Nsryjdtyk

The following function produces compiler errors:

func iterateColumnsAlongGravity<S: Sequence>(using block: (_ indexes: S) -> ())

    where S.Element == Int {



        switch gravityDirection {

        case .bot:

            for i in stride(from: 0, to: w * h, by: h) {

                // 'Range<Int>' is not convertible to 'S'

                block(i..<(i + h))

            }

        case .top:

            for i in stride(from: 0, to: w * h, by: h) {

                // 'ReversedCollection<(Range<Int>)>' is not convertible to 'S'

                block((i..<(i + h)).reversed())

            }

        case .left:

            for y in 0..<h {

                let indexes = stride(from: y, to: w * h, by: h).reversed()

                // '([Int]) -> ()' is not convertible to '(S) -> ()'

                block(indexes)

            }

        case .right:

            for y in 0..<h {

                // '(StrideTo<Int>) -> ()' is not convertible to '(S) -> ()'

                let indexes = stride(from: y, to: w * h, by: h)

                block(indexes)

            }

        }

}

I don't understand why the compiler doesn't converts Range<Int> to S (and other types as well) while Range is obviously conforms to Sequence and it's element is Int.

It's even more peculiar, that if I replace block with a class method of similar signature, there are no any errors:

func printIntSequence<S: Sequence>(_ s: S) where S.Element == Int {

    for i in s {

        print(i)

    }

}



func typeConversionTest() {

    switch gravityDirection {

    case .bot:

        for i in stride(from: 0, to: w * h, by: h) {

            printIntSequence(i..<(i + h))

        }

    case .top:

        for i in stride(from: 0, to: w * h, by: h) {

            printIntSequence((i..<(i + h)).reversed())

        }

    case .left:

        for y in 0..<h {

            let indexes = stride(from: y, to: w * h, by: h).reversed()

            printIntSequence(indexes)

        }

    case .right:

        for y in 0..<h {

            let indexes = stride(from: y, to: w * h, by: h)

            printIntSequence(indexes)

        }

    }

}

The only difference between iterateColumnsAlongGravity(using:) and typeConversionTest() that the first one takes the block as a parameter.

During the discussion on Swift forums the following information was revealed:

1. What I trying to implement is called higher rank polymorphism and this big feature isn't added to Swift yet.


2. 
   

   Probably the easiest solution is to discard generics and use StrideTo<Int> here.

   
   
   

   func iterateColumnsAlongGravity(using block: (_ indexes: StrideTo<Int>) -> ()) {
   
       switch gravityDirection {
   
       case .bot:
   
           for i in stride(from: 0, to: w * h, by: h) {
   
               block(stride(from: i, to: i + h, by: 1))
   
           }
   
       case .top:
   
           for i in stride(from: 0, to: w * h, by: h) {
   
           block(stride(from: i + h - h, to: i - h, by: -1))
   
           }
   
       case .left:
   
           for y in 0..<h {
   
               block(stride(from: (w - 1) * h + y, to: -1, by: -h))
   
           }
   
       case .right:
   
           for y in 0..<h {
   
               block(stride(from: y, to: w * h, by: h))
   
           }
   
       }
   
   }