Class type required but T found

Question

I can't create an actor for some reason (here is a simple version of my class hierarchy):

abstract class Class1[T <: Class2[_]: ClassTag] extends Actor {
   //....
   val res = List(1, 2, 3) map { x => context actorOf Props(new T(x)) } // error
}

abstract class Class2[U <: Class3 : ClassTag](a: Int) extends Actor { ... }
abstract class Class3(b: Int) extends Actor

But there is an error saying class type required but T found.

Answer 1

You can't call new T(x) with type parameter T. There could be no such constructor for T:

class WithoutSuchConstructor extends Class2[Class3](1)

You should specify method to create T explicitly:

abstract class Class1[T <: Class2[_]: ClassTag] extends Actor {
  //....
  def createT(i: Int): T
  val res = List(1, 2, 3) map { x => context actorOf Props(createT(x)) }
}

Alternatively:

abstract class Class1[T <: Class2[_]: ClassTag](createT: Int => T) extends Actor {
  //....
  val res = List(1, 2, 3) map { x => context actorOf Props(createT(x)) }
}

Answer 2

One approach I have used involves creating an 'instantiator' trait which can be used to create instances of types for which an implicit instantiator exists:

trait Instantiator[+A] {
  def apply(): A
}

object Instantiator {
  def apply[A](create: => A): Instantiator[A] = new Instantiator[A] {
    def apply(): A = create
  }
}

class Foo() { ... }

object Foo {
  implicit val instantiator: Instantiator[Foo] = Instantiator { new Foo() }
}

// def someMethod[A]()(implicit instantiator: Instantiator[A]): A = {
def someMethod[A : Instantiator](): A = {
  val a = implicitly[Instantiator[A]].apply()
  ...
}

someMethod[Foo]()

Answer 3

I think this is because of JVM restriction also known as "type erasure". http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

also see "Cannot Create Instances of Type Parameters" at http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html

By the way C# allows you to write:

new T()

when you define a restriction

where T: new()

but unfortunately constructor must be parameterless