How can I invoke the constructor of a Scala abstract type?

Question

I\'m trying to figure out how to invoke a constructor for a Scala abstract type:

class Journey(val length: Int)
class PlaneJourney(length: Int) extends Journey(l         


        

Answer 1

There is no direct way to invoke the constructor or access the companion object given only a type. One solution would be to use a type class that constructs a default instance of the given type.

trait Default[A] { def default: A }

class Journey(val length: Int)
object Journey {
  // Provide the implicit in the companion
  implicit def default: Default[Journey] = new Default[Journey] {
    def default = new Journey(0)
  }
}

class Port[J <: Journey : Default] {
  // use the Default[J] instance to create the instance
  def startJourney: J = implicitly[Default[J]].default
}

You will need to add an implicit Default definition to all companion objects of classes that support creation of a default instance.