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

[Thorsten Seitz]

While I am a big fan of union types, I think an enum based solution for your problem is not too bad. Just use the enums only for the intersection results, i.e.

enum Intersection1d {
    case point(Point)
    case line(Line)
}

Same for Intersection2d, but adding case plane(Plane) and so on for higher dimensions.

This way the purpose of these types is clear which was not the case for a more general  GeometryValue.

-Thorsten 



> Am 15.08.2016 um 16:29 schrieb Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org>:
> 
> I considered the enum approach but it is very tedious to use "space.add(.point(point))" which also doesn't add any clarity.
> The other approach where we add the properties of a Point as associated values also has a major drawback:
> 
> // you cannot create a point without "erasing" it's type
> let point = GeometryValue.point(x: 4, y: 5)
> 
> // if you already have a point instance
> "space.add(.point(x: point.x, y: point.y))"
> 
> If the union type feature is implemented the intersection of a Point and a Line returns (Point | Line) which is a subtype of a general GeometryValue eg. (Point | Line | Plane) and additionally cannot be checked for a Plane since it doesn't make sense.
> 
> The problem with a protocol is that there are only a few methods in common and again in a method like intersect there is no exhaustiveness check in switches.
> 
> Best regards
> Maximilian
> 
>> Am 12.08.2016 um 17:16 schrieb Sean Heber <sean at fifthace.com>:
>> 
>> As an aside, you could use an enum instead of a protocol to avoid the problem of not having exhaustive switches:
>> 
>> enum GeometryValue {
>> case point(Point)
>> case line(Line)
>> }
>> 
>> And perhaps (depending on circumstances) you might not even need a Point and Line struct, so you could just put those values inside the enum:
>> 
>> enum GeometryValue {
>> case point(x: Float, y: Float)
>> case line(x: Float, y: Float, angle: Float)
>> }
>> 
>> However personally, I think using a protocol is the more Swifty approach to this problem. If I end up not having any common properties or functions that apply to that one umbrella protocol, then I consider that a signal that I’m modeling my solution incorrectly.
>> 
>> l8r
>> Sean
>> 
>> 
>>> On Aug 12, 2016, at 6:24 AM, Maximilian Hünenberger via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 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
>>> Maximilian
>>> 
>>>> 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
>>>> 
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution