[swift-evolution] [Revision] [Pitch] Rename `T.Type`
Chris Lattner
clattner at apple.com
Thu Jul 21 18:14:03 CDT 2016
> On Jul 21, 2016, at 4:07 PM, L. Mihalkovic via swift-evolution <swift-evolution at swift.org> wrote:
>
> As unfocussed as original
Please keep comments *constructive* on this list.
-Chris
> Regards
> (From mobile)
>
> On Jul 22, 2016, at 12:40 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>> https://github.com/DevAndArtist/swift-evolution/blob/rename_t_dot_type/proposals/0126-rename-t-dot-type.md <https://github.com/DevAndArtist/swift-evolution/blob/rename_t_dot_type/proposals/0126-rename-t-dot-type.md>
>> Rename T.Type
>>
>> Proposal: SE–0126 <x-msg://594/0126-refactor-metatypes-repurpose-t-dot-self-and-mirror.md>
>> Authors: Adrian Zubarev <https://github.com/DevAndArtist>, Anton Zhilin <https://github.com/Anton3>
>> Status: Revision
>> Review manager: Chris Lattner <http://github.com/lattner>
>> Revision: 2
>> Previous Revisions: 1 <https://github.com/apple/swift-evolution/blob/83707b0879c83dcde778f8163f5768212736fdc2/proposals/0126-refactor-metatypes-repurpose-t-dot-self-and-mirror.md>
>> 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 <applewebdata://1A857E6E-5ECA-433A-9669-6DB2B5E23BD0>
>> [Review] SE–0126: Refactor Metatypes, repurpose T[dot]self and Mirror <applewebdata://1A857E6E-5ECA-433A-9669-6DB2B5E23BD0>
>> [Proposal] Refactor Metatypes, repurpose T[dot]self and Mirror <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160718/024772.html>
>> [Discussion] Seal T.Type into Type<T> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160704/023818.html>
>> 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] <https://twitter.com/jckarter/status/754420461404958721> [2] <https://twitter.com/jckarter/status/754420624261472256> [3] <https://twitter.com/jckarter/status/754425573762478080>
>> 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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