Swift µframework of Either, which represents two alternatives.
Swift Ruby C
Latest commit 74079a6 Dec 2, 2016 @guidomb guidomb Bumps version to 2.0.0.



This is a Swift microframework providing Either<Left, Right> and EitherProtocol, with generic implementations of ==/!= where Left & Right: Equatable.

Either allows you to specify that a value should be one of two types, or that that a value of a single type should have one of two semantics. For example, Either<Int, NSError> might be the result of a computation which could fail, while Either<String, String> might mean that you want to handle a string in one of two different ways.

EitherProtocol is an easy-to-adopt protocol (it requires one method and two constructor functions) which allows clients to generically use Either or conforming Result, etc. types.


Constructing an Either:

// Wrap:
let left = Either<Int, String>.left(4)
let right = Either<Int, String>.right("four")

Extracting the value:

// Unwrap:
let value = left.either(ifLeft: { $0 }, ifRight: { $0.characters.count })

Representing success/failure:

let result = someComputation() // result has type `Either<Error, T>`
let success = result.right // success has type `T?`
let error = result.left    // error has type `Error?`

However, you might instead prefer to use a more tailored Result type. Even if it doesn’t conform to EitherProtocol already, you can implement conformance in your application:

extension Result: EitherProtocol { // Result<T, Error>
    static func toLeft(_ value: Error) -> Result {
        return Result(error: value)

    static func toRight(value: T) -> Result {
        return Result(value: value)

    func either<Result>(ifLeft: (Error) -> Result, ifRight: (T) -> Result) -> Result {
        switch self {
        case let .success(x):
            return g(x)
        case let .failure(error):
            return f(error)

Now you can use generic functions like ==, !=, and any you might write with both Either and Result.

API documentation is in the source.


  1. Add this repository as a submodule and check out its dependencies, and/or add it to your Cartfile if you’re using carthage to manage your dependencies.
  2. Drag Either.xcodeproj into your project or workspace, and do the same with its dependencies (i.e. the other .xcodeproj files included in Either.xcworkspace). NB: Either.xcworkspace is for standalone development of Either, while Either.xcodeproj is for targets using Either as a dependency.
  3. Link your target against Either.framework and each of the dependency frameworks.
  4. Application targets should ensure that the framework gets copied into their application bundle. (Framework targets should instead require the application linking them to include Either and its dependencies.)