[swift-users] Experimenting with conditional conformances

[Antonino Ficarra]

I’have a protocol:

protocol Inertial {
	func	convexHull( _ t:AffineTransform? ) -> [CGPoint]
	func	area( _ t:AffineTransform? ) -> CGFloat
	func	firstOrderMomentums( _ t:AffineTransform? ) -> (x:CGFloat,y:CGFloat)
	func	secondOrderMomentums( _ t:AffineTransform? ) -> (xx:CGFloat,xy:CGFloat,yy:CGFloat)
}

I make [CPoint] (a poligon) conforms to the protocol:

extension Array : Inertial where Element == CGPoint {
    func convexHull(_ t: AffineTransform?) -> [CGPoint] {
        return ConvexHull.convexHull( points:self ).map { $0 * t }
    }
    
    func area(_ t: AffineTransform?) -> CGFloat {
        return InertialUti.area( polig: self,t )
    }
    
    func firstOrderMomentums(_ t: AffineTransform?) -> (x: CGFloat, y: CGFloat) {
        return InertialUti.firstOrderMomentums( polig:self,t )
    }
    
    func secondOrderMomentums(_ t: AffineTransform?) -> (xx: CGFloat, xy: CGFloat, yy: CGFloat) {
        return InertialUti.secondOrderMomentums( polig:self,t )
    }
}


and work like a charm.

Then I try to make [[CPoint]] (an array of poligons) conforms to the same protocol:

extension Array : Inertial where Element == [CGPoint] {
    var allVertexs : [CGPoint] {
        return flatMap({$0})
    }
    
    func convexHull(_ t: AffineTransform?) -> [CGPoint] {
        return ConvexHull.convexHull( points:allVertexs ).map { $0 * t }
    }
    
    func area(_ t: AffineTransform?) -> CGFloat {
        return InertialUti.area( poligs: self,t )
    }
    
    func firstOrderMomentums(_ t: AffineTransform?) -> (x: CGFloat, y: CGFloat) {
        return InertialUti.firstOrderMomentums( poligs:self,t )
    }
    
    func secondOrderMomentums(_ t: AffineTransform?) -> (xx: CGFloat, xy: CGFloat, yy: CGFloat) {
        return InertialUti.secondOrderMomentums( poligs:self,t )
    }
}

but this isn’t permitted: Redundant conformance of 'Array<Element>' to protocol ‘Inertial’ !

I think it's a severe limitation: array with different element’s type (i.e. different specializations of the same generic type) should be treated as distinct types and not as the same type.











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