The first thing to understand is that the head field inside of scalaz.concurrent.Actor is not the "head" of the message queue in any traditional sense of the word. A better description would be "last". The there are no pointers to the head of the queue, which one of the very clever things about this implementation.
Consider the case where the actor has no outstanding messages. This new message will go into the following code:
def !(a: A): Unit = {