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

Thu Aug 18 04:05:50 CDT 2016

While purpose of the types are clear in this case there is not only intersection. I also want to find out the distance between different GeometryTypes and other properties like angels between two lines or a Line and a Plane but this doesn't make sense for a Point and some other GeometryType.

Therefore a GeometryType with subtypes is almost the perfect solution. I imagine the perfect solution would be to have something like a "newtype" feature which is similar to a protocol but the types which "conform" to it are known at compile time.

Best regards
Maximilian

> Am 18.08.2016 um 07:14 schrieb Thorsten Seitz <tseitz42 at icloud.com>:
> 
> 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
>>>>> 
>>>>> 
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