Type-safety with ADT and Aux pattern

问题

I'm designing type-safe code with ADT and Aux-pattern and cannot get rid of some asInstanceOf. Here is the example:

  sealed trait Source
  case object FileSystem extends Source
  case object Network extends Source

  sealed trait Data {
    type S <: Source
  }
  object Data {
    type Aux[T <: Source] = Data { type S = T }
  }
  case class RegularFile(path: String) extends Data { type S = FileSystem.type }
  case class Directory(path: String) extends Data { type S = FileSystem.type }
  case class UnixDevice(path: String) extends Data { type S = FileSystem.type }
  case class Remote(ip: String) extends Data { type S = Network.type }

  //Lots of asInstanceOf
  def availableData[S <: Source](src: Source): List[Data.Aux[S]] = {
    src match {
      case FileSystem => List(
        RegularFile("/tmp/test").asInstanceOf[Data.Aux[S]],
        Directory("/home/somename").asInstanceOf[Data.Aux[S]],
        UnixDevice("/dev/null").asInstanceOf[Data.Aux[S]],
      )
      case Network => List(
        Remote("192.168.0.1").asInstanceOf[Data.Aux[S]]
      )
    }
  }

In this case it is pretty obvious that asInstanceOf is correct, but is there a way to get rig of it?

I was considering S <: Source: ClassTag, but it doesn't seem to be useful here. Maybe other reflection trick?

回答1:

Please see explanation why the signature

def availableData[S <: Source](src: S): List[Data.Aux[S]] = {
  src match {
    case FileSystem => List(
      RegularFile("/tmp/test"),
      Directory("/home/somename"),
      UnixDevice("/dev/null"),
    )
    case Network => List(
      Remote("192.168.0.1")
    )
  }
}

doesn't work and several approachs how to fix it:

Generic function where the return type depends on the input type in Scala?

Why can't I return a concrete subtype of A if a generic subtype of A is declared as return parameter?

Type mismatch on abstract type used in pattern matching

For example try a type class

trait AvailableData[S <: Source] {
  def availableData(src: S): List[Data.Aux[S]]
}
object AvailableData {
  implicit val fileSystem: AvailableData[FileSystem.type] = _ =>
    List[Data.Aux[FileSystem.type]](
      RegularFile("/tmp/test"),
      Directory("/home/somename"),
      UnixDevice("/dev/null"),
    )
  implicit val network: AvailableData[Network.type] = _ =>
    List[Data.Aux[Network.type]](
      Remote("192.168.0.1")
    )
}

def availableData[S <: Source](src: S)(implicit 
  ad: AvailableData[S]
): List[Data.Aux[S]] = ad.availableData(src)

来源：https://stackoverflow.com/questions/64467354/type-safety-with-adt-and-aux-pattern

标签

scala

type-safety

algebraic-data-types