How to preserve original type in match without resorting to asInstanceOf?

Question

The below code is using ZIO, so I've added the scalaz tag, though that may be a bit beside the point. I have a function that takes a type J with a typeclass constraint (Job):

  def execJvm2[J: Job](cmdIn: J): IO[Nothing, Future[RunResult]] = {
    type IOJob = IO[Nothing, J]
    val cmd0: IOJob = omitted(cmdIn)
    val cmd1: IOJob = cmd0.map {
      case cmd : OneShot =>
        memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
        cmd.asInstanceOf[J]
      case cmd: Repl =>
        memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
        cmd.asInstanceOf[J]
      case cmd: ExecFile =>
        memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
        cmd.asInstanceOf[J]
      case _ => ???
    }
    cmd1.map { cmd => poll(cmd.id) }
  }

The examples in the pattern match (OneShot, Repl, ExecFile) all are instances of Job and have their implicit typeclass instances in scope, though I guess that is a bit beside the point. The main reason this should work without using asInstanceOf, to my thinking, is that the type has only been narrowed down in the pattern match from J to e.g. OneShot, but I would think the compiler would know it is still a J as well.

Answer 1

It looks a bit worse, but I think

val cmd1: IOJob = cmd0.map { cmdJ => cmdJ match {
  case cmd: OneShot =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmdJ
  case cmd: Repl =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmdJ
  case cmd: ExecFile =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmdJ
  case _ => ???
}}

should work. And cheating with type erasure a bit, I think this should compile and work (but try it):

val cmd1: IOJob = cmd0.map {
  case cmd: OneShot with J @unchecked =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmd
  case cmd: Repl with J @unchecked =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmd
  case cmd: ExecFile with J @unchecked =>
    memStorage(cmd.id) = JobWithResult(cmd, runIO(runInSystem(cmd)))
    cmd
  case _ => ???
}