Custom sort in scala - scala

Question

I cannot make this work:
abstract class AlternativeSortingType[T, B](implicit val ord: scala.math.Ordering[B]) {
def convert(t: T): B
}
case class LengthSortingType() extends AlternativeSortingType[String, Int] {
def convert(t: String): Int = t.length
}
class ProduceResult {
var c: AlternativeSortingType[String, _] = LengthSortingType()
def sort(l: List[String]) = l.sortBy(c.convert(_))(c.ord)
}
It complains with the following:
<console>:20: error: type mismatch;
found : scala.math.Ordering[_$1]
required: scala.math.Ordering[Any]
Note: _$1 <: Any, but trait Ordering is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
def sort(l: List[String]) = l.sortBy(c.convert(_))(c.ord)
^
I already tried to put the type B to the method only, but I don't see how to get rid of the wilcard. I want to be able to sort by specifying any kind of converter.
How to make the type match?

Answer 1

I found a workaround in which I can even now make composition of orderings.
abstract class AlternativeSortingType[T] extends Ordering[T] { self =>
def &&(other: AlternativeSortingType[T]): AlternativeSortingType[T] = new AlternativeSortingType[T] {
def compare(e: T, f: T): Int = {
val ce = self.compare(e, f)
if(ce == 0) other.compare(e, f) else ce
}
}
}
case class LengthSortingType() extends AlternativeSortingType[String] {
def compare(e: String, f: String): Int = e.length - f.length
}
case class DictionarySortingType() extends AlternativeSortingType[String] {
def compare(e: String, f: String): Int = if(e < f) -1 else if(e==f) 0 else 1
}
class ProduceResult {
var c: AlternativeSortingType[String] = LengthSortingType() && DictionarySortingType()
def sort(l: List[String]) = l.sortWith((e,f) => c.compare(e, f) < 0)
}