Trait Parameters

Scala 3 enables traits to have parameters, just like a class.

For example, here is a trait Greeting:

trait Greeting(val name: String):
  def msg = s"How are you, $name"

A class, enum, or object can extend Greeting as follows:

class Greet extends Greeting("Bob"):
  println(msg)

However if another trait extends Greeting then it must not pass arguments:

trait FormalGreeting extends Greeting:
  override def msg = s"How do you do, $name"

If you want a class to greet Bob formally, then you should extend both FormalGreeting and Greeting:

class GreetFormally extends FormalGreeting, Greeting("Bob"):
  println(msg)