[swift-evolution] [Pre-Proposal-Discussion] Union Type - Swift 4

[Xiaodi Wu]

But you can do that already with protocols, can't you?
On Fri, Aug 19, 2016 at 2:24 AM Haravikk via swift-evolution <
swift-evolution at swift.org> wrote:

> I'm a +1 for union types.
>
> My main reason for wanting it is to eliminate (some) function overloads;
> behind the scenes the compiler may still produce one compiled function per
> union type (for performance), but at a high level we only need to worry
> about one implementation, and one call signature, which I think is a good
> thing.
>
> On 11 Aug 2016, at 02:28, Cao Jiannan via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hi all,
>
> I want to make a discussion about union type for swift 4.
> See
> https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md
>
> Add union type grammar, represents the type which is one of other types.
>
> var stringOrURL: String | URL = "https://www.apple.com"
>
> Now, if we using the new union type feature, we can declare type
> conveniently, No other type declaration, and compiler will automatically
> calculate the common interface.
>
> func input(value: A | B | C) {
>     print(value.commonProperty) // type checker will calculate the common interface, developer just use it out of box
>     switch value {
>     case let value as A:
>         // value is type A
>         print(value.propertyInA)
>     case let value as B:
>         // value is type B
>         print(value.propertyInB)
>     case let value as C:
>         // value is type C
>         print(value.propertyInC)
>     }
>     // there is no default case other than A, B or C. we already declared that.
> }
>
> Note: A, B, C can be either class or protocol, or any other types. This
> leaves developer more freedom.
>
> Impact on existing code
>
>    - This is a new feature, developer who need declare common type will
>    alter to this new grammar.
>    - Enum based version optional or IUO will be replaced by Union-based
>    ones. Any optional type will automatically replaced by union type
>
>
>
> <https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md#detailed-design>
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