in which Self in protocol declarations is inconsistent.  
>> The requirements on conforming types are different - invariance for Self 
>> parameters and covariance for Self return types.
>> 
>> IMO it would be much more clear if this distinction was explicit rather than 
>> implicit based on the location of Self.  It would also be extremely useful 
>> in some cases to be able to specify an invariant `ConformingSelf` return 
>> type.
>> 
>> 
>>> 
>>> ~Robert Widmann
>>> 
>>> 2015/12/28 11:49、Matthew Johnson via swift-evolution 
>>> > のメッセージ:
>>> 
 I have brought up the idea of a non-covarying Self a few times.  
 
 I was surprised to realize that Self is actually non-covarying when used 
 for parameters in protocol declarations!
 
 Here is an example demonstrating this:
 
 protocol P {
func foo(s: Self)
 }
 protocol Q {
func bar() -> Self
 }
 
 class C: P {
// this works!  Self as an argument type in the protocol declaration 
 does not covary
func foo(c: C) {}
 }
 
 class D: C {}
 
 extension C: Q {
// method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform 
 to protocol ‘Q'
func bar() -> C { return self } 
 }
 
 
 It doesn’t make sense to allow a co-varying Self for parameters so I can 
 understand how the current state might have arisen.  At the same time, 
 using Self to mean two different things is inconsistent, confusing and it 
 doesn’t allow us to specify a non-covarying Self as a return type in 
 protocol requirements.  
 
 As I have pointed out before, the ability to specify a non-covarying Self 
 as a return type would make it possible to design a protocol that can be 
 retroactively conformed to by non-final 

Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Matthew Johnson via swift-evolution]

> On Dec 28, 2015, at 12:43 PM, Joe Groff  wrote:
> 
>> 
>> On Dec 28, 2015, at 10:31 AM, Matthew Johnson > > wrote:
>> 
>>> 
>>> On Dec 28, 2015, at 12:02 PM, Joe Groff >> > wrote:
>>> 
 
 On Dec 28, 2015, at 8:49 AM, Matthew Johnson via swift-evolution 
 > wrote:
 
 I have brought up the idea of a non-covarying Self a few times.  
 
 I was surprised to realize that Self is actually non-covarying when used 
 for parameters in protocol declarations!
 
 Here is an example demonstrating this:
 
 protocol P {
   func foo(s: Self)
 }
 protocol Q {
   func bar() -> Self
 }
 
 class C: P {
   // this works!  Self as an argument type in the protocol declaration 
 does not covary
   func foo(c: C) {}
 }
 
 class D: C {}
 
 extension C: Q {
   // method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform 
 to protocol ‘Q'
   func bar() -> C { return self } 
 }
 
 
 It doesn’t make sense to allow a co-varying Self for parameters so I can 
 understand how the current state might have arisen.  At the same time, 
 using Self to mean two different things is inconsistent, confusing and it 
 doesn’t allow us to specify a non-covarying Self as a return type in 
 protocol requirements.  
 
 As I have pointed out before, the ability to specify a non-covarying Self 
 as a return type would make it possible to design a protocol that can be 
 retroactively conformed to by non-final classes (such as those in Apple’s 
 frameworks).
 
 I think it would be a very good idea to introduce a non-covarying Self 
 which would specify the type that adds conformance to the protocol and 
 require this Self to be used in places where covariance is not possible, 
 such as parameter types.  It would also be allowed elsewhere, such as 
 return types, making it easier to conform non-final classes when 
 covariance is not required by the protocol.
 
 One possible name is `ConformingSelf`.  One thing I like about this name 
 is that it makes it very clear that it is the type that introduces 
 protocol conformance.
 
 I’m interested in hearing thoughts on this.
>>> 
>>> I proposed this a while back — see "controlling protocol conformance 
>>> inheritance" from a while back. The current rules preserve inheritability 
>>> of the protocol conformance in all cases. `Self` in argument positions maps 
>>> to the root class of the conformance, since a method taking `Base` can also 
>>> take any `Derived` class and thereby satisfy the conformance for `Derived`. 
>>> In return positions, the derived type must be produced. I think there's 
>>> value in controlling this behavior, but the control belongs on the 
>>> conformer's side, not the protocol's. For some class hierarchies, the 
>>> subclasses are intended to be API themselves in order to extend behavior. 
>>> Every UIView subclass is interesting independently, for example, and ought 
>>> to satisfy `NSCoding` and other requirements independently. In other class 
>>> hierarchies, the base class is intended to be the common API, and 
>>> subclasses are just implementation details. NSString, NSURL, etc. exemplify 
>>> this—for most purposes the common `NSCoding` implementation is sufficient 
>>> for all NSStrings. Likewise, you probably want NSURL to conform to 
>>> `StringLiteralConvertible` but don't particularly care what subclass you 
>>> get out of the deal. I had proposed the idea of modifying a class's 
>>> conformance declaration to allow it control whether the conformance is 
>>> inherited:
>>> 
>>> extension NSString: static NSCoding { } // NSCoding conformance statically 
>>> applies to only NSString, Self == NSString
>>> extension UIView: required NSCoding { } // NSCoding conformance is required 
>>> of all subclasses, Self <= UIView
>>> 
>> 
>> If I understand correctly, what you’re saying is that you don’t think there 
>> is a reason why a protocol would want to specifically require invariance.  I 
>> would have to think about this some more but you may well be right.  The 
>> conforming-side solution would definitely work in every use case I know of.
>> 
>> I figured out how Self protocol requirements are actually treated 
>> consistently from a particular perspective.  Self is effectively treated as 
>> a covariant requirement, but becomes an invariant requirement when used in 
>> positions where covariance is not possible (parameters and return type for 
>> structs and final classes).  This makes sense and is consistent, if not 
>> totally obvious.  Is this reasonably accurate?
> 
> Yeah. It might help to think of `Self` within the protocol as referring to 
> the full range of 

