A Future is a simplified take on Promises, written in Swift. It aims to provide the following for it's users:

1. A clean separation of success & error handlers, with no optional values in completion blocks.
2. Type-safe error handling
3. An easy to use call site that better matches how we describe code.

When calling a method which returns a Future:

doAsyncThing().then { result in 
	// do something with result
}

Result, here, will be non-optional and the then block will only be called if the future has completed successfully.

Error handling is done similarly:

doAsyncThingThatErrors().catch { error in 
	// do something with error
}

Errors here are also non-optional, and of the type defined by the Future itself (as opposed to non-typed throws calls)

Futures rely heavily on generics to provide type safety.

func doAsyncThing() -> Future<Int, MyError> {
}

The first value of the Future<> definition denotes the type expected with a successful Future, the second denotes the type expected with a failing Future. The error type must conform to Swift's own Error type.

The Future's initializer aims to retain type safety, but eliminate as much of the tedious type annotations as possible.

func doAsyncThing() -> Future<Int, MyError> {
	return Future { resolver in 
		// ...
	}
}

The resolver value here passed into the block is a Result.Resolver, which has two methods:

resolver.resolve(value:)

and

resolver.reject(error:)

Which will either resolve or reject the associated Future appropriately.

Some methods to compose Futures exist on the Futures namespace. They are:

Futures.all()

Which returns a Future which will be resolved with a Collection of Results if, and only if, all of the passed Futures complete successfully.

Futures.any()

Which returns a Future which will be resolved with a Collection of Results if one or more of the passed Futures complete successfully.

Futures.first()

Which returns a Future which will resolve or reject with the value of the first passed Future which completes.

Right now the preferred way to try out Futures is via the Swift Package Manager.

.package(url: "https://github.com/genius/future", from: "0.0.1")

You can also copy the Future.swift file from the Sources directory of your own project if installation via the Swift Package Manager is not available to you.

Bug reports, constructive feedback and pull requests are always welcome. If you're using Futures in your own apps, we'd love to hear about it.