Ordering thenBy

ordering-then-by.scala

class RichOrdering[A](oa: Ordering[A]) {
  def thenBy[B: Ordering](f: A => B): Ordering[A] = new Ordering[A] {
    def compare(a1: A, a2: A) = oa.compare(a1, a2) match {
      case 0 => Ordering[B].compare(f(a1), f(a2))
      case x => x
    }
  }
}

implicit def ToRichOrdering[A](oa: Ordering[A]): RichOrdering[A] = new RichOrdering(oa)

case class Person(name: String, age: Int)
Ordering.by((_: Person).name).thenBy(_.age)

scalaz-order-thenby.scala

scala> import scalaz._, Scalaz._

scala> case class Person(name: String, age: Int)
defined class Person 

scala> val byName = orderBy((_: Person).name)byName: scalaz.Order[Person] = scalaz.Orders$$anon$2@37cae761 

scala> val byAge = orderBy((_: Person).age)
byAge: scalaz.Order[Person] = scalaz.Orders$$anon$2@64ad8f2b

// Should really come with Scalaz
// Order#compare returns Ordering; Semigroup[Ordering] provides
// lexicographical sorting:
// https://github.com/scalaz/scalaz/blob/6.0.3/core/src/main/scala/scalaz/Semigroup.scala#L44
scala> implicit def OrderSemigroup[A]: Semigroup[Order[A]] = 
  semigroup((o1, o2) => order((a1: A, a2: A) => o1.order(a1, a2) |+| o2.order(a1, a2)))
OrderingSemigroup: [A]=> scalaz.Semigroup[scalaz.Order[A]]

scala> val byAgeThenName = byAge |+| byName
byAgeThenName: scalaz.Order[Person] = scalaz.Orders$$anon$2@19163ae3

scala> byAgeThenName.order(Person("bob", 12), Person("bob", 13))
res5: scalaz.Ordering = LT

Jason,

would you mind if I borrow this example (with credit of course) for learn you a scalaz? It would fit perfectly in the simple typeclasses section.

Jordan

Go for it.

I committed the type class instance for Semigroup[Order] to Scalaz master, too.