This makes it more convenient to create them, sure, but how would you pass them
around or extract the value in a type-safe manner?
e.g. now I can write:
enum Thing<T, U> {
case thingOne(T)
case thingTwo(U)
}
// How do I require thingOne<String> or thingTwo<Int>?
func handle(thing: Thing<String, Int>) {
switch thing {
case .thingOne(let s): print("string \(s)")
case .thingTwo(let i): print("int \(i)")
}
}
With your proposed syntax:
enum Thing {
case thingOne<T>(T)
case thingTwo<T>(T)
}
func handle(thing: Thing) {
switch thing {
case thingOne(let s):
// What is the type of s?
case thingTwo<Int>(let i):
// is it even possible to write an exhaustive switch?
}
}
> On Apr 24, 2017, at 6:57 AM, Joshua Alvarado via swift-evolution
> <[email protected]> wrote:
>
> Here is my pitch on adding generics to enum cases and not to the enum type
> itself. Let me know if you have an improvements or modifications lets open it
> to discussion thank you swiftys! :)
>
> Enum with generic cases
>
> Proposal: SE-NNNN
> <https://github.com/lostatseajoshua/swift-evolution/blob/master/NNNN-enum-generic-cases.md>
> Authors: Joshua Alvarado <https://github.com/alvaradojoshua0>
> Review Manager: TBD
> Status: PITCH
> During the review process, add the following fields as needed:
>
> Decision Notes: Rationale
> <https://lists.swift.org/pipermail/swift-evolution/>, Additional Commentary
> <https://lists.swift.org/pipermail/swift-evolution/>
> Bugs: SR-NNNN <https://bugs.swift.org/browse/SR-NNNN>, SR-MMMM
> <https://bugs.swift.org/browse/SR-MMMM>
> Previous Revision: 1
> <https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md>
> Previous Proposal: SE-XXXX
> <https://github.com/lostatseajoshua/swift-evolution/blob/master/XXXX-filename.md>
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#introduction>Introduction
>
> This proposal adds a change to the enumeration type that allows an enum case
> to cast a generic on its associated value.
>
> Swift-evolution thread: Discussion thread topic for that proposal
> <https://lists.swift.org/pipermail/swift-evolution/>
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#motivation>Motivation
>
> Enums currently support generics, but they are added onto the type itself.
> This can cause adverse syntax when implementing generics for associated
> values to be stored along each case. The enum case holds the associated value
> (not the enum type itself) so should create its own value constraints.
>
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#proposed-solution>Proposed
> solution
>
> The generic is to be casted on the case of the enum and not on the enum
> itself.
>
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#detailed-design>Detailed
> design
>
> Current implementation:
>
> // enum with two generic types
> enum Foo<T: Hashable, U: Collection> {
> case bar(obj: T)
> case baz(obj: U)
> }
>
> // U is to be casted but it is not even used
> let foo: Foo<String, [String]> = .bar(obj: "hash")
>
> // Creating an optional enum, the generics have to be casted without a value
> set
> // The casting is really not needed as the values should be casted not the
> enum
> var foo1: Foo<String, [String]>?
>
> // Collections don’t look great either
> var foos = [Foo<String, [String]>]()
> foos.append(.bar(obj:"hash"))
> Proposed solution
>
> enum Foo {
> case bar<T: Hashable>(obj: T)
> case baz<U: Collection>(obj: U)
> }
>
> // generic type inferred on T
> var foo: Foo = .bar(obj: "hash")
>
> // doesn’t need to cast the generic on the optional enum
> // the associated value will hold the cast
> var foo1: Foo?
>
> // This also gives better syntax with collections of enums with associated
> types
> var foos = [Foo]()
> foos.append(.bar(obj: "hey")
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#source-compatibility>Source
> compatibility
>
> This may cause subtle breaking changes for areas in code with generic enum
> cases. The compiler could help with the change by finding the associated
> generic and updating the case with the new syntax.
>
>
> <https://github.com/lostatseajoshua/swift-evolution/tree/master#alternatives-considered>Alternatives
> considered
>
> An alternative would be to extend the associatedtype keyword to the enum type.
>
> enum Foo {
> associatedtype T = Hashable
> case bar(obj: T)
> }
>
> Copy of proposal can be found here Swift proposal on github
> <https://github.com/lostatseajoshua/swift-evolution/blob/master/NNNN-enum-generic-cases.md>
>
> --
> Joshua Alvarado
> [email protected]
> <mailto:[email protected]>_______________________________________________
> swift-evolution mailing list
> [email protected]
> https://lists.swift.org/mailman/listinfo/swift-evolution
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