Question

A class of mine is a generic and defines iv as MHashMap[DateTime, T]. MHashMap is an alias for scala.collection.mutableHashMap. Then I use this code to expose a minimum finding functionality

def minimum: BigDecimal = {
  iv match {
    case dtMcM: MHashMap[DateTime, MyCustomClass] => dtMcM.minBy(_._2.bdField)._2.bdField
    case dtBdM: MHashMap[DateTime, BigDecimal] => dtBdM.minBy(_._2)._2
    case dtDlM: MHashMap[DateTime, Double] => dtDlM.minBy(_._2)._2.toBigDecimal
    case dtItM: MHashMap[DateTime, Int] => dtItM.minBy(_._2)._2
    case _ => throw new IllegalArgumentException("Unsupported underlying type")
  }
}

But the complier complains:

No implicit Ordering defined for T with Int.
case dtItM: MHashMap[DateTime, Int] => dtItM.minBy(_._2)._2

And the same for Double and BigDecimal cases.

How is that the compiler can not even recognize Int to apply default ordering? Curious thing is that it desn’t seem to have any problem with a BigDecimal field of my own custom class.

Answer 1

Well, Int <: T with Int. Now, if you are going to use Ordering[Int] for it, then you need Ordering[T with Int] <: Ordering[Int], which means Ordering must be contravariant. Unfortunately, Ordering is not contravariant (even though it could — and it is in Scalaz).