swift - 将 Range 和 StrideTo 转换为 Sequence 时出现特殊错误

Question

以下函数会产生编译器错误:

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)
            }
        }
}

我不明白为什么编译器不转换 Range<Int>至 S (以及其他类型)同时 Range显然是符合Sequence它的元素是Int .

它更特别，如果我替换block使用具有相似签名的类方法，没有任何错误:

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)
        }
    }
}

iterateColumnsAlongGravity(using:)之间的唯一区别和 typeConversionTest()第一个将 block 作为参数。

Answer 1

期间 the discussion在 Swift 论坛上透露了以下信息:

1. 我尝试实现的是高阶多态性，这个重要的特性还没有添加到 Swift 中。

2. 可能最简单的解决方案是放弃泛型并使用 StrideTo<Int>这里。

   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))
           }
       }
   }