无法为特定类型扩展通用结构 [英] Can't extend generic struct for specific type

问题描述

想在 Swift3 中添加一些糖.基本上，我希望能够执行以下操作:

Wanted to toy with adding some sugar in Swift3. Basically, I wanted to be able to do something like:

let randomAdjust = (-10...10).random

为此，我决定需要扩展ClosedRange.但后来发现它对我的情况可能会更好，我现在真的只是打算做 Int，使用 CountableClosedRange.我最近的多次尝试看起来像:

To do that, I decided I would need to extend ClosedRange. But then found it would probably be even better for my case, I really just plan on doing Int's for now, to use CountableClosedRange. My latest of multiple attempts looked like:

extension CountableClosedRange where Bound == Int {
    var random:Int {
        return Int(arc4random_uniform(UInt32(self.count) + 1)) + self.lowerBound
    }
}

但是操场抱怨:

error: same-type requirement makes generic parameter 'Bound' non-generic
extension CountableClosedRange where Bound == Int {

我什至不知道它在那里告诉我什么.

I don't even know what it's telling me there.

推荐答案

通常遇到这种障碍的方式是尝试扩展 Array.这是合法的:

The way this roadblock is commonly encountered is when attempting to extend Array. This is legal:

extension Array where Element : Comparable {
}

但这是非法的:

extension Array where Element == Int {
}

编译器抱怨:

相同类型的要求使得泛型参数 'Element' 非泛型

Same-type requirement makes generic parameter 'Element' non-generic

问题是这里使用了==和Array的参数化类型Element，因为Array是一个泛型的struct.

The problem is the use of == here in combination with Array's parameterized type Element, because Array is a generic struct.

Array 的一种解决方法是提升 Array 继承的层次结构，以达到非通用结构的内容:

One workaround with Array is to rise up the hierarchy of Array's inheritance to reach something that is not a generic struct:

extension Sequence where Iterator.Element == Int {
}

这是合法的，因为 Sequence 和 Iterator 是通用的协议.

That's legal because Sequence and Iterator are generic protocols.

不过，另一种解决方案是从目标类型(即 Int)向上提升层次结构.如果我们能找到一个 Int 符合的协议，那么我们可以使用 : 操作符代替 ==.嗯，有一个:

Another solution, though, is to rise up the hierarchy from the target type, namely Int. If we can find a protocol to which Int conforms, then we can use the : operator instead of ==. Well, there is one:

extension CountableClosedRange where Bound : Integer {
}

这是我们两次尝试在一个范围内实现 random 的真正区别.您的尝试遇到了障碍而 我的 不是因为您使用的是 ==而我使用的是 :.我可以这样做，因为有一个 Double 符合的协议 (FloatingPoint).

That's the real difference between our two attempts to implement random on a range. The reason your attempt hits a roadblock and mine doesn't is that you are using == whereas I am using :. I can do that because there's a protocol (FloatingPoint) to which Double conforms.

但是，正如你被告知的那样，幸运的是，所有这些诡计很快就会成为过去.

But, as you've been told, with luck all this trickery will soon be a thing of the past.

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