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Implement the ask pattern in ActorRef. The basic signature is already there.

Implementation details. Whenever someone asks an actor about something, the following happens:

• a temporary actor is created under /system/queries
• this actor poses the question and awaits the answer (for a certain amount of time)
• once the response msg arrives it completes the callback
• on timeout, it should complete with error.

Here in the callback signature, it's OK to use Swift 5's result type.

Whenever a message can't be delivered to actor, or is sent to Nobody, or is not handled by an actor, it needs to end up delivered to a special actor called /system/deadLetters. This actor is present in all ActorSystems and initially, just logs the dead letters in a nice way.

It may also be a good idea to envelope the messages sent to the DeadLetters actor, so that the intended recipient (its path) is also known.

Find a way how an Actor can signal an unhandled message with minimum ceremony.

It is quite common for an actor to ignore messages it is not able to handle at a given moment. This should be handled with minimum ceremony within the receive method. On the other hand if a message has not been handled by an actor, its cell should report it to the dead letters mailbox.

This poses a requirement on the actor's receive method to signal whether a message has been handled or not. The question is how to do it properly.

Akka makes use of Scala's partial functions and the built in syntactic sugar for them to solve this elegantly (the cell can inspect whether the receive method is defined at a certain value).

In Swift we have to be more explicit. Possible solutions:

• The cell will assume that a message is unhandled unless the receive method explicitly consumes it.
• The cell will assume that the message is handled unless the recieve methods raises a specific error to signal otherwise
• The cell doesn't assume anythign. The receive method returns Handled|Unhandled to signal the outcome.

Think about other possibilities (return the message if unhandled, etc...)

Implement ActorRef.forward. The method should allow message forwarding, i.e.

// within an actor method
func receive(msg: Any) {
  let ref: ActorRef  = // ...
  ref.forward(msg: msg)
}

In this case, the actor behind ref will receive the message with the original sender rather than the actor that forwards it (as it would happen if one goes for ref.tell(msg). The call is equivalent to

ref.tell(msg: msg, sender: sender)

executed in an actor's receive method.

ActorRef.tell(msg:sender:) should be made internal to the project.

A forwarding operator might also be added, e.g. !<<, as in

ref !<< msg

Implement the become and unbecome methods for actors (via the actor context). See Akka's documentation for details.

Setup CI, using Circle CI.

this might be confusing in the swift context. Perhaps ref or refToSelf or selfRef or something more pleasing.

Also provide:

• CocoaPods podspec incl upload to repo
• Carthage file

Use Coveralls.io

Implement the stashing / unstashing feature as described here.