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

Maximilian H├╝nenberger m.huenenberger at me.com
Fri Aug 12 06:24:12 CDT 2016

Hi Cao,

I would be in favor until I find another approach to this problem:

Consider you have a geometry framework and two types: Point and Line

An intersection between two lines can be either none, a point or a line (if both are identical).

The return type would probably be (Point | Line)?

I've modeled it with an empty protocol "GeometryType". However this has a major disadvantage:
If you have a general "GeometryType?" you have to cast it in a switch to the specific type.
In case of (Point| Line)? the switch statement can be checked for exhaustiveness.

For future directions:

There should also be a subtype relationship:

let tu: (T | U) = T()
let tuv: (T | U | V) = tu // works

Overloaded functions/operators could also take Union types based on their overloads:

func take(_ i: Int) -> String { ... }

func take(_ s: String) -> Int? { ... }

let value: (Int | String) = "1234"
let value2 = take(value) // returns (String | Int?)

Best regards

> Am 11.08.2016 um 03:28 schrieb Cao Jiannan via swift-evolution <swift-evolution at swift.org>:
> 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
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