Hello,
I’ve come again with another proposal directly from the Generics Manifesto.
Please let me know if it needs any modifications before sending the pull
request.
Typealiases in protocols and protocol extensions
Proposal: SE-XXXX
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md>
Authors: David Hart <https://github.com/hartbit>, Doug Gregor
<https://github.com/DougGregor>
Status: TBD
Review manager: TBD
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#introduction>Introduction
This proposal is from the Generics Manifesto
<https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md> and
brings the typealias keyword back into protocols for type aliasing.
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#motivation>Motivation
In Swift versions prior to 2.2, the typelias keyword was used outside of
protocols to declare type aliases and in protocols to declare associated types.
Since SE-0011
<https://github.com/apple/swift-evolution/blob/master/proposals/0011-replace-typealias-associated.md>
and Swift 2.2, associated type now use the associatedtype keyword and typelias
is available for implementing true associated type aliases.
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#proposed-solution>Proposed
solution
The solution allows the creation of associated type aliases. Here is an example
from the standard library:
protocol Sequence {
associatedtype Iterator : IteratorProtocol
typealias Element = Iterator.Element
}
The example above shows how this simplifies referencing indirect associated
types:
func sum<T: Sequence where T.Element == Int>(sequence: T) -> Int {
return sequence.reduce(0, combine: +)
}
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#detailed-design>Detailed
design
The following grammar rules needs to be added:
protocol-member-declaration → protocol-typealias-declaration
protocol-typealias-declaration → typealias-declaration
<https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#impact-on-existing-code>Impact
on existing code
This will have no impact on existing code, but will probably require improving
the Fix-It that was created for migrating typealias to associatedtype in Swift
2.2._______________________________________________
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