blah.scala

object blah { 
  case class OptionT[F[_], A](x: F[Option[A]])
  
  trait Functor[F[_]]
  object Functor {
    implicit val OptionFunctor: Functor[Option] =
      sys.error("todo")
  }
  object OptionT {
    implicit def OptionTFunctor[F[_]: Functor]: Functor[({ type l[a] = OptionT[F, a] })#l] =
      sys.error("todo")
  }
  case class Writer[W, A](w: W, a: A)
  object Writer {
    implicit def WriterFunctor[W]: Functor[({ type l[a] = Writer[W, a] })#l] =
      sys.error("todo")
  }
  type WWW[A] = Writer[String, A]
  type OOO[A] = OptionT[WWW, A]

  implicitly[Functor[WWW]]
  implicitly[Functor[OOO]]
  implicitly[Functor[({ type l[a] = Writer[String, a] })#l]]
  implicitly[Functor[({ type l[a] = OptionT[Option, a] })#l]]
  // implicitly[Functor[({ type l[a] = OptionT[WWW, a] })#l]]
}