Pill 0.9

Pill 0.9

LangLanguage SwiftSwift
License MIT
ReleasedLast Release Nov 2018
SPMSupports SPM

Maintained by kean.

Pill 0.9

A streamlined Future<Value, Error> implementation.


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 callbacks into a function that performs a computation.

Futures are easily composable. Future<Value, Error> provides a set of functions like map, flatMap, zip, reduce and more to compose futures.

Quick Start

To attach a callback to the Future use on(success:failure:completion:) method:

let user: Future<User, Error>

user.on(success: { print("received entity: \($0)" },
        failure: { print("failed with error: \($0)" })
// As an alternative observe a completion:
user.on(completion: { print("completed with result: \($0)" })   

By default, all of the callbacks are executed on the main queue (DispatchQueue.main). To change the queue pass one into on method:

future.on(queue: .global(), success: { print("value: \($0)" })

Mapping Values

Use familiar map and flatMap function to transform the future's values and chain futures:

let user: Future<User, Error>
func loadAvatar(url: URL) -> Future<UIImage, Error> {}

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

Mapping Errors

Future has typed errors. To convert from one error type to another use mapError:

let request: Future<Data, URLError>

Use flatMapError to "recover" from an error.


Use zip to combine the result of up to three futures in a single future:

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"

Creating Futures

Using Promise:

func someAsyncOperation(args) -> Future<Value, Error> {
    let promise = Promise<Value, Error>()
    someAsyncOperationWithCallback(args) { value, error in
        // when finished...
        promise.succeed(result: value)
        // if error...
        promise.fail(error: error)
    return promise.future

Using a convenience init method:

let future = Future<Int, Error> { succeed, fail in
    someAsyncOperationWithCallback { value, error in
        // succeed or fail

With a value or an error:

Future<Int, Error>(value: 1)
Future<Int, Error>(error: Error.unknown)


Use wait method to block the current thread and wait until the future receives a result:

let result = future.wait()

Synchronous Inspection

class Future<Value, Error> {
    var isPending: Bool { get }
    var value: Value? { get }
    var error: Error? { get }
    var result: Result<Value, Error> { get }


Pill considers cancellation to be a concern orthogonal to Future. There are multiple cancellation approaches. There are arguments for failing futures with an error on cancelation, there is also an argument for never resolving futures when the associated work gets canceled. In order to implement cancelation you might want to consider CancellationToken or other similar patterns.


Avoid nesting futures, as this is the issue that futures are designed to solve:

loadUser().on(success: { user in
    loadAvatar(url: user.avatarUrl).on(success: { avatar in
        print("avatar loaded")


  • iOS 9.0 / watchOS 2.0 / OS X 10.11 / tvOS 9.0
  • Xcode 10
  • Swift 4.2


Pill is available under the MIT license. See the LICENSE file for more info.