Unable to map on HList
===

I was trying to solve [this problem][1] with shapeless. However I am for some reason unable to map on the `HList`. I'll let the code speak for itself.

    import shapeless._
    import HList._
    
    case class Foo(a: Option[Int], b: Option[Int])
    
    val a = Foo(Some(3), None)
    
    val b = Foo(Some(22), Some(1))
    
    implicit val fooIso = HListIso(Foo.apply _, Foo.unapply _)
    
    val mapper = new (({ type O2[+A] = (Option[A], Option[A]) })#O2 ~> Option) {
      def apply[A](x: (Option[A], Option[A])): Option[A] = x._1.orElse(x._2)
    }
    
    fooIso.fromHList(fooIso.toHList(a).zip(fooIso.toHList(b)).map(mapper))

Error message is:

    <console>:55: error: could not find implicit value for parameter mapper: shapeless.Mapper[java.lang.Object with shapeless.~>[[+A](Option[A], Option[A]),Option],shapeless.::[(Option[Int], Option[Int]),shapeless.::[(Option[Int], Option[Int]),shapeless.HNil]]]
                  fooIso.fromHList(fooIso.toHList(a).zip(fooIso.toHList(b)).map(mapper))
                                                                               ^

Why doesn't the mapping work?

  [1]: http://stackoverflow.com/questions/11438277/update-operations-on-a-scala-case-class
