Wip sip23 √

That is a fair point, and one I was expecting. Since we created SIP-14 I've been quite exposed to people's questions on the Akka, Scala ML as well as StackOverflow. One of the most frequent issues is the use of callbacks and closing over mutable state. When using transformation methods like map, filter etc, it is very infrequent that this problem manifests itself.

I think that having both a callback (onFailure, onComplete and onSuccess), a poll-based (value/isCompleted) and a monadic/transformational API in the same entity was a mistake, and I would argue that encouraging the users to use the monadic/transformational API is the right way to go. I'm curious as to what you think of that idea.