Erica Sadun and I have written a proposal are following up the recent discussion thread "[RFC] #Self” with a proposal to introduce StaticSelf, an invariant Self.
The recent discussion can be found here: http://thread.gmane.org/gmane.comp.lang.swift.evolution/16565
The proposal can be found here: swift-evolution/NNNN-static-self.md at static-self · anandabits/swift-evolution · GitHub
We look forward to continuing the discussion. We plan to submit a PR in the near future after incorporating your final feedback.
Thanks,
Matthew
Introducing StaticSelf, an Invariant Self
Proposal: TBD
Authors: Matthew Johnson <https://github.com/anandabits>, Erica Sadun <https://github.com/erica>
Status: TBD
Review manager: TBD
Introduction
This proposal introduces a new keyword that provides consistent invariant type semantics in all contexts.
The Swift-evolution thread about this topic can be found here: [RFC] #Self <http://thread.gmane.org/gmane.comp.lang.swift.evolution/16565>
Motivation
The distinction between covariant and non-covariant type references come into play when
conforming non-final classes to protocols. Fixing a protocol requirement to a covarying type
means that a method returning Self must be overriden by all subclasses in order to return
the correct, matching type.
This proposal builds on the covariant construct Self accepted in SE–0068 <https://github.com/apple/swift-evolution/blob/master/proposals/0068-universal-self.md>
to introduce an invariant type identifier. It enables protocol declarations to consistently
refer to a type that is fixed at compile time. This ensures that subclasses can inherit
protocol implementations without having to re-implement that code at each level of
inheritance.
Under this proposal, a new identifier keyword is fixed in use at the point of protocol conformance
to the static type of that construct.
class A: MyProtocol
The invariant StaticSelf identifier will always refer to A, unlike Self, which is covarying and refers to
the type of the actual instance. Since multiple inheritance for non-protocol types is disallowed,
this establishes this invariant type identifier with no possibility for conflict.
Consider the following example, under the current system:
protocol StringCreatable {
static func createWithString(s: String) -> Self
}
extension NSURL: StringCreatable {
// cannot conform because NSURL is non-final
// error: method 'createWithString' in non-final class 'NSURL' must return `Self` to conform to protocol 'A'
}
Introducing a static, invariant version of Self permits the desired conformance:
protocol StringCreatable {
static func createWithString(s: String) -> StaticSelf
}
extension NSURL: StringCreatable {
// can now conform conform because NSURL is fixed and matches the static
// type of the conforming construct. Subclasses need not re-implement
// NOTE: the return type can be declared as StaticSelf *or* as NSURL
// they are interchangeable
static func createWithString(s: String) -> StaticSelf {
// ...
}
}
Additional Utility
The utility of StaticSelf is not limited to protocols. A secondary use enables code to refer to the lexical context’s current type without explicitly mentioning its name. This provides a useful shortcut when referencing static type members with especially long names and when re-purposing code between types.
class StructWithAVeryLongName {
static func foo() -> String {
// ...
}
func bar() {
// ...
let s = StaticSelf.foo()
//
}
}
Detailed Design
This proposal introduces StaticSelf, a new keyword that may be used in protocols to refer to the invariant static type of a conforming construct. StaticSelf may also be used in the lexical context of any type declaration. In such use, the keyword is identical to spelling out the full name of that type.
Impact on existing code
Being additive, there should be no impact on existing code.
Alternatives considered
The keyword is not fixed at this time. Alternatives that have been discussed include StaticType, InvariantSelf, SelfType, or Type. The community is welcome to bikeshed on the most clear and concise name for this keyword.