NumericOption.scala

object NumericOption {
  implicit def numericOption[A](implicit N: Numeric[A]): Numeric[Option[A]] =
    new Numeric[Option[A]] {
      def fromInt(x: Int): Option[A] = Option(N.fromInt(x))
      def negate(x: Option[A]): Option[A] = x.map(N.negate(_))

      // Not sure whether these should return zero or throw an error...
      def toDouble(x: Option[A]): Double = x.map(N.toDouble(_)).getOrElse(0)
      def toFloat(x: Option[A]): Float = x.map(N.toFloat(_)).getOrElse(0)
      def toInt(x: Option[A]): Int = x.map(N.toInt(_)).getOrElse(0)
      def toLong(x: Option[A]): Long = x.map(N.toLong(_)).getOrElse(0)

      def minus(x: Option[A], y: Option[A]): Option[A] = for (a <- x; b <- y) yield N.minus(a, b)
      def plus(x: Option[A], y: Option[A]): Option[A] = for (a <- x; b <- y) yield N.plus(a, b)
      def times(x: Option[A], y: Option[A]): Option[A] = for (a <- x; b <- y) yield N.times(a, b)

      def compare(x: Option[A], y: Option[A]): Int = {
        (x, y) match {
          case (Some(x), Some(y)) => N.compare(x, y)
          case (Some(_), None) => 1
          case (None, Some(_)) => -1
          case (None, None) => 0
        }
      }
    }

  def main(args: Array[String]): Unit = {
    println(List[Option[Double]](Some(2.5), Some(3.5)).sum) // Some(6.0)
    println(List[Option[Double]](Some(2.5), None).sum) // None
  }
}