[swift-users] Type alias with a "where" clause specifying required associated type

[Rudolf Adamkovič]

I see. TBH, I don’t understand why it says “Any” in the "generalized existentials” but everything else is clear. 

Thank you Slava!

R+

> On 5 Sep 2017, at 01:52, Slava Pestov <spestov at apple.com> wrote:
> 
> Hi Rudolf,
> 
> What you are describing is not possible right now. The protocol PointType cannot be used as a type at all, because it has an associated type requirement. Also it is not clear what a ‘where’ clause attached to a type alias would mean.
> 
> There has been some discussion of ‘generalized existentials’ on the evolution list. So eventually, you might be able to say something like this:
> 
> typealias Point = Any<Point : PointType where Point.Float == Float>
> 
> Slava
> 
>> On Sep 4, 2017, at 6:12 PM, Rudolf Adamkovič via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> I have the following ProceduralDrawing type:
>> 
>> public struct ProceduralDrawing<Float, Point: PointType> where Point.Float == Float {
>>     // ... code that used Float and Point ...
>> }
>> 
>> public protocol PointType {
>>     associatedtype Float: FloatingPoint
>>     var x: Float { get }
>>     var y: Float { get }
>> }
>> 
>> I would like to avoid adding Point as a generic parameter but the following doesn’t work:
>> 
>> public struct ProceduralDrawing<Float: FloatingPoint> {
>>     // ERROR: 'where' clause cannot be attached to a non-generic declaration
>>     typealias Point = PointType where Point.Float == Float
>>     // ... code that uses Float and Point ...
>> }
>> 
>> Is there a way to do this? If not, why?
>> 
>> Thanks!
>> 
>> R+
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> 

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