What is an associated type in a protocol?

Middle PRO

The idea

An associatedtype is a type placeholder inside a protocol. The protocol states that such a type exists and what constraints it satisfies, and each conforming type supplies its concrete type (explicitly via typealias or inferred from signatures). Sequence.Element, Collection.Index and IteratorProtocol.Element work this way.

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

struct IntStack: Container {
    private var items: [Int] = []
    var count: Int { items.count }
    // Item = Int is inferred from the signature
    mutating func append(_ item: Int) { items.append(item) }
    subscript(i: Int) -> Int { items[i] }
}

func contains<C: Container>(_ c: C, _ x: C.Item) -> Bool {
    (0..<c.count).contains { c[$0] == x }
}
// Swift 5.7+: primary associated type
func sum(_ c: some Container<Int>) -> Int {
    (0..<c.count).reduce(0) { $0 + c[$1] }
}
let boxes: [any Container<Int>] = [IntStack()]