> On Dec 28, 2015, at 12:02 PM, Joe Groff <[email protected]> wrote:
>
>>
>> On Dec 28, 2015, at 8:49 AM, Matthew Johnson via swift-evolution
>> <[email protected]> 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?
>From that perspective, it seems Self should also behave this way when used as
>a return type in method signatures:
protocol P {
func bar() -> Self
}
final class C: P {
// this doesn’t work, but maybe it should because C is final and can’t
covary:
func bar() -> Self { return C() }
// this works:
// func bar() -> C { return C() }
}
struct S: P {
// this doesn’t work, but maybe it should because S is a struct and can’t
covary:
func bar() -> Self { return S() }
// this works:
// func bar() -> S { return S() }
}
This thread was prompted by work on a proposal for protocol forwarding which
requires careful consideration of Self requirements.
One of the things I have considered is whether it would be possible and
desirable to wrap and forward the entire interface of a type rather than just
specific protocols. As it turns out, this is not possible in a robust manner
without using Self in the signature where the type should be promoted to the
forwarding type (i.e. just because a method on Double takes a Double parameter
you don’t necessarily want to promote the type of that parameter to Kilograms
when wrapping Double).
Matthew
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