[swift-evolution] [Proposal] Union Type

[Cao Jiannan]

I can't compile your code. I'm really not sure your code will pass the complex generic check 🙃

Union version more complex or generic version?
I think union version is more normal.
> For example:
> 
> typealias ABC = A | B | C
> 
> typealias ABCD = ABC | D
> 
> we just use an existed type ABC to construct ABCD
> 
> But how about generic wrap?
> 
> Bikeshedding:
> 
> // we generate the boundary with `A | B` or directly OneOf<A, B>
> enum OneOf<...T> {
> 
>     ...case $#(T)
> 
>     // Bikeshedding variadic enum casses:
>     // we might need an index to set the value?
>     init(index: Int, value: T) {
> 
>         self = .$index(value)
>     }
> }
> 
> /// Usage:
> /// A | B | C == OneOf<A, B, C>
> func |<T, U>(_: T.Type, _: U.Type) -> OneOf<T, U>.Type {
> 
>     // I believe the usage of `type` like this was prposed by Joe Groff
>     // I also use the proposal to remove `.self` magic here
>     return OneOf<T, U>
> }
> 
> // Here is how to merge OneOf into a single dimension
> func |<...T, U>(_: OneOf<...T>.Type, _: U.Type) -> OneOf<...T, U>.Type {
> 
>     // Copy and merge types into the new `OneOf` type
>     return OneOf<...T, U>
> }
> 
> func |<T, ...U>(_: T.Type, _: OneOf<...U>.Type) -> OneOf<T, ...U>.Type {
> 
>     // Copy and merge types into the new `OneOf` type
>     return OneOf<T, ...U>
> }
> 
> func |<...T, ...U>(_: OneOf<...T>.Type, _: OneOf<...U>.Type) -> OneOf<...T, ...U>.Type {
> 
>     // Copy and merge types into the new `OneOf` type
>     return OneOf<...T, ...U>
> }
> Your example will become:
> 
> typealias ABC = A | B | C // or OneOf<A, B, C>
> typealias ABCD = ABC | D // merging lhs OneOf with D to OneOf<A, B, C, D>
> Mission accomplished.
> 
> 
>> 在 2016年7月1日，17:06，Cao Jiannan <frogcjn at 163.com> 写道：
>> 
>> 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>
>> 
>> Hi,
>> 
>> I'm now officially proposal the union type feature for Swift. Please see:
>> 
>> 
>> https://github.com/apple/swift/commit/eb7311de065df7ea332cdde8782cb44f9f4a5121 <https://github.com/apple/swift/commit/eb7311de065df7ea332cdde8782cb44f9f4a5121>
>> Introduction
>> 
>> 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/>"
>> 
>> 
>> I would be thankful if someone support this idea and give some advice. Thanks!
>> 
>> 
>> --
>> Jiannan
>> 
>> 
> 
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