Manifest for higher-kinded type missing

Question

I am having trouble with scalac not finding a manifest for a higher-kinded internal type I want to use. Consider some list like type with a constructor:

trait L[S, A]

def emptyList[S, A: Manifest]: L[S, A] = ???

Now another type which gets constructed from its companion object like this:

object Tree {
  private class Leaf[S, A]
  private class Impl[S, A](l: L[S, Leaf[S, A]]) extends Tree[S, A]

  def empty[S, A]: Tree[S, A] = new Impl(emptyList[S, Leaf[S, A]])
}
trait Tree[S, A]

It fails because scalac wants a manifest for type Leaf[S, A]. Why isn't that available? I don't want to relax visibility of Leaf and clutter the constructor by asking to pass in a manifest argument.

I don't understand this at all—I thought that from the JVM perspective, the arrays that are constructed in L boil down to Array[Leaf[_, _]] aka Array[java.lang.Object], so what's the point of this refusal?

Any ways to find the manifest?

Answer 1

You have to provide manifest for all your parameters, in effect creating a complete manifest chain:

def emptyList[ S, A : Manifest ] {}
class Leaf[ S, A ]
def emptyTree[ S: Manifest, A : Manifest ] { emptyList[ S, Leaf[ S, A ]]}