An implicit pearl in Scala

gistfile1.scala

/** 
 * An answer from Jason Zaugg on the scala-internals mailing list
 * 
 *  Implicit parameters can have defaults.
 */

scala> implicit def OptionalImplicit[A <: AnyRef](implicit a: A = null) = Option(a)
OptionalImplicit: [A <: AnyRef](implicit a: A)Option[A]

scala> implicitly[Option[Ordering[Int]]]
res0: Option[Ordering[Int]] = Some(scala.math.Ordering$Int$@1443628)

scala> implicitly[Option[Ordering[Any]]]
res1: Option[Ordering[Any]] = None

/**
 * A variation on the theme, proposed here: http://stackoverflow.com/questions/4364601/adding-a-validity-check-dependent-on-a-typeclass
 */

class My2Col[A](a1 : A, a2 : A)(implicit num: Numeric[A] = null){
  for{check <- Option(num); if(check.gteq(a1, a2))}
     throw new IllegalArgumentException
}