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

Notice the word "implicit" in the diagnostic. The parameter is declared implicit meaning the compiler will try to find a suitable value in scope at the point you invoke the constructor. If you make your Ordering an implicit value, it will be eligible for this treatment by the compiler:

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

scala> val tm1 = new collection.immutable.TreeMap[A, String]()
tm1: scala.collection.immutable.TreeMap[A,String] = Map()

Edit:

That example works in the REPL because the REPL encloses your code in invisible class definitions. Here's one that works free-standing:

case class A(val i:Int) extends Ordered[A] { def compare(o:A) = i - o.i }

object A { implicit object AOrdering extends Ordering[A] { def compare(o1: A, o2: A) = o1.i - o2.i } }

class B {
    import A.AOrdering

    val tm1 = new collection.immutable.TreeMap[A, String]()
}

Answer 2

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))

Answer 3

Instead of extending Ordering[A], try extending Ordered[A]. Like so:

scala> case class A(val i:Int) extends Ordered[A] {def compare(o:A) = i-o.i}
defined class A

scala> A(1)<A(2)
res0: Boolean = true

scala> A(1)<A(0)
res1: Boolean = false

scala> new collection.immutable.TreeMap[A, String]()
res3: scala.collection.immutable.TreeMap[A,String] = Map()