为什么协议的关联类型不在 Swift 中使用泛型类型语法？

我对用于协议关联类型的语法与用于泛型类型的语法之间的区别感到困惑。

例如，在 Swift 中，可以使用类似于

struct Stack<T> {
var items = [T]()
mutating func push(item: T) {
items.append(item)
}
mutating func pop() -> T {
return items.removeLast()
}
}

而使用类似于

protocol Container {
associatedtype T
mutating func append(item: T)
var count: Int { get }
subscript(i: Int) -> T { get }
}

为什么后者不仅仅是:

protocol Container<T> {
mutating func append(item: T)
var count: Int { get }
subscript(i: Int) -> T { get }
}

这种语言没有采用后一种语法，是否有某种深层次的(或者只是显而易见的，我没有意识到)原因？

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最佳答案

This has been covered a few times on the devlist. The basic answer is that associated types are more flexible than type parameters. While you have a specific case here of one type parameter, it is quite possible to have several. For instance, Collections have an Element type, but also an Index type and a Generator type. If you specialized them entirely with type parameterization, you'd have to talk about things like Array<String, Int, Generator<String>> or the like. (This would allow me to create arrays that were subscripted by something other than Int, which could be considered a feature, but also adds a lot of complexity.)

It's possible to skip all that (Java does), but then you have fewer ways that you can constrain your types. Java in fact is pretty limited in how it can constrain types. You can't have an arbitrary indexing type on your collections in Java. Scala extends the Java type system with associated types just like Swift. Associated types have been incredibly powerful in Scala. They are also a regular source of confusion and hair-tearing.

Whether this extra power is worth it is a completely different question, and only time will tell. But associated types definitely are more powerful than simple type parameterization.

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RobNapier's answer is (as usual) quite good, but just for an alternate perspective that might prove further enlightening...

On Associated Types

A protocol is an abstract set of requirements — a checklist that a concrete type must fulfill in order to say it conforms to the protocol. Traditionally one thinks of that checklist of being behaviors: methods or properties implemented by the concrete type. Associated types are a way of naming the things that are involved in such a checklist, and thereby expanding the definition while keeping it open-ended as to how a conforming type implements conformance.

When you see:

protocol SimpleSetType {
associatedtype Element
func insert(_ element: Element)
func contains(_ element: Element) -> Bool
// ...
}

What that means is that, for a type to claim conformance to SimpleSetType, not only must that type contain insert(_:) and contains(_:) functions, those two functions must take the same type of parameter as each other. But it doesn't matter what the type of that parameter is.

You can implement this protocol with a generic or non-generic type:

class BagOfBytes: SimpleSetType {
func insert(_ byte: UInt8) { /*...*/ }
func contains(_ byte: UInt8) -> Bool { /*...*/ }
}


struct SetOfEquatables<T: Equatable>: SimpleSetType {
func insert(_ item: T) { /*...*/ }
func contains(_ item: T) -> Bool { /*...*/ }
}

Notice that nowhere does BagOfBytes or SetOfEquatables define the connection between SimpleSetType.Element and the type used as the parameter for their two methods — the compiler automagically works out that those types are associated with the right methods, so they meet the protocol's requirement for an associated type.

On Generic Type Parameters

Where associated types expand your vocabulary for creating abstract checklists, generic type parameters restrict the implementation of a concrete type. When you have a generic class like this:

class ViewController<V: View> {
var view: V
}

It doesn't say that there are lots of different ways to make a ViewController (as long as you have a view), it says a ViewController is a real, concrete thing, and it has a view. And furthermore, we don't know exactly what kind of view any given ViewController instance has, but we do know that it must be a View (either a subclass of the View class, or a type implementing the View protocol... we don't say).

Or to put it another way, writing a generic type or function is sort of a shortcut for writing actual code. Take this example:

func allEqual<T: Equatable>(a: T, b: T, c: T) {
return a == b && b == c
}

This has the same effect as if you went through all the Equatable types and wrote:

func allEqual(a: Int, b: Int, c: Int) { return a == b && b == c }
func allEqual(a: String, b: String, c: String) { return a == b && b == c }
func allEqual(a: Samophlange, b: Samophlange, c: Samophlange) { return a == b && b == c }

As you can see, we're creating code here, implementing new behavior — much unlike with protocol associated types where we're only describing the requirements for something else to fulfill.

TLDR

Associated types and generic type parameters are very different kinds of tools: associated types are a language of description, and generics are a language of implementation. They have very different purposes, even though their uses sometimes look similar (especially when it comes to subtle-at-first-glance differences like that between an abstract blueprint for collections of any element type, and an actual collection type that can still have any generic element). Because they're very different beasts, they have different syntax.

Further reading

The Swift team has a nice writeup on generics, protocols, and related features here.

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To add to the already great answers, there's another big difference between generics and associated types: the direction of the type generic fulfilment.

In case of generic types, it's the client that dictates which type should be used for the generic, while in case of protocols with associated types that's totally in the control of the type itself. Which means that types that conform to associated types are in liberty to choose the associated type that suits them best, instead of being forced to work with some types they don't know about.

As others have said, the Collection protocol is a good example of why associated types are more fit in some cases. The protocol looks like this (note that I omitted some of the other associated types):

protocol Collection {
associatedtype Element
associatedtype Index


...
}

If the protocol would've been defined as Collection<Element, Index>, then this would've put a great burden on the type conforming to Collection, as it would've have to support any kind of indexing, many of them which don't even make sense (e.g. indexing by a UIApplication value).

So, choosing the associated types road for protocol generics it's also a matter of empowering the type that conforms to that protocol, since it's that type the one that dictates what happens with the generics. And yes, that might sound less flexible, but if you think about it all types that conform to Collection are generic types, however they only allow generics for the types that make sense (i.e. Element), while "hardcoding" the other associated types (e.g. Index) to types that make sense and are usable in their context.


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