[swift-evolution] [Revision] [Pitch] Rename `T.Type`

[L. Mihalkovic]

As unfocussed as original
Regards
(From mobile)

> On Jul 22, 2016, at 12:40 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> https://github.com/DevAndArtist/swift-evolution/blob/rename_t_dot_type/proposals/0126-rename-t-dot-type.md
> 
> Rename T.Type
> 
> Proposal: SE–0126
> Authors: Adrian Zubarev, Anton Zhilin
> Status: Revision
> Review manager: Chris Lattner
> Revision: 2
> Previous Revisions: 1
> Introduction
> 
> This proposal renames the current metatype T.Type notation and the global function from SE–0096 to match the changes.
> 
> Swift-evolution threads:
> 
> [Pitch] Rename T.Type
> [Review] SE–0126: Refactor Metatypes, repurpose T[dot]self and Mirror
> [Proposal] Refactor Metatypes, repurpose T[dot]self and Mirror
> [Discussion] Seal T.Type into Type<T>
> Motivation
> 
> In Swift metatypes have the following notation: T.Type
> 
> As already showed in SE–0096 and SE–0090 the Swift community strongly is in favor of (re)moving magical intstance or type properties.
> 
> SE–0096 moves instanceOfT.dynamicType to type<T>(of: T) -> T.Type.
> 
> SE–0090 aims to remove .self completely.
> 
> We propose to rename T.Type to a generic-like notation Metatype<T>. To be able to achieve this notation we have to resolve a few issues first.
> 
> Known issues of metatypes:
> 
> Assume this function that checks if an Int type conforms to a specific protocol. This check uses current model of metatypes combined in a generic context:
> 
> func intConforms<T>(to _: T.Type) -> Bool {
>    return Int.self is T.Type
> }
> 
> intConforms(to: CustomStringConvertible.self) //=> false
> 
> Int.self is CustomStringConvertible.Type      //=> true
> [1] When T is a protocol P, T.Type is the metatype of the protocol type itself, P.Protocol. Int.self is not P.self.
> 
> [2] There isn’t a way to generically expression P.Type yet.
> 
> [3] The syntax would have to be changed in the compiler to get something that behaves like .Type today.
> 
> Written by Joe Groff: [1] [2] [3]
> A possible workaround might look like the example below, but does not allow to decompose P.Type:
> 
> func intConforms<T>(to _: T.Type) -> Bool {
>   return Int.self is T
> }
> 
> intConforms(to: CustomStringConvertible.Type.self) //=> true
> We can extend this issue and find the second problem by checking against the metatype of Any:
> 
> func intConforms<T>(to _: T.Type) -> Bool {
>     return Int.self is T
> }
> 
> intConforms(to: Any.Type.self) //=> true
> 
> intConforms(to: Any.self)      //=> true
> 
> Int.self is Any.Type           //=> Always true
> When using Any the compiler does not require .Type at all and returns true for both variations.
> 
> The third issue will show itself whenever we would try to check protocol relationship with another protocol. Currently there is no way (that we know of) to solve this problem:
> 
> protocol P {}
> protocol R : P {}
> 
> func rIsSubtype<T>(of _: T.Type) -> Bool {
>     return R.self is T
> }
> 
> rIsSubtype(of: P.Type.self) //=> false
> 
> R.self is Any.Type //=> Always true
> R.self is P.Type   //=> true
> R.self is R.Type   //=> true
> We also believe that this issue is the reason why the current global functions sizeof, strideof and alignof make use of generic <T>(_: T.Type) declaration notation instead of (_: Any.Type).
> 
> Proposed solution
> 
> Rename any occurrence of T.Type and T.Protocol to Metatype<T>.
> 
> Revise metatypes internally.
> 
> When T is a protocol, T.self should always return an instance of Metatype<T> (old T.Type) and never a T.Protocol. Furthermore, metatypes should reflect the same type relationship behavior like the actual types themselves.
> 
> To match the correct meaning and usage of the noun ‘Metatype’ from this proposal, we also propose to rename the global function from SE–0096:
> 
> before: public func type<T>(of instance: T) -> T.Type
> after: public func metatype<T>(of instance: T) -> Metatype<T>
> Examples:
> 
> protocol P {}
> protocol R : P {}
> class A : P {}
> class B : A, R {}
> 
> func `is`<T>(metatype: Metatype<Any>, also _: Metatype<T> ) -> Bool {
>     return metatype is Metatype<T>
> }
> 
> `is`(metatype: R.self, also: Any.self) //=> true | Currently: false
> `is`(metatype: R.self, also: P.self)   //=> true | Currently: false
> `is`(metatype: R.self, also: R.self)   //=> true
> 
> `is`(metatype: B.self, also: Any.self) //=> true | Currently: false
> `is`(metatype: B.self, also: P.self)   //=> true | Currently: false
> `is`(metatype: B.self, also: R.self)   //=> true | Currently: false
> `is`(metatype: B.self, also: A.self)   //=> true
> `is`(metatype: B.self, also: B.self)   //=> true
> 
> func cast<T>(metatype: Metatype<Any>, to _: Metatype<T>) -> Metatype<T>? {
>     return metatype as? Metatype<T>
> }
> 
> cast(metatype: R.self, to: Any.self)     //=> an Optional<Metatype<Any>> | Currently: nil
> cast(metatype: R.self, to: P.self)       //=> an Optional<Metatype<P>>   | Currently: nil
> cast(metatype: R.self, to: R.self)       //=> an Optional<Metatype<R>>   | Currently: an Optional<R.Protocol>
> 
> let anyR: Any.Type = R.self
> let r = cast(metatype: anyR, to: R.self) //=> an Optional<Metatype<R>>   | Currently: an Optional<R.Protocol>
> 
> cast(metatype: B.self, to: Any.self)     //=> an Optional<Metatype<Any>> | Currently: nil
> cast(metatype: B.self, to: P.self)       //=> an Optional<Metatype<P>>   | Currently: nil
> cast(metatype: B.self, to: R.self)       //=> an Optional<Metatype<R>>   | Currently: nil
> cast(metatype: B.self, to: A.self)       //=> an Optional<Metatype<A>>
> cast(metatype: B.self, to: B.self)       //=> an Optional<Metatype<B>>
> 
> let pB: P.Type = B.self
> let b = cast(metatype: pB, to: B.self)   //=> an Optional<Metatype<B>>
> Impact on existing code
> 
> This is a source-breaking change that can be automated by a migrator. Any occurrence of T.Type or T.Protocol will be simply renamed to Metatype<T>.
> 
> Alternatives considered
> 
> Alternatively it’s reasonable to consider to rename T.self to T.metatype.
> It was considered to reserve Type<T> for different usage in the future.
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
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> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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