Trait Parameters

Dotty allows traits to have parameters, just like classes have parameters.

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

class C extends Greeting("Bob") {
  println(msg)
}

Arguments to a trait are evaluated immediately before the trait is initialized.

One potential issue with trait parameters is how to prevent ambiguities. For instance, you might try to extend Greeting twice, with different parameters.

class D extends C with Greeting("Bill") // error: parameter passed twice

Should this print "Bob" or "Bill"? In fact this program is illegal, because it violates the second rule of the following for trait parameters:

1. If a class C extends a parameterized trait T, and its superclass does not, C must pass arguments to T.

2. If a class C extends a parameterized trait T, and its superclass does as well, C may not pass arguments to T.

3. Traits may never pass arguments to parent traits.

Here's a trait extending the parameterized trait Greeting.

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

As is required, no arguments are passed to Greeting. However, this poses an issue when defining a class that extends FormalGreeting:

class E extends FormalGreeting // error: missing arguments for `Greeting`.

The correct way to write E is to extend both Greeting and FormalGreeting (in either order):

class E extends Greeting("Bob") with FormalGreeting

Reference

For more info, see Scala SIP 25.