In Java, we can abstract over type variables. After all, we don't write a length function for List<String>, List<Integer>, List<Person> and so on.

With Scala, we can abstract over type constructors.

trait Mappable[F[_]] {
  def map[A, B](fa: F[A], f: A => B): F[B]
}
          

Some type constructors that are Mappable: