Scala self-type and generic class

问题

abstract class Bar[M] {
  def print(t: M): Unit = {
    println(s"Bar: ${t.getClass()}")
  }
}

trait Foo[M] {
  this: Bar[M] =>
  def print2(t: M): Unit = {
    println(s"Foo: ${t.getClass()}")
  }
}

object ConcreteBar extends Bar[Int] with Foo[Int] {}
object ConcreteFooBar extends Bar[Int] with Foo[Int] {}

object Test {
  def main(args: Array[String]): Unit = {
    ConcreteBar.print(1)
    ConcreteFooBar.print2(1)
  }

In the example above, is there a way so that we don't have to repeat the type in the self-typed "bar" trait? Therefore we could declare ConcreteFooBar like this:

object ConcreteFooBar extends Bar[Int] with Foo {}

回答1:

You can use an abstract type instead of a type parameter for Foo, like this:

abstract class Bar[M] {
  type Base = M
  def print(t: M): Unit = {
    println(s"Bar: ${t.getClass()}")
  }
}

trait Foo {
  type Base
  def print2(t: Base): Unit = {
    println(s"Foo: ${t.getClass()}")
  }
}

来源：https://stackoverflow.com/questions/47554414/scala-self-type-and-generic-class