Scala 具有通用返回类型的Spark数据集转换方法赢得'；不编译

我有以下case类，它们是从trait扩展而来的

package com.mypackage.spark.event

case class TypedEvent(id: String, timestamp: Long, `type`: String)

sealed trait Event{
  def id: String
  def timestamp: Long
}

case class CreationEvent(id: String, timestamp: Long) extends Event

case class DeleteEvent(id: String, timestamp: Long) extends Event

我需要将指定类型的数据集

TypedEvent

转换为从

Event trait

扩展而来的另一个数据集类型，使用数据集类中的方法和模式匹配机制，如下所示（我使用的是Spark 2.3.1）：

因此，我尝试向

transfrom

方法本身添加一个类型参数，如下所示：

.transform[_
import spark.implicits._

val jsonDF = spark.read.json(pathToJsonFile)
val typedEventsDS = jsonDF.select("id", "timestamp", "type").as[TypedEvent]

val eventTypes = Array("CreateEvent", "DeleteEvent" , ...)

eventTypes.foreach(eventType => {
  val result = typedEventsDS.filter($"type" <=> eventType)
.transform(featurize(spark, eventType)) // line 61
  /**
   * ...
   */
})

def featurize (spark: SparkSession, eventType: String): Dataset[TypedEvent] => Dataset[_ <: Event] = dataset => {

  import spark.implicits._

  eventType match {
    case "CreateEvent" => dataset.as[CreationEvent]
    case "DeleteEvent" => dataset.as[DeleteEvent]
    ...
  }
}

Error:(61, 12) no type parameters for method transform: (t:
org.apache.spark.sql.Dataset[com.mypackage.spark.event.TypedEvent] =>
org.apache.spark.sql.Dataset[U])org.apache.spark.sql.Dataset[U] exist so 
that it can be applied to arguments 
org.apache.spark.sql.Dataset[com.mypackage.spark.event.TypedEvent] =>
org.apache.spark.sql.Dataset[_ <: com.mypackage.spark.event.Event])
--- because ---
argument expression's type is not compatible with formal parameter type;

found:
org.apache.spark.sql.Dataset[com.mypackage.spark.event.TypedEvent] =>
org.apache.spark.sql.Dataset[_ <: com.mypackage.spark.event.Event]
required:
org.apache.spark.sql.Dataset[com.mypackage.spark.event.TypedEvent] =>
org.apache.spark.sql.Dataset[?U]
      .transform(featurize(spark, eventType))

  def featurize[T <: Event](spark: SparkSession, eventType: String): Dataset[TypedEvent] => Dataset[T] = 
dataset => { /* ... same ... */}