具有关联类型的Swift子协议 [英] Swift subprotocol with associated type

问题描述

我想知道如何在Swift中表达这种类型的关系(例如kotlin中的例子)

I'd like to know how this type of relation (example in kotlin) would be expressed in Swift

interface Index<K, V> {
  fun getAll(key: K): Sequence<V>
}

我试图使用具有关联类型的协议，例如:

I tried to use protocols with associated types, like this:

protocol Index {
    associatedtype Key
    associatedtype Value
    associatedtype Result: Sequence where Sequence.Element == Value

    func getAll(key: Key) -> Result
}

但这不起作用(Associated type 'Element' can only be used with a concrete type or generic parameter base)

然后，作为一种解决方法，我尝试了此操作:

Then, as a workaround, i tried this:

protocol Index {
    associatedtype Key
    associatedtype Value

    func get<T: Sequence>(key: Key) -> T where T.Element == Value
}

但是，这似乎并不是正确/惯用的方式.

But this doesn't really seem like the right / idiomatic way to do it.

只有两个约束:

1. 序列不能是具体类型
2. Index上的任何方法都没有有意义的实现

注意:

• 将存在一个实现Sequence的类/类型，该类/类型特定于Index的每种实现

• There will be a class / type that implements Sequence that is specific to each implementation of Index

我愿意接受任何其他结构性改变！ 预先感谢.

I'm open to any other structural change! Thanks in advance.

推荐答案

您需要使用关联类型的名称，而不是继承协议的名称:

You need to use the associated type's name, not the inherited protocol's name:

associatedtype Result: Sequence where Result.Element == Value
//                                    ^^^^^^

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