Using trait method in the class constructor - scala

Question

I have a trait and a class that extends the trait. I can use the methods from the trait as follows:
trait A {
def a = ""
}
class B(s: String) extends A {
def b = a
}
However, when I use the trait's method in the constructor like this:
trait A {
def a = ""
}
class B(s: String) extends A {
def this() = this(a)
}
then the following error appears:
error: not found: value a
Is there some way to define default parameters for the construction of classes in the trait?
EDIT: To clarify the purpose: There is the akka-testkit:
class TestKit(_system: ActorSystem) extends { implicit val system = _system }
And each test looks like this:
class B(_system: ActorSystem) extends TestKit(_system) with A with ... {
def this() = this(actorSystem)
...
}
because I want to create common creation of the ActorSystem in A:
trait A {
val conf = ...
def actorSystem = ActorSystem("MySpec", conf)
...
}

Answer 1

It's a little bit tricky because of Scala initialization order. The simplest solution I found is to define a companion object for your class B with apply as factory method:
trait A {
def a = "aaaa"
}
class B(s: String) {
println(s)
}
object B extends A {
def apply() = new B(a)
def apply(s: String) = new B(s)
}