Trait Inheritance

28 March 2015 - Reykjavik, Iceland

You can add multiple traits that invoke eachother, starting with the last one.

trait WarnLogger extends Logger {
  override def log(msg: String) { super.log(s"[WARN] $msg") }
}

trait ShortLogger extends Logger {
  val maxLength = 10
  override def log(msg: String) {
    super.log(
      if (msg.length <= maxLength) msg else msg.substring(0, maxLength - 3) + "..."
    )
  }
}

Both modify message being passed to Logger.

super.log for traits is not the same as super.log for classes.

Instead, super.log calls the next trait in the trait hierarchy, which is dependent on the order with which traits are added. FILO order, unless something hairy comes up. So inheriting from both traits in different order results in different logs:

val foo1 = new Foo with ConsoleLogger with WarnLogger with ShortLogger
foo1.log('Testing logger')
// [WARN] Testing lo...

val foo2 = new Foo with ConsoleLogger with ShortLogger with WarnLogger
foo2.log('Testing logger')
// [WARN] Tes...

So beware the fact that you can’t tell from a trait’s definition which method will be invoked by a call to a super method. However, if you find that you do need to control which super method is called, you can specify it in brackets. With the WarnLogger, it would look like:

trait WarnLogger extends Logger {
  override def log(msg: String) {
    super[ConsoleLogger].log(s"[WARN] $msg")
  }
}

This only works with traits/classes that are immediate supertypes. Anything further away in the inheritance tree won’t work.