Listing 4.8:
val sessionUser = actor {
  while (true) {
    self.receiveWithin (1800 * 1000) {
    ...
    }
  }
}

self.receiveWithin (1800 * 1000) should be self.receiveWithin (180 *
1000) for 3 minutes

"In using ! to denote message sending, Scala follows the tradition of
Erlang:"

The use of "!" for message sending in occam (1983) predates the usage in
Erlang (1986).
I am not 100% sure, but if I'm not mistaken some variants of CSP already
used the "!" earlier.