How to use Scala's Ordering#reverse with a TreeMap?

Scala's Ordering trait has a method reverse which seems to be the "official" way to get a TreeMap which is sorted "the wrong" way.

The snippet of the trait looks like this:

trait Ordering[T] extends开发者_开发百科 Comparator[T] with PartialOrdering[T] with Serializable {
  outer =>

  override def reverse: Ordering[T] = new Ordering[T]{
    override def reverse = outer
    def compare(x: T, y: T) = outer.compare(y, x)
  }
  /*snip*/
}

I thought it would work comparable to Java's Collections.reverseOrder, but Ordering.reverse doesn't work of course.

How can I use the reverse Ordering with a TreeMap, e. g.:

new TreeMap[Foo, Bar](/*???*/)

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new TreeMap[Foo, Bar]()(implicitly[Ordering[Foo]].reverse)

Assuming, of course, that Ordering[Foo] is implicitly available (such as Ordering[Int] or Ordering[String]). If you have it defined as object X, just pass X.reverse instead.

Note that the first set of parenthesis here must be empty -- it is only the second set of parenthesis that receive the implicit parameter in this particular case.

---

new TreeMap[Foo,Bar]()(Ordering[Foo].reverse)

also works. Nice! Succinct and without implicitly magic.

What's really going on here, if I'm not mistaken:

• Odering[Foo] calls Ordering.apply, which has an implicit parameter (which is itself of type Ordering)
• the implicit def Ordering.ordered is found (I'm not sure why) for that parameter and creates a new Ordering object
• that Ordering object is passed to and returned by Ordering.apply
• reverse is called on that Ordering object and creates another new Ordering object

I'm mixing run-time and compile-time lingo here, but you'll catch my drift.

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new TreeMap[Foo,Bar]()(Ordering.ordered[Foo].reverse) should work if there is no object for Ordering[Foo], as long as Foo extends Ordered[Foo] (or more generally Foo extends Comparable[Foo]).