Swift Academy · Lesson

Primary Associated Types and Typed Throws

Constraining protocol existentials and Swift 6 typed error propagation.

Primary Associated Types

Swift 5.7 lets you specify primary associated types on protocols, enabling constrained existentials.

protocol Container<Element> {
  associatedtype Element
  var items: [Element] { get }
}
struct Bag<T>: Container { var items: [T] = [] }

With primary associated types you can write any Container to narrow the existential.

func printItems(_ c: any Container<Int>) {
  c.items.forEach { print($0) }
}
printItems(Bag(items: [1,2,3]))