How to use reflection on parameterized trait in Scala? - scala

Question

The access on Manifest seems to be tricky from a trait in scala.
How could this code compile in scala ?
trait SomeTraitOf[+A] {
def newInstanceOfA : A = /* necessary code to make it work */
}
(Related, it works fine as a parametized class :
class SomeTraitOf[A : Manifest] {
def newInstanceOfA(implicit m : Manifest[A]) : A =
m.erasure.newInstance.asInstanceOf[A]
}
but not with a covariant type parameter (+A))
Edit : The real stuff
sealed trait RootPeerProxy[+A] extends Proxy {
def peer: A
def self = peer
def peerManifest[B >: A](): Option[Manifest[B]]
private[scalavaadin] def newInstance() : Option[A]
}
trait PeerProxy[+A] extends RootPeerProxy[A] {
override def peerManifest[B >: A](): Option[Manifest[B]]
override def peer(): A = this.newInstance match {
case None => {throw new IllegalStateException("oups")}
case Some(a) => a
}
private[scalavaadin] override def newInstance() : Option[A] = peerManifest map { m => m.erasure.newInstance.asInstanceOf[A] }
}
Since traits cannot provide manifest for the parametized trait, the class implementing the trait should, but I am not getting it.

Answer 1

About covariance:
Since Manifest[A] is invariant in the parameter A, you can't do what you want directly. The usual strategy is to weaken the return type,
trait SomeTraitOf[+A] {
def newInstance[B >: A](implicit m: Manifest[B]): B = {
m.erasure.newInstance.asInstanceOf[B]
}
}
You can use the trait as follows,
class Parent
class Child extends Parent
val childBuilder = new SomeTraitOf[Child] {}
val parentBuilder: SomeTraitOf[Parent] = childBuilder
parentBuilder.newInstance // returns a Parent!
About View Bounds:
From your comment below, I guess you're also asking about "view bounds", which are just a concise way of declaring an implicit parameter. Your declaration
class SomeTraitOf[A : Manifest] { ...
basically translates to
class SomeTraitOf[A]()(implicit m0: Manifest[A]) { ....
Traits can't have view bounds because they can't take any (value) parameters. But that's not really a problem here, because in your example
class SomeTraitOf[A : Manifest] {
def newInstanceOfA(implicit m : Manifest[A]) : A =
m.erasure.newInstance.asInstanceOf[A]
}
you're not using the view bound! (You're using the parameter m instead.) If you wanted to use the view bound, you could do it this way:
class SomeTraitOf[A : Manifest] {
def newInstanceOfA : A =
implicitly[Manifest[A]].erasure.newInstance.asInstanceOf[A]
}