codecIsEasy.scala

// define the ADT
trait Tag {
  val id: Byte
}
case object Empty extends Tag {
  val id = -1.toByte
}
case object Initial extends Tag {
  val id = 0.toByte
}
case object Authenticated extends Tag {
  val id = 1.toByte
}

// create a smart constructor
object Tag {
  def apply(id: Byte): Tag = id match {
    case 0 => Initial
    case 1 => Authenticated
    case _ => Empty
  }
}

// here is a partial function that acts like a finagle filter
val TagFilter: PartialFunction[Tag, String] = {
  case Authenticated => "auth"
  case Initial => "login"
  case _ => "unknown"
}

// here is are some incoming tag ids
println(TagFilter(Tag(0.toByte))) // login
println(TagFilter(Tag(1.toByte))) // auth
println(TagFilter(Tag(2.toByte))) // unknown

// here is the great part
// now you can define interfaces that *only* accept a specific Tag type!
val authedFilter: Authenticated.type => String = t => "you are authenticated!!!!"

println(authedFilter(Authenticated)) // you are authenticated!!!!
//println(authedFilter(Empty)) // !! DOES NOT COMPILE !!