Swift 4.2: conforming to protocol with generic func?

Question

Consider the following:

import Cocoa

protocol Mark {}

protocol Detector: class {
  func check<M: Mark>(mark: M)
}

enum ColorTint: Mark {
  case red, green, blue
}

class Vision: Detector { // Error: type does not conform to protocol
  func check(mark: ColorTint) {
       switch mark {
            case .red:
                 print("I see reddish")
            case .green:
                 print("I see greenish")
            case .blue:
                 print("I see blueish")
       }
  }
}

I have two protocols, one of which has generic function with parameter constrained to another protocol. In the result I cannot make a type conformant to the protocol with the generic function. Is it even possible?

Swift

4.5k

Posted by

ViktorCode

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Answer 1

Still, I wonder if it was possible to do with generics.

The answer is NO.

When you declare a prtocol with generic method, without typealias like your Detector:

protocol Detector: class {
    func check<M: Mark>(mark: M)
}

the type conforming to the protocol needs to implement the generic method:

class Vision: Detector {
    func check<M: Mark>(mark: M) {
        print("I see \(mark)ish")
        //...
    }
}

If you have found that you can do it with associated type, it's not a workaround, but the right solution.

Posted by

OOPer

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Answer 2

Okay, I was able to work around that by getting rid of the generic protocol function in favour of associated type, which then can be explicitly specified in Vision class.

Still, I wonder if it was possible to do with generics.

Posted by

ViktorCode

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Answer 3

Still, I wonder if it was possible to do with generics.

The answer is NO.

When you declare a prtocol with generic method, without typealias like your Detector:

protocol Detector: class {
    func check<M: Mark>(mark: M)
}

the type conforming to the protocol needs to implement the generic method:

class Vision: Detector {
    func check<M: Mark>(mark: M) {
        print("I see \(mark)ish")
        //...
    }
}

If you have found that you can do it with associated type, it's not a workaround, but the right solution.

Posted by

OOPer

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Answer 4

To be precise I tried to make Vision class implementing generic method, but I couldn't find syntax to make compiler happy. Specifing generic parameter for class and/or method didn't work.

Is there any way to make Vision class from my example work without changing given protocols and enum?
EDIT: With the check function constrained to ColorTint type, of course, not to any Mark.

Posted by

ViktorCode

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Answer 5

Is there any way to make Vision class from my example work without changing given protocols and enum?

EDIT: With the check function constrained to ColorTint type, of course, not to any Mark.

I need to repeat NO. When a method declares generic parameters of its own, I mean not using associatedtype,

the type conforming to the method needs to implement the generic method.

You need to modify the protocol, if your `Vision` class implements only a method specific to `ColorTint`.

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OOPer

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Answer 6

Thank you for clarifying.

My original idea behind the protocols with generic functions was to have a single class that can conform to two or more specializations of the same protocol. I would just have to provide required 'check' functions specialized for concrete types. Associated types do not allow for that, as a conforming class can be specialised for one type only. So, I will have to resort to using single 'check' function with runtime type matching.

Posted by

ViktorCode

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Swift 4.2: conforming to protocol with generic func?

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