Future<Value, Error>

Inspired by the Swift core team’s recent activity in the proposal to add Result<Value, Error> to the Swift Standard Library, I started working on a new version of FutureX – a Future implementation. The initial goal was to provide a small and simple Future<Value, Error> class with typed errors, but eventually, I challenged myself to code the Future that would feel like a part of the Swift Standard Library. For me, that meant building an idiomatic Swift implementation with a streamlined functional API and good performance.


A future represents a result of computation which may be available now, or in the future, or never. Essentially, a future is an object to which you attach callbacks, instead of passing them into a function that performs a computation.

Futures are easily composable. For example, Future<Value, Error> in FutureX provides a set of functions like map, flatMap, zip, reduce and more to compose futures (more on it later).

Prior Art

There are plenty of great Future implementations available in Swift. I’ve looked at mxcl/PromiseKit, google/Promises, SwiftNIO/EventLoopFuture, BrightFutures, khanlou/Promise and many more. Most don’t provide typed errors. And many are just too big which makes including them in a project a hard sell. So I still decided to continue working on Pill because none of them “were quite my tempo”.

Future<Value, Error>

The first change was an addition of typed errors: instead of Promise<T> we now have Future<Value, Error> (and also Promise<Value, Error> which is now a separate struct). The typed errors give you extra confidence when dealing with errors.

There seem to be an opinion that typed errors are inconvenient and just add unnecessary complexity. I used to share this opinion but after working with typed errors for a while I found the opposite to be true. Typed errors are not only safer, but they often make error handling more ergonomic since you no longer need to cast them.

Streamlined Functional API

I reviewed all of the terminology used in FutureX: resolve, fulfill, reject, then, recover – I didn’t feel that Swift developers were accustomed to these terms. So I decided to replace them with a set of functional methods that we all know and love.

Map and FlatMap

Instead of then there is map and flatMap:

// Before
func then<U>(on queue: DispatchQueue = .main, _ closure: @escaping (T) throws -> U) -> Promise<U>
func then<U>(on queue: DispatchQueue = .main, _ closure: @escaping (T) throws -> Promise<U>) -> Promise<U>

// After
func map<NewValue>(_ closure: @escaping (Value) -> NewValue) -> Future<NewValue, Error>
func flatMap<NewValue>(_ closure: @escaping (Value) -> Future<NewValue, Error>) -> Future<NewValue, Error>

Transforming values and chaining futures is now as Swifty as it gets:

let avatar = user
    .map { $0.avatarURL }

Zip and Reduce

I decided not to stop there and added two other incredibly useful functions: zip and reduce.

Use zip to combine the result of up to three futures into a single tuple:

let user: Future<User, Error>
let avatar: Future<UIImage, Error>

Future.zip(user, avatar).on(success: { user, avatar in
    // use both values

Or to wait for the result of multiple futures:

Future.zip([future1, future2]).on(success: { values in
    // use an array of values

Use reduce to combine the results of multiple futures:

let future1 = Future<Int, Error>(value: 1)
let future2 = Future<Int, Error>(value: 2)

Future.reduce(0, [future1, future2], +).on(success: { value in
    print(value) // prints "3"

What’s Next

Working on Future<Value, Error> was a lot of fun, especially implementing functional parts like zip and reduce. I’m extremely satisfied with the result and the fact that even with the new additions it is still just under 150 lines of code – this truly shows the power of composition.

If you’d like to give Future<Value, Error> a try, FutureX is a great place to start.