Swift Opaque and Existential Types

How it works

some Protocol in a result hides one concrete return type while preserving its identity for the type system. any Protocol is an existential container that can hold different conforming types. They solve different API problems and can have different available operations and performance characteristics.

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 opaque-existential-types.swift in a new learning folder. On Linux/macOS compile with swiftc -swift-version 6 -parse-as-library opaque-existential-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 opaque-existential-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

protocol Labelled { var text: String { get } }
struct RoomLabel: Labelled { let text = "Cedar" }
struct CountLabel: Labelled { let text = "6 places" }
func makeLabel() -> some Labelled { RoomLabel() }

@main
struct Lesson {
    static func main() {
        print(makeLabel().text)
        let mixed: [any Labelled] = [RoomLabel(), CountLabel()]
        print(mixed.map { $0.text })
    }
}

Expected result

Cedar
["Cedar", "6 places"]

makeLabel always returns the same concrete RoomLabel type. The mixed array deliberately stores two concrete implementations behind their shared readable property. Neither form performs network type discovery or dynamic validation.