A question came up during a talk I gave about how to provide a witness for the below `Functor` implementation. This is one (terrible) way to do it and should highlight how implicits can be used to provide the witness. There are definitely better ways to do this.

canigetawitness.scala

sealed trait Option2[+A]
case object None2 extends Option2[Nothing]
case class Some2[A](value: A) extends Option2[A]

trait Functor[F[_]] {

  def map[A,B](f: A => B): F[A] => F[B]

}

implicit object optionFunctor extends Functor[Option2] {
  def map[A,B](f: A => B): Option2[A] => Option2[B] = (oa: Option2[A]) => oa match {
    case None2 => None2
    case Some2(a) => Some2(f(a))
  }
}

implicit def getFunctor[A[_] : Functor](a: A[_]): Functor[A] = {
  implicitly[Functor[A]]
}

object MainProgram {

  def doThing(i: Int): String = i.toString

  def main(args: Array[String]) {

    val tmp: Option2[Int] = Some2(1)

    val opFunc: Option2[Int] => Option2[String] = tmp.map(doThing)

    println("*** the result: %s".format(opFunc(tmp)))

  }
}