生成器中的Scala类型范围

Question

I think I narrowed my problem down to the following:

trait Thing

case class SpecificThing(i: Int) extends Thing

trait ThingSource[T <: Thing] {
  def  next: T
}

class SpecificThingSource() extends ThingSource[SpecificThing] {
  override def next = SpecificThing(1)
}

object ThingSource {
  def apply[A <: Thing, B <: ThingSource[A]](sourceType: String): B = {
    sourceType match {
      case "specific" => new SpecificThingSource()
    }
  }
}

val a = ThingSource("specific").next

The compiler seems to be happy about most of it, except for the builder part. Here it gives this error:

Expression of type SpecificThingSource doesn't conform to the expected type B

I would expect that with SpecificThing being a subtype of type Thing , and SpecificThingSource being of type ThingSource[SpecificThing] , there is no room for ambiguity. What am I missing? Is it even possible to use a builder in this way? Or are there better ways to achieve what I want?

Answer 1

If you want to "return a subclass of ThingSource " , then you need existential , not universal quantification:

  def apply(sourceType: String)
  : B forSome { type B <: ThingSource[A] forSome { type A <: Thing}} = {
    sourceType match {
      case "specific" => new SpecificThingSource()
    }
  }

which can be written shorter as

  def apply(sourceType: String): ThingSource[_] = {
    sourceType match {
      case "specific" => new SpecificThingSource()
    }
  }