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

[Haravikk]

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 <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 <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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