Swift Generics and Associated Types

How it works

A generic parameter names a placeholder type at a use site. An associated type names part of a protocol’s contract chosen by each conformer. Constraints such as Equatable let the algorithm use the operations it needs without replacing everything with Any.

Run this Swift example

Install the toolchain for your platform from Swift.org, then check swift --version. The reviewed portable baseline is Swift 6.4.2 in Swift 6 mode on Ubuntu 26.04 x86_64 through the PC’s existing WSL installation. These are real Linux executions, not claims of Windows-native or Apple-platform test results. Windows Swift has separate C++/SDK prerequisites in its official setup guide.

Save the complete source as generics-associated-types.swift in a new learning folder. On Linux/macOS compile with swiftc -swift-version 6 -parse-as-library generics-associated-types.swift -o lesson, then run ./lesson. On a configured Windows toolchain use an output such as lesson.exe and run .\lesson.exe. The parse-as-library option supports the explicit @main entry point used by these sources. Compile each ordinary lesson separately.

Bhylu has no Swift browser runner. The JavaScript/Python editors do not run these sources; Apache, PHP and MySQL are not required. Downloads are source text, never executable teaching endpoints.

Download the complete generics-associated-types.swift · Download all source files with hash checks

For the optional collection, save download-examples.mjs in a new folder and run node download-examples.mjs. A local preview uses node download-examples.mjs http://127.0.0.1:8000/examples/swift/. It refuses an existing bhylu-swift-examples folder and installs or executes no Swift automatically. Apple-only files remain separate Xcode learning examples, not a portable Swift package target.

Example

func firstMatch<T: Equatable>(in values: [T], equalTo target: T) -> T? { values.first { $0 == target } }
protocol FirstReadable { associatedtype Element; func first() -> Element? }
struct Queue<T>: FirstReadable {
    let values: [T]
    func first() -> T? { values.first }
}

@main
struct Lesson {
    static func main() {
        print(firstMatch(in: ["Cedar", "Ash", "Cedar"], equalTo: "Ash") ?? "missing")
        print(Queue(values: [2, 3]).first() ?? -1)
    }
}

Expected result

Ash
2

The equality constraint permits == for the actual element type. Queue chooses its Element through the protocol conformance. The empty result stays optional, keeping absence separate from a valid zero or empty string.