[swift-evolution] [Pitch] Enum with generic cases

Joshua Alvarado alvaradojoshua0 at gmail.com
Mon Apr 24 08:57:19 CDT 2017

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
   - 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
   - Previous Revision: 1
   - Previous Proposal: SE-XXXX


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

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.

The generic is to be casted on the case of the enum and not on the enum

Current implementation:

// enum with two generic typesenum Foo<T: Hashable, U: Collection> {
    case bar(obj: T)
    case baz(obj: U)
// U is to be casted but it is not even usedlet 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 enumvar foo1: Foo<String, [String]>?
// Collections don’t look great eithervar foos = [Foo<String, [String]>]()

Proposed solution

enum Foo {
    case bar<T: Hashable>(obj: T)
    case baz<U: Collection>(obj: U)
// generic type inferred on Tvar foo: Foo = .bar(obj: "hash")
// doesn’t need to cast the generic on the optional enum// the
associated value will hold the castvar foo1: Foo?
// This also gives better syntax with collections of enums with
associated typesvar foos = [Foo]()
foos.append(.bar(obj: "hey")


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.

An alternative would be to extend the associatedtype keyword to the enum

enum Foo {
    associatedtype T = Hashable
    case bar(obj: T)

Copy of proposal can be found here Swift proposal on github

Joshua Alvarado
alvaradojoshua0 at gmail.com
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