具有ADT和Aux模式的类型安全

问题描述

我正在使用ADT和Aux-pattern设计类型安全的代码，无法摆脱某些asInstanceOf。这是示例：

  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]]
      )
    }
  }

在这种情况下，很明显asInstanceOf是正确的，但是有没有办法操纵它？

我当时正在考虑S <: Source: classtag，但在这里似乎没有用。也许还有其他反思技巧？

解决方法

请参阅说明为什么签名

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")
    )
  }
}

不起作用，有几种解决方法：

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

例如，尝试类型类

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"),)
  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)