[swift-evolution] [Pitch] Enum with generic cases
Kevin Nattinger
swift at nattinger.net
Mon Apr 24 15:56:22 CDT 2017
> On Apr 24, 2017, at 1:34 PM, Joshua Alvarado <alvaradojoshua0 at gmail.com> wrote:
>
> Well in your case thing one and thing two will be the same type as you are using the same T generic type on both.
It’s two different and unrelated generics unless you’re intending to change the way generics work entirely.
func foo<T>(x: T)
func bar<T>(x: T)
They both use “T” as the generic, but the T’s are unrelated.
You can even have a generic function type shadow the containing class’s generic type:
enum Thing<T, U> {
case thingOne(T)
case thingTwo(U)
func foo<T>(x: T) -> T { return x }
}
let x = Thing<String, Int>.thingOne("").foo(x: [5.0])
type(of: x) // Array<Double>.Type
(Not recommended, obviously, just demonstrating that generic types are independent regardless of the name)
>
> To achieve your case you can do an extension on the enum and use two different generics:
>
> enum Thing {
> case thingOne<T>(T)
> case thingTwo<U>(U)
> }
>
> extension Thing where T == String, U == Int {
Ah, but there is no T or U on the enum, just on the cases. And what if the function needs to be in another class for encapsulation reasons?
Furthermore, how would you even call this function? How do you create a variable that can hold either a thingOne with a string or a thingTwo with an Int, and nothing else?
> func handle(thing: Thing) {
> switch thing {
> case thingOne(let s):
> // s is type String
>
> case thingTwo(let i):
> // i is an Int
> }
> }
> }
>
> This can actually be achieved in Swift 3.1, you can run this in a playground.
>
> enum Foo<T, U> {
> case bar(obj: T)
> case baz(obj: U)
>
> func handle() {
> switch self {
> case .bar(obj: let x):
> break
> case .baz(obj: let y):
> break
> }
> }
> }
>
> extension Foo where T == String, U == Int {
> func handle() {
> switch self {
> case .bar(obj: let str):
> print(str)
> case .baz(obj: let aNum):
> print(aNum)
> }
> }
> }
>
> let foo = Foo<String, Int>.baz(obj: 1)
> foo.handle() // prints 1
>
>
>
> On Mon, Apr 24, 2017 at 2:15 PM, Kevin Nattinger <swift at nattinger.net <mailto:swift at nattinger.net>> wrote:
> 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 <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> 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
>> alvaradojoshua0 at gmail.com <mailto:alvaradojoshua0 at gmail.com>_______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>
>
>
>
> --
> Joshua Alvarado
> alvaradojoshua0 at gmail.com <mailto:alvaradojoshua0 at gmail.com>
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