Listing 9.4 would cause an error as:

..
  case Exit(from, reason) =>

          //Retrieve assigned work:
          //from: type mismatch, expected Actor, actual: AbstractActor
          val (key, value) = assignedMappers(from)

a1 ! Broadcast("Hello", Set(a1, a2, a3, a4)) lines at page 122 should be:

a1 ! BCast("Hello", Set(a1, a2, a3, a4)) 

(see BCast case class definition at page 120)

In Fig 9.1, in the left table the heading should be V1=List[String], not
List[File].

Listing 9.7 uses the orElse method which has not been explained yet.