What’s the difference between any and some in Swift 5.7?

Published on: June 8, 2022

In Swift 5.1 Apple introduced the some keyword. This keyword was key in making SwiftUI work because the View protocol defines a Self requirement:

protocol View {
  associatedtype Body: View
  @ViewBuilder @MainActor var body: Self.Body { get }
}

If you’d write var body: View instead of var body: some View you’d see the following compiler error in Swift 5.7:

Use of protocol 'View' as a type must be written 'any View’

Or in older versions of Swift you’d see the following:

protocol can only be used as a generic constraint because it has Self or associated type requirements

The some keyword fixes this by hiding the concrete associated type from whoever interacts with the object that has some Protocol as its type. More on this later.

In Swift 5.6, the any keyword was added to the Swift language.

While it sounds like the any keyword acts as a type erasing helper, all it really does is inform the compiler that you opt-in to using an existential (a box type that conforms to a protocol) as your type.

Code that you would originally write as:

func getObject() -> SomeProtocol {
  /* ... */
}

Should be written as follows in Swift 5.6 and above:

func getObject() -> any SomeProtocol {
  /* ... */
}

This makes it explicit that the type you return from getObject is an existential (a box type) rather than a concrete object that was resolved at compile time. Note that using any is not mandatory yet, but you should start using it. Swift 6.0 will enforce any on existentials like the one that's used in the example you just saw.

Since both any and some are applied to protocols, I want to put them side by side in this blog post to better explain the problems they solve, and how you should decide whether you should use any, some, or something else.

Understanding the problems that any and some solve

To explain the problems solved by any we should look at a somewhat unified example that will allow us to cover both keywords in a way that makes sense. Imagine the following protocol that models a Pizza:

protocol Pizza {
    var size: Int { get }
    var name: String { get }
}

It’s a simple protocol but it’s all we need. In Swift 5.6 you might have written the following function to receive a Pizza:

func receivePizza(_ pizza: Pizza) {
    print("Omnomnom, that's a nice \(pizza.name)")
}

When this function is called, the receivePizza function receives a so-called box type for Pizza. In order to access the pizza name, Swift has to open up that box, grab the concrete object that implements the Pizza protocol, and then access name. This means that there are virtually no compile time optimizations on Pizza, making the receivePizza method more expensive than we’d like.

Furthermore, the following function looks pretty much the same, right?

func receivePizza<T: Pizza>(_ pizza: T) {
    print("Omnomnom, that's a nice \(pizza.name)")
}

There’s a major difference here though. The Pizza protocol isn’t used as a type here. It’s used as a constraint for T. The compiler will be able to resolve the type of T at compile time and receivePizza will receive a concrete instance of a type rather than a box type.

Because this difference isn’t always clear, the Swift team has introduced the any keyword. This keyword does not add any new functionality. Instead, it forces us to clearly communicate “this is an existential”:

func receivePizza(_ pizza: any Pizza) {
    print("Omnomnom, that's a nice \(pizza.name)")
}

The example that uses a generic <T: Pizza> does not need the any keyword because Pizza is used as a constraint and not as an existential.

Now that we have a clearer picture regarding any, let’s take a closer look at some.

In Swift, many developers have tried to write code like this:

let someCollection: Collection

Only to be faced by a compiler error to tell them that Collection has a Self or associated type requirement. In Swift 5.1 we can tell the compiler that anybody that accesses someCollection should not concern themselves with any of that. They should just know that this thing conforms to Collection and that’s all.

This mechanism is essential to making SwiftUI’s View protocol work.

The downside of course is that anybody that works with a some Collection, some Publisher, or some View can’t access any of the generic specializations. That problem is solved by primary associated types which you can read more about right here.

However, not all protocols have associated type requirements. For example, our Pizza protocol does not have an associated type requirement but it can benefit from some in certain cases.

Consider this receivePizza version again:

func receivePizza<T: Pizza>(_ pizza: T) {
    print("Omnomnom, that's a nice \(pizza.name)")
}

We defined a generic T to allow the compiler to optimize for a given concrete type of Pizza. The some keyword also allows the compiler to know at compile time what the underlying type for the some object will be; it just hides this from the user of the object. This is exactly what <T: Pizza> also does. We can only access on T what is exposed by Pizza. This means that we can rewrite receivePizza<T: Pizza>(_:) as follows:

func receivePizza(_ pizza: some Pizza) {
    print("Omnomnom, that's a nice \(pizza.name)")
}

We don’t need T anywhere else, so we don’t need to “create” a type to hold our pizza. We can just say “this function takes some Pizza" instead of “this function takes some Pizza that we’ll call T". Small difference, but much easier to write. And functionally equivalent.

Choosing between any and some

Once you understand the use cases for any and some, you’ll realize that it’s not a matter of choosing one over the other. They each solve their own very similar problems and there’s always a more correct choice.

Generally speaking you should prefer using some or generics over any whenever you can. You often don’t want to use a box that conforms to a protocol; you want the object that conforms to the protocol.

Or sticking with our pizza analogy, any will hand the runtime a box that says Pizza and it will need to open the box to see which pizza is inside. With some or generics, the runtime will know exactly which pizza it just got, and it’ll know immediately what to do with it (toss if it’s Hawaii, keep if it’s pepperoni).

In lots of cases you’ll find that you actually didn’t mean to use any but can make some or a generic work, and according to the Swift team, we should always prefer not using any if we can.

Making the decision in practice

Let’s illustrate this with one more example that draws heavily from my explanation of primary associated types. You’ll want to read that first to fully understand this example:

class MusicPlayer {
    var playlist: any Collection<String> = []

    func play(_ playlist: some Collection<String>) {
        self.playlist = playlist
    }
}

In this code, I use some Collection<String> instead of writing func play<T: Collection<String>>(_ playlist: T) because the generic is only used in one place.

My var playlist is an any Collection<String> and not a some Collection<String> for two reasons:

  1. There would be no way to ensure that the concrete collection that the compiler will deduce for the play method matches the concrete collection that’s deduced for var playlist; this means they might not be the same which would be a problem.
  2. The compiler can’t deduce what var playlist: some Collection<String> in the first place (try it, you’ll get a compiler error)

We could avoid any and write the following MusicPlayer:

class MusicPlayer<T: Collection<String>> {
    var playlist: T = []

    func play(_ playlist: T) {
        self.playlist = playlist
    }
}

But this will force us to always use the same type of collection for T. We could use a Set, an Array, or another Collection but we can never assign a Set to playlist if T was inferred to be an Array. With the implementation as it was before, we can:

class MusicPlayer {
    var playlist: any Collection<String> = []

    func play(_ playlist: some Collection<String>) {
        self.playlist = playlist
    }
}

By using any Collection<String> here we can start out with an Array but pass a Set to play, it’s all good as long as the passed object is a Collection with String elements.

In Summary

While some and any sound very complex (and they honestly are), they are also very powerful and important parts of Swift 5.7. It’s worth trying to understand them both because you’ll gain a much better understanding about how Swift deals with generics and protocols. Mastering these topics will really take your coding to the next level.

For now, know that some or generics should be preferred over any if it makes sense. The any keyword should only be used when you really want to use that existential or box type where you’ll need to peek into the box at runtime to see what’s inside so you can call methods and access properties on it.

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