如何避免在 Scala 中调用 asInstanceOf

Question

这是我的代码的简化版本。

我怎样才能避免打电话给asInstanceOf（因为这是一个糟糕的设计解决方案的味道）？

sealed trait Location
final case class Single(bucket: String)     extends Location
final case class Multi(buckets: Seq[String]) extends Location

@SuppressWarnings(Array("org.wartremover.warts.AsInstanceOf"))
class Log[L <: Location](location: L, path: String) { // I prefer composition over inheritance
  // I don't want to pass location to this method because it's a property of the object
  // It's a separated function because there is another caller
  private def getSinglePath()(implicit ev: L <:< Single): String = s"fs://${location.bucket}/$path"

  def getPaths(): Seq[String] =
    location match {
      case _: Single => Seq(this.asInstanceOf[Log[_ <: Single]].getSinglePath())
      case m: Multi  => m.buckets.map(bucket => s"fs://${bucket}/$path")
    }
}

Answer 1

尝试类型类

class Log[L <: Location](location: L, val path: String) {
  def getSinglePath()(implicit ev: L <:< Single): String = s"fs://${location.bucket}/$path"
  def getPaths()(implicit gp: GetPaths[L]): Seq[String] = gp.getPaths(location, this)
}

trait GetPaths[L <: Location] {
  def getPaths(location: L, log: Log[L]): Seq[String]
}
object GetPaths {
  implicit val single: GetPaths[Single] = (_, log) => Seq(log.getSinglePath())
  implicit val multi:  GetPaths[Multi]  = (m, log) => m.buckets.map(bucket => s"fs://${bucket}/${log.path}")
}

通常，类型类是模式匹配的编译时替换。

我必须将 getSinglePath 设为 public 并将 path 设为 val，以便在 GetPaths 中提供对它们的访问权限。如果您不想这样做，可以将类型类嵌套到Log

class Log[L <: Location](location: L, path: String) {
  private def getSinglePath()(implicit ev: L <:< Single): String = s"fs://${location.bucket}/$path"
  def getPaths()(implicit gp: GetPaths[L]): Seq[String] = gp.getPaths(location)

  private trait GetPaths[L1 <: Location] {
    def getPaths(location: L1): Seq[String]
  }
  private object GetPaths {
    implicit def single(implicit ev: L <:< Single): GetPaths[L] = _ => Seq(getSinglePath())
    implicit val multi: GetPaths[Multi] = _.buckets.map(bucket => s"fs://${bucket}/$path")
  }
}

---

实际上我们不必显式传递location，也不需要L1

class Log[L <: Location](location: L, path: String) {
  private def getSinglePath()(implicit ev: L <:< Single): String = s"fs://${location.bucket}/$path"
  def getPaths()(implicit gp: GetPaths): Seq[String] = gp.getPaths()

  private trait GetPaths {
    def getPaths(): Seq[String]
  }
  private object GetPaths {
    implicit def single(implicit ev: L <:< Single): GetPaths = () => Seq(getSinglePath())
    implicit def multi(implicit ev: L <:< Multi):   GetPaths = () => location.buckets.map(bucket => s"fs://${bucket}/$path")
  }
}

现在GetPaths 是零参数type class，与magnet pattern 略有相似。