Make the Preferences trait contravariant in its type parameter

[davidrpugh]

Preference orderings are naturally contravariant in their type parameters. Suppose that A is a sub-type of B, then a preference ordering defined over type B can be used anywhere such an ordering defined over type A is needed implying that Ordering[B] is a sub-type of Ordering[A]. In the standard library Ordering is actually invariant in its type parameter (see nice discussion on SO for the technical reasons for this as well as the possibility that it will be fixed in the future).

One way to make Preferences contravariant is as follows...

trait Preferences[-A] {
  def ordering[B](implicit ev: B <:< A): Ordering[B]
}

Another approach would be to make A and upper bound.

trait Preferences[-A] {
  def ordering[B <: A]: Ordering[B]
}

[davidrpugh]

I posted this question on SO and apparently the second solution is considered best practice.

[davidrpugh]

Another possible implementation for the Preferences trait that might be useful...

trait Preference[-A] {
  self =>
  
  // should be defined as described in the Ordering[A] trait in Scala std lib
  def compare[B <: A]: (B, B) => Int
  
  def ordering[B <: A]: Ordering[B] = {
    new Ordering[B] {
      def compare(b1: B, b2: B): Int = {
        self.compare(b1, b2)
      } 
    }
  }
  
  // should return b1 if b1 is weakly preferred to b2; else b2 
  def prefers[B <: A](b1: B, b2: B): B = {
    if (compare(b1, b2) <= 0) b2 else b1
  }
  
}