MessageCompanion.scala

trait MessageCompanion[S <: Message, T] /* ... */ {
  def unapply(in: Input): Option[T] = {
    if (isDefinedAt(in)) {
      val result = Some(read(in))
      checkComplete(in)
      result
    } else None
  }

  def read(in: Input): T

  // ...
}

sillyExample.scala

class Request(val text: String, num: Int) extends Message { /* ... */ }
class Ping extends Message { /* ... */ }

object RequestCompanion extends MessageCompanion[Request, (String, int)] { /* ... */ }
object PingCompanion extends MessageCompanion[Ping, ???] { /* ... */ }


val in: Input = // ...
in match {
  case RequestMessage(text, num) => println(num + " times " + text + "!")
  case PingMessage() => println("ping")
  // ..
}