ordered.scala

def binarySearch[B, A <% Ordered[B]](a: Array[A], v: B) = {
  def recurse(low: Int, high: Int): Option[Int] = (low + high) / 2 match {
    case _ if high < low => None
    case mid if a(mid) > v => recurse(low, mid - 1)
    case mid if a(mid) < v => recurse(mid + 1, high)
    case mid => Some(mid)
  }
  recurse(0, a.size - 1)
}

case class Thing(val n:Int)

implicit val compThingToThing: Thing => Ordered[Thing] =
  t => new Ordered[Thing] {
    def compare(that: Thing): Int = { t.n - that.n }
  }
  
implicit val compThingToInt: Thing => Ordered[Int] =
  t => new Ordered[Int] {
    def compare(that: Int): Int = { t.n - that }
  }
  
  
val a:Array[Thing] = Array(Thing(1), Thing(3), Thing(5), Thing(6))

binarySearch(a, Thing(5)) // compiles, compThingToThing is available
binarySearch(a, 5) // compiles, compThingToInt is available
binarySearch(a, "test") // doesn't compile, it isn't known how to compare Things to strings