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Async+ for Swift provides a simple chainable interface for your async and throwing code, similar to promises and futures. Have the best of both worlds: you can use the async solution built into the language, but keep all the useful features of promises.

✏️ Usage

Basic chaining operations are:

  • .then arranges blocks one after another, passing along any values

  • .recover recovers from a thrown error with a backup value (or block to run)

  • .catch catches any errors (and allows you to throw new ones for later catch blocks)

  • attempt { ... } kicks off a chain as in the example below:

attempt {
    return try await getThing()
}.recover {
    error in
    return try await backupGetThing(error)
}.then {
    thing in
    await thing.doYour()
}.catch {
    error in

For comparison, if we tried to write the above flow without Async+ we’d get something like this:

Task.init {
    do {
        let thing: Thing
        do {
            thing = try await getThing()
        } catch {
            thing = try await backupGetThing(error)
        await thing.doYour()
    } catch {
        error in

Async+ allows async and/or throwing code to remain unnested, modular, and concise. For a full list of operations see the documentation.

Want to still use chained code within a do/catch block, Task.init, or similar context? Easy: chains are fully interoperable with async and/or throwing contexts via the operations .async(), and .asyncThrows() at the end of the chain, for example:

let foo = await attempt{ ... }.then{ ... }.async() // non-throwing chain
let foo = try await attempt{ ... }.then{ ... }.asyncThrows() // throwing chain

If the chain doesn’t throw you will not be able to call asyncThrows on it (it is a Guarantee<T> type rather than a Promise<T> type), and vice versa. Similarly, chains with potential for uncaught errors will raise an unused value warning at compilation time.

💾 Installation

Install the Async+ SwiftPM package in Xcode by going to <your project> -> <ProjectName> -> Package Dependencies -> "+" and entering:

Or modify your Package.swift file:

dependencies: [
    .Package(url: "", majorVersion: 0, minor: 1),

To use Async+ in a Swift file you must add import AsyncPlus to the top of the file.

📘 Documentation


Getting Started


Using chains from async or throwing contexts

Motivation and common patterns


Migrating from PromiseKit

Frequently asked questions (FAQ)

🚀 Feedback and Contributing

This package is in its initial release: please provide feedback and suggestions in order to help shape the API, either by submitting an issue on Github or sending a message on Discord.


View Github