Asynchronous, Functional and automatically Concurrent

The following code fragment is from an earlier post on using Scala actors and AMQP. I thought I would bring this snippet up once again to highlight some of the goodness that functional Scala offers in modeling actor model of concurrent computation.

import scala.actors.Actor

case class Trade(id: Int, security: String, principal: Int, commission: Int)
case class TradeMessage(message: Trade)
case class AddListener(a: Actor)

class TradingService extends Actor {

  def act = loop(Nil)

  def loop(traders: List[Actor]) {
    react {
    case AddListener(a) => loop(a :: traders)
    case msg@TradeMessage(t) => traders.foreach(_ ! msg); loop(traders)
    case _ => loop(traders)
    }
  }
}


An implementation of the Observer design pattern using message passing. Interested traders can register as observers and observe every trade that takes place. But without any mutable state for maintaining the list of observers. Not a very familiar paradigm to the programmers of an imperative language. The trick is to have the list of observers as an argument to the loop() function which is tail called.

Nice .. asynchronous, referentially transparent, side-effect-free functional code. No mutable state, no need of explicit synchronization, no fear of race conditions or deadlocks, since no shared data are being processed concurrently by multiple threads of execution.