Scala traits hierarchy doesn't compile

Question

In my venture to learning Scala, I created the following hierarchy:

trait Animal {
  val name: String = "Animal"
}

trait HasLegs {
  this: Animal =>
  val numLegs: Int
}

abstract class Terrestrial extends Animal with HasLegs {
  val name: String = "Terrestrial"
}

class Dog(val name: String) extends Terrestrial {
  def numLegs = 4

  override def toString = {
    f"My name is: $name%s. I'm a Dog and I've $numLegs%d legs. I'm from the ${super.name}%s family."
  }
}

However, the compiler complains that in Dog.toString "super may be not be used on value name". What am I doing wrong?

Arne Claassen · Accepted Answer · 2015-05-10T00:12:04.407

There were a couple of issues, primarily stemming from using val in superclasses, which locks it down to a value, rather than something that can be overridden and accessed via super. There were also some problems with not specifying override:

trait Animal {
  // in order for super to be called on this it has to be a def
  def name: String = "Animal"
}

trait HasLegs {
  this: Animal =>
  val numLegs: Int
}

abstract class Terrestrial extends Animal with HasLegs {

  // this needs to be an override of Animal.name
  override def name: String = "Terrestrial"
}

class Dog(override val name: String) extends Terrestrial {

  // since the trait specified this as a val, the implementation
  // also needs to be a val
  val numLegs = 4

  override def toString = {
    f"My name is: $name%s. I'm a Dog and I've $numLegs%d legs. I'm from the ${super.name}%s family."
  }
}

I've commented on changes inline.

As for the reason why super cannot be used on val see SI-899

This is the intended behavior, and it was changed so that traits could override vals to be more uniform with classes.

So here the name from Terrestrial will be invoked. Is there a way to get the name from the Animal? Or does super work for immediate superclass only? — Abhijit Sarkar, May 10 '15 at 02:11
See Jon Skeet's answer on why there is no super.super (he talks about Java but the same holds true for scala http://stackoverflow.com/questions/586363/why-is-super-super-method-not-allowed-in-java — Arne Claassen, May 10 '15 at 02:19

Scala traits hierarchy doesn't compile

1 Answers1