Scala TypeTag缺少类名

Question

I'm trying to get some information using Scala's reflect library :

abstract class Model

class Person extends Model

class Car extends Model

abstract class AbstractDao[T <: Model]

object PersonDao extends AbstractDao[Person]

object CarDao extends AbstractDao[Car]

object DataLoader {

  val daos = Seq(PersonDao, CarDao)
  val modelToString = daos.map(genericImportEntities(_))

  val modelToString2 = Seq(genericImportEntities(PersonDao), genericImportEntities(CarDao))

  private def genericImportEntities[T <: Model](dao: AbstractDao[T])
                                               (implicit
                                                t2: TypeTag[T]
                                               ): String = {

    t2.tpe.toString
  }
}

If I call modelToString, the output is List(_1, _1)

With modelToString2, it is List(Person, Car)

Any idea how can I make modelToString work?

Answer 1

The issue is that type of daos is Seq[AbstractDao[_]] . So when calling daos.map(genericImportEntities(_)) , T is an unknown type which the compiler calls _1 . Generally, TypeTag s are only useful when you know the static types at the point where the compiler should insert them, and in this case you don't.

The easiest way to fix this would be to move TypeTag into AbstractDao :

abstract class AbstractDao[T <: Model](implicit val tag: TypeTag[T])

private def genericImportEntities[T <: Model](dao: AbstractDao[T]) = 
  dao.tag.tpe.toString

Then the compiler inserts the tags at definition of PersonDao and CarDao and they can be used later in genericImportEntities .