Minimal integration of State and Actor

StateActor.scala

import akka.actor._
import scalaz.State

/** 
 * Minimal integration between State monad and Akka's Actor.
 * 
 * Usage: upon receiving a message, create a State instance and then call runState(st). The state value
 * is persisted in an actor field between invocations of runState.
 */
abstract class StateActor[S] extends Actor {

  protected def initialState: S
  private var lastState: S = initialState

  protected def runState[A](st: State[S, A]): A = {
    val (newState, result) = st.run(lastState)
    lastState = newState
    result
  }
}

Nice!