v4.0 Beta 1: Architecture Rap Battle

Deferred 4.0 brings the latest and greatest of Swift to Deferred, and is optimized to be faster and easier-to-use than ever before.

• Deferred supports and requires Swift 4.1 or better. (#200, #201, #222, #237, #250, #251)
• Deferred<Value> uses up to 70% lower memory and is up to 80% faster. (#229)
• Task<SuccessValue> and its related types have been redesigned to address common pain points. (#243)
  • Collection.allSucceeded() returns a combined array of all successes as its success value. (#217)
  • TaskResult<Value> provides common-case map(_:) and flatMap(_:) methods. (#244)
  • Task.fallback(upon:to:) can throw from within its handler. (#245, #255)
  • Task() is renamed Task.never. (#247)
  • Task(upon:flags:onCancel:execute:) is renamed Task.async(upon:flags:onCancel:execute:). (#248)
  • The functionality of Task<SuccessValue> (map(upon:transform:), andThen(upon:start:), recover(upon:substituting:) fallback(upon:to:), and so on) are conditionally available on Future and Deferred. (#253)
  • Task<SuccessValue> convenience initializers are more aligned with the Swift API Design Guidelines. (#254)
  • Task.repeat(upon:count:continuingIf:to:) can be used to simply re-attempt work if it fails. (#255)
• Future<Value> and its related types benefits from changes to Task<SuccessValue> and Deferred<Value>.
  • Implementing a custom Future requires fewer method implementations. (#215, #216)
  • Future() is renamed Future.never. (#247)
  • Future.async(upon:flags:execute:) can be used to capture the result of asynchronous work. (#248)
  • Future and Deferred conditionally have init(success:) and init(failure:), instead of requiring init(value: .success(...)). (#254)
• Improvements to debugging, reflection, and playgrounds. (#202, #212, #214, #223)
• Protected<Value> and TaskResult<Value> are annotated for binary resilience in Swift. Their use across module boundaries should be much faster. (#226, #243)