hm.scala

sealed trait MaybeNumeric[A]
case class  YesNumeric[A](n: Numeric[A]) extends MaybeNumeric[A]
case class  NoNumeric[A]() extends MaybeNumeric[A]

object MaybeNumeric {
  implicit def instance[A](implicit ev: Numeric[A]): MaybeNumeric[A] =
    YesNumeric(ev)
}

def add[A](a: A, b: A)(implicit ev: MaybeNumeric[A] = NoNumeric[A]()): Option[A] =
  ev match {
    case NoNumeric() => None
    case YesNumeric(ev) => Some(ev.plus(a, b))
  }



scala> add(1.0, 2.0)
res3: Option[Double] = Some(3.0)

scala> add(3,4)
res4: Option[Int] = Some(7)

scala> add("five", "six")
res5: Option[String] = None