Failed to run simple akka sample using sbt

Question

I'm just following akka sample but could not run the program.

1. I've installed akka, sbt(0.13), scala(2.10.3) using homebrew(OSX Mountail Lion)
2. make empty directory named akka_test
3. create build.sbt and Hello.scala file
4. run sbt in akka_test directory and compile command worked well
5. sbt's run command complains No main class detected

What should I do to run the program?

here's my code

build.sbt

name := "My Project"

version := "1.0"

scalaVersion := "2.10.2"

resolvers += "Typesafe Repository" at "http://repo.typesafe.com/typesafe/releases/"

libraryDependencies +=
  "com.typesafe.akka" %% "akka-actor" % "2.2.1"

Hello.scala

import akka.actor.Actor
import akka.actor.Props

class HelloWorld extends Actor {

  override def preStart(): Unit = {
    // create the greeter actor
    val greeter = context.actorOf(Props[Greeter], "greeter")
    // tell it to perform the greeting
    greeter ! Greeter.Greet
  }

  def receive = {
    // when the greeter is done, stop this actor and with it the application
    case Greeter.Done ⇒ context.stop(self)
  }
}

object Greeter {
  case object Greet
  case object Done
}

class Greeter extends Actor {
  def receive = {
    case Greeter.Greet ⇒
      println("Hello World!")
      sender ! Greeter.Done
  }
}

Answer 1

sbt has run-main command which accepts main class from command line, so full command is

sbt "run-main akka.Main HelloWorld"