如何在 Scala 中通過動態輸入來選擇 typeclass

Question

有一個帶有列的光滑表：

def name: Rep[Option[String]] = ???
def email: Rep[String] = ???
def fraudScores: Rep[Int] = ???

還有 typeclass 來計算不同字段的速率：

trait Rater[T] {
  def apply(rep: T): Result
}

object Rater {
  def getRater[T](t: T)(implicit rater: Rater[T]): Result = rater(t)

  implicit val int: Rater[Rep[Int]] = v => calculateRate(v, _)
  implicit val str: Rater[Rep[String]] = v => calculateRate(v, _)
  implicit val strOpt: Rater[Rep[Option[String]]] = v => calculateRate(v, _)
}

和 map：

val map: Map[String, Rep[_ >: Option[String] with String with Int]] = Map(
  "name" -> name,
  "email" -> email,
  "scores" -> fraudScores
)

我想做的是根據動態輸入獲取正確的實例，例如：

val fname = "scores" // value getting from http request
map.get(fname).fold(default)(f => {
    val rater = getRater(f)
    rater(someVal)
})

但是收到錯誤，即Rep[_ >: Option[String] with String with Int]沒有隱式，是否有一些解決方法？

Answer 1

我認為，您的問題是Map是一種錯誤的表示方式。 使用案例 class：

    case class Foo(
       name: Rep[Option[String]], 
       email: Rep[String], 
       score: Rep[Int]
    )

    def applyRater[T : Rater](t: T) = implicitly[Rater[T]](t)
    def rate(foo: Foo, what: String) = what match {
      case "name" => applyRater(foo.name)
      case "email" => applyRater(foo.email)
      case "score" => applyRater(foo.score)
      case _ => default
    }