Get name of abstract type

Question

I'm trying to create a trait that would provide the name of the abstract type that is added in the sub-class:

trait T {
  type T
  def myClassOf[T:ClassTag] = implicitly[ClassTag[T]].runtimeClass
  def getType = {
    myClassOf[T].getSimpleName
  }
}

class TT extends T {
  type T = String
}

However, this fails to compile:

Error:(7, 15) not enough arguments for method myClassOf: (implicit evidence$1: scala.reflect.ClassTag[T.this.T])Class[_].
Unspecified value parameter evidence$1.
    myClassOf[T].getSimpleName
             ^

But it works fine if I move the getType method to the sub-class. Can someone explain why and whether there is a way to do this call from the sub-class?

Answer 1

At the point where you call myClassOf[T] T is still abstract, so the compiler can not generate a ClassTag for it. You can fix it by delaying the generation of the ClassTag[T] until T is known.

trait Trait {
  type T
  def myClassOf[A:ClassTag] = implicitly[ClassTag[A]].runtimeClass
  def getType(implicit tag: ClassTag[T]) = {
    myClassOf[T].getSimpleName
  }
}

class Sub extends Trait {
  type T = String
}

If adding an implicit parameter is impossible for some reason I think the best way is probably requiring some method getClassT to be implemented in subclasses. Since its return type is Class[T] it's difficult to provide a wrong implementation in a subclass.

trait Trait {
  type T
  def getType = {
    getClassT.getSimpleName
  }
  def getClassT: Class[T]
}

class Sub extends Trait {
  type T = String
  def getClassT = classOf[T]
}