在Swift 3的Collection扩展中无法使用index.contains() [英] Unable to use indices.contains() in a Collection extension in Swift 3

问题描述

我在Swift 2.3中编写了以下扩展名:

I wrote following extension in Swift 2.3:

extension CollectionType {
    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Generator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

但是，事实证明Swift 3.0不具有contains()功能.相反，它为我提供了此方法的以下语法:

However, it turns out that Swift 3.0 does not have contains() function. Instead, it offers me following syntax for this method:

indices.contains(where: { (<#Self.Indices.Iterator.Element#>) -> Bool in
    <# code ??? what should it do??? #>
})

问题是我不知道该块内应包含什么.请提供有关迁移的任何帮助?

The problem is that I don't know what should it contain inside the block. Any help with migrating it, please?

推荐答案

Swift 4更新

在Swift 4中，感谢能够在关联类型上包含where子句，Collection现在强制

Swift 4 Update

In Swift 4, thanks to the ability to have where clauses on associated types, Collection now enforces that Indices's Element type is the same type as the Collection's Index.

因此，这意味着我们只能说:

This therefore means that we can just say:

extension Collection {

    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

---

雨燕3

Swift 3中的Sequence协议仍然具有 contains(_:) 方法，如果该序列属于Equatable个元素，则该方法将接受该序列的一个元素:

---

Swift 3

The Sequence protocol in Swift 3 still has a contains(_:) method, which accepts an element of the sequence if the sequence is of Equatable elements:

extension Sequence where Iterator.Element : Equatable {
    // ...
    public func contains(_ element: Self.Iterator.Element) -> Bool
    // ...
}

您遇到的问题是由于Collection的indices属性要求类型的变化.在Swift 2中，它的类型是Range<Self.Index> –但是在Swift 3中，它的类型是 Indices (Collection协议的关联类型):

The problem you're encountering is due to the change in the type of Collection's indices property requirement. In Swift 2, it was of type Range<Self.Index> – however in Swift 3, it is of type Indices (an associated type of the Collection protocol):

/// A type that can represent the indices that are valid for subscripting the
/// collection, in ascending order.
associatedtype Indices : IndexableBase, Sequence = DefaultIndices<Self>

由于Swift中目前尚无法使用Collection协议本身来表示Indices的Iterator.Element类型为

As there's currently no way in Swift for the Collection protocol itself to express that Indices's Iterator.Element is of type Index (this will however be possible in a future version of Swift), there's no way for the compiler to know that you can pass something of type Index into contains(_:). This is because it's currently fully possible for a type to conform to Collection and implement Indices with whatever element type it wants.

因此解决方案是仅限制您的扩展名以确保Indices 确实具有类型为Index的元素，从而允许您将index传递给contains(_:):

Therefore the solution is to simply constrain your extension to ensure that Indices does have elements of type Index, allowing you to pass index into contains(_:):

extension Collection where Indices.Iterator.Element == Index {

    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Iterator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

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