Scala 2.8 TreeMap and custom Ordering

Question

I'm switching from scala 2.7 and ordered to scala 2.8 and using ordering. It looks quite straight forward but I was wondering could I make it a little less verbose. For example:

scala> case class A(i: Int)
defined class A
scala> object A extends Ordering[A] { def compare(o1: A, o2: A) = o1.i - o2.i}
defined module A

If I then try to create a TreeMap I get an error

scala> new collection.immutable.TreeMap[A, String]()
<console>:10: error: could not find implicit value for parameter ordering: Ordering[A]
       new collection.immutable.TreeMap[A, String]()
       ^

However if I explicitly specify the object A as the ordering it works fine.

scala> new collection.immutable.TreeMap[A, String]()(A)
res34: scala.collection.immutable.TreeMap[A,String] = Map()

Do I always have to explicitly specify the ordering or is there a shorter format?

Thanks

Answer 1

Mind you, there's a slightly less verbose way of creating an Ordering:

implicit val OrderingA = Ordering.by((_: A).i)

The main advantage of Ordering being you can provide many of them for the same class. If your A class is truly Ordered, then you should just extend that. If not, instead of using implicits, you may pass an Ordering explicitly:

new collection.immutable.TreeMap[A, String]()(Ordering.by(_.i))