Scala covariant type T occurs in invariant position

Question

I have this trait:

trait Delivery[T] {
  def form(): Form[T]
}

where Form is from play2 framework.

Then I have object that implements Delivery trait:

case class NewPostValidator(town: String, number: Int)

object NewPost extends Delivery[NewPostValidator]{
  def form(): Form[NewPostValidator] = Form(mapping(
    "town" -> nonEmptyText,
    "number" -> number)(NewPostValidator.apply)(NewPostValidator.unapply))
}

Now I want to write function which accepts List of objects that implements trait Delivery. And I can't write type for parameter of it. If I try to write this like

list: List[Delivery[AnyRef]]

I've got type mismatch error and if I change Delivery trait to:

trait Delivery[+T] {
    def form(): Form[T]
}

I've got Scala covariant type T occurs in invariant position error. How can I describe type for this parameter?

Answer 1

I'm not sure if you want all the forms inside the method to be of the same underlying type or if you care about the return type inside Form . It would be good to understand what's going on here.

trait Delivery[T] {
  def form(): Form[T]
}

case class NewPostValidator(town: String, number: Int)

object NewPost extends Delivery[NewPostValidator]{
  def form(): Form[NewPostValidator] = Form(mapping(
    "town" -> nonEmptyText,
    "number" -> number)(NewPostValidator.apply)(NewPostValidator.unapply))

  def accept[T <: Delivery[_]](list: List[T]): List[Form[_]] = {
    list.map(_.form())
  }

}