[swift-users] A somewhat stupid question about protocol conformances

[Antonino Ficarra]

Adrian,
thanks, but don’t work :-(


import Foundation

protocol AreaProtocol {
	// required
	func	area() -> CGFloat
	
	// implemented in protocol extension
	func	volume( height:CGFloat ) -> CGFloat
	// ....
}

extension AreaProtocol {
	func	volume( height:CGFloat ) -> CGFloat {
		return height * area()
	}
	// ....
	// ....
}

protocol CGPointSequence: Sequence, AreaProtocol where Element == CGPoint {}

extension CGPointSequence {
	func area()	-> CGFloat	{
		return 0.0	// ... poligon area
	}
}


extension Array:CGPointSequence {} // !'CGPointSequence' requires the types 'Element' and 'CGPoint' be equivalent




> Il giorno 25 nov 2017, alle ore 09:11, Adrian Zubarev <adrian.zubarev at devandartist.com> ha scritto:
> 
> This is correct. None of your types does conform to your protocol because you never conformance explicitly. 
> 
> The extenstion you wrote is just a conditional extension which says if the Self type conforms to AreaProtocol and the associated typed Element is a CGPoint then add the area method to it. Nothing more, nothing less.
> 
> Now you may think that you have to rewrite that extension to `extension Sequence : AreaProtocol where Element == CGPoint`, but that won't work because retroactively conforming protocols to other protocols is not supported yet. Nor are conditional conformances included yet, but they will in Swift 4.1 or 5.
> 
> You have one option left.
> protocol MySequence : Sequence, AreaProtocol where Element == CGPoint {}
> 
> extension Array : MySequence {}
> 
> Then also conform other sequences you need to support that functionality.
> 
> Double check if the code compiles, I wrote it on my iPhone, but you get the idea now.
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> Am 25. November 2017 um 07:40:25, Antonino Ficarra via swift-users (swift-users at swift.org <mailto:swift-users at swift.org>) schrieb:
> 
>> In this code example:
>> 
>> import Foundation
>> 
>> protocol AreaProtocol {
>> func area() -> CGFloat
>> 
>> // implemented in protocol extension
>> func volume( height:CGFloat ) -> CGFloat
>> // ....
>> }
>> 
>> extension AreaProtocol {
>> func volume( height:CGFloat ) -> CGFloat {
>> return height * area()
>> }
>> // ....
>> // ....
>> }
>> 
>> 
>> // conform CGPoint sequences to AreaProtocol
>> extension Sequence
>> where Self : AreaProtocol, Element == CGPoint
>> {
>> func area() -> CGFloat {
>> return 0.0 // ... poligon area
>> }
>> }
>> 
>> let p0 = CGPoint(x: 0.0, y: 0.0)
>> let p1 = CGPoint(x: 2.0, y: 0.0)
>> let p2 = CGPoint(x: 2.0, y: 2.0)
>> let p3 = CGPoint(x: 0.0, y: 2.0)
>> 
>> let poligon = [p0,p1,p2,p3]
>> let a  = poligon.area()// ! Type '[CGPoint]' does not conform to protocol 'AreaProtocol'
>> let v  = poligon.volume( height:10.0 )// ! Value of type '[CGPoint]' has no member 'volume'
>> 
>> 
>> An array of CGPoint is a CGPoint sequence? Why the array don't gets automatic conformance to AreaProtocol?
>> How can conform an array of CGPoint to AreaProtocol?
>> 
>> Sorry for my bad english,
>> Antonino
>> 
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users

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