Re: [swift-evolution] Self behaves inconsistently in protocol method signatures

[Joe Groff via swift-evolution]

> On Dec 28, 2015, at 10:31 AM, Matthew Johnson  wrote:
> 
>> 
>> On Dec 28, 2015, at 12:02 PM, Joe Groff > > wrote:
>> 
>>> 
>>> On Dec 28, 2015, at 8:49 AM, Matthew Johnson via swift-evolution 
>>> > wrote:
>>> 
>>> I have brought up the idea of a non-covarying Self a few times.  
>>> 
>>> I was surprised to realize that Self is actually non-covarying when used 
>>> for parameters in protocol declarations!
>>> 
>>> Here is an example demonstrating this:
>>> 
>>> protocol P {
>>>   func foo(s: Self)
>>> }
>>> protocol Q {
>>>   func bar() -> Self
>>> }
>>> 
>>> class C: P {
>>>   // this works!  Self as an argument type in the protocol declaration does 
>>> not covary
>>>   func foo(c: C) {}
>>> }
>>> 
>>> class D: C {}
>>> 
>>> extension C: Q {
>>>   // method ‘bar()’ in non-final class ‘C’ must return ‘Self’ to conform to 
>>> protocol ‘Q'
>>>   func bar() -> C { return self } 
>>> }
>>> 
>>> 
>>> It doesn’t make sense to allow a co-varying Self for parameters so I can 
>>> understand how the current state might have arisen.  At the same time, 
>>> using Self to mean two different things is inconsistent, confusing and it 
>>> doesn’t allow us to specify a non-covarying Self as a return type in 
>>> protocol requirements.  
>>> 
>>> As I have pointed out before, the ability to specify a non-covarying Self 
>>> as a return type would make it possible to design a protocol that can be 
>>> retroactively conformed to by non-final classes (such as those in Apple’s 
>>> frameworks).
>>> 
>>> I think it would be a very good idea to introduce a non-covarying Self 
>>> which would specify the type that adds conformance to the protocol and 
>>> require this Self to be used in places where covariance is not possible, 
>>> such as parameter types.  It would also be allowed elsewhere, such as 
>>> return types, making it easier to conform non-final classes when covariance 
>>> is not required by the protocol.
>>> 
>>> One possible name is `ConformingSelf`.  One thing I like about this name is 
>>> that it makes it very clear that it is the type that introduces protocol 
>>> conformance.
>>> 
>>> I’m interested in hearing thoughts on this.
>> 
>> I proposed this a while back — see "controlling protocol conformance 
>> inheritance" from a while back. The current rules preserve inheritability of 
>> the protocol conformance in all cases. `Self` in argument positions maps to 
>> the root class of the conformance, since a method taking `Base` can also 
>> take any `Derived` class and thereby satisfy the conformance for `Derived`. 
>> In return positions, the derived type must be produced. I think there's 
>> value in controlling this behavior, but the control belongs on the 
>> conformer's side, not the protocol's. For some class hierarchies, the 
>> subclasses are intended to be API themselves in order to extend behavior. 
>> Every UIView subclass is interesting independently, for example, and ought 
>> to satisfy `NSCoding` and other requirements independently. In other class 
>> hierarchies, the base class is intended to be the common API, and subclasses 
>> are just implementation details. NSString, NSURL, etc. exemplify this—for 
>> most purposes the common `NSCoding` implementation is sufficient for all 
>> NSStrings. Likewise, you probably want NSURL to conform to 
>> `StringLiteralConvertible` but don't particularly care what subclass you get 
>> out of the deal. I had proposed the idea of modifying a class's conformance 
>> declaration to allow it control whether the conformance is inherited:
>> 
>> extension NSString: static NSCoding { } // NSCoding conformance statically 
>> applies to only NSString, Self == NSString
>> extension UIView: required NSCoding { } // NSCoding conformance is required 
>> of all subclasses, Self <= UIView
>> 
> 
> If I understand correctly, what you’re saying is that you don’t think there 
> is a reason why a protocol would want to specifically require invariance.  I 
> would have to think about this some more but you may well be right.  The 
> conforming-side solution would definitely work in every use case I know of.
> 
> I figured out how Self protocol requirements are actually treated 
> consistently from a particular perspective.  Self is effectively treated as a 
> covariant requirement, but becomes an invariant requirement when used in 
> positions where covariance is not possible (parameters and return type for 
> structs and final classes).  This makes sense and is consistent, if not 
> totally obvious.  Is this reasonably accurate?

Yeah. It might help to think of `Self` within the protocol as referring to the 
full range of types [BaseClass, Self] that are required to conform to the 
protocol. Covariance and contravariance push the interval in opposite 
directions; in argument position, you need to cover the range by specifying its 
upper bound