Design protocol interfaces in Swift

Description: Learn how you can use Swift 5.7 to design advanced abstractions using protocols. We'll show you how to use existential types, explore how you can separate implementation from interface with opaque result types, and share the same-type requirements that can help you identify and guarantee relationships between concrete types. To get the most out of this session, we recommend first watching “Embrace Swift generics" from WWDC22.

We will use this protocol definition as an example:

protocol Animal {
  associatedtype CommodityType: Food
  associatedtype FeedType: AnimalFeed

  func produce() -> CommodityType
  func eat(_: FeedType)
  var isHungry: Bool { get }

Understand type erasure

  • when you call a method returning an associated type on an existential type, the compiler will use type erasure to determine the result type of the call
  • type erasure replaces these associated types with corresponding existential types that have equivalent constraints

Type erasure semantics

Producing position

  • associatedtypes appearing in the result of a protocol method declaration are in producing position, because calling the method will produce a value of this type
    • e.g., the produce() return type in the Animal protocol definition above
  • The type any Food is called the upper bound of the associated CommodityType
  • the actual concrete type that is returned from produce() can safely convert to the upper bound:
let animals: [any Animal] = [Cow()] { animal in
  animal.produce() // we're calling `produce()` on an `any Animal` that holds a `Cow` at runtime.

Consuming position

  • associatedtypes appearing in the parameter list of a protocol method declaration are in consuming position, because calling the method will produce a value of this type
  • the upper bound cannot safely convert to the actual concrete type, because the concrete type is unknown
let animals = [Cow()] { animal in // given an arbitrary `any AnimalFeed`, there is no way to statically guarantee that it stores what Cow needs
  • type erasure does not allow us to work with associated types in consuming position
  • instead, you must unbox the existential any type by passing it to a function that takes an opaque some type

Hide implementation details

Constrained opaque result type

  • new in swift 5.7
  • written by applying type arguments in angle brackets after the protocol name - e.g., some Collection<any Animal>

Identify type relationships

  • every protocol has a Self type, which stands for the concrete conforming type
  • we can express the relationship between associatedtypes using a same-type requirement, written in a where clause
  • A same-type requirement expresses a static guarantee that two different, possibly nested associated types must in fact be the same concrete type
protocol AnimalFeed{
  associatedtype CropType: Crop
    where CropType.FeedType == Self // 👈🏻 same-type requirement
  static func grow() -> CropType

protocol Crop {
  associatedtype FeedType: AnimalFeed
    where FeedType.CropType == Self // 👈🏻 same-type requirement
  func harvest() -> FeedType
  • By understanding your data model, you can use same-type requirements to define equivalences between these different nested associated types
  • Generic code can then rely on these relationships when chaining together multiple calls to protocol requirements

Missing anything? Corrections? Contributions are welcome 😃


Written by

Federico Zanetello

Federico Zanetello

Software engineer with a strong passion for well-written code, thought-out composable architectures, automation, tests, and more.