blah.scala

object blah {
  case class Id[+A](a: A)
  object Id {
    implicit def IdFunctor: Functor[Id] = ???
  }

  case class OptionT[F[+_], +A](x: F[Option[A]])

  trait Functor[F[+_]]

  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[ ({ type m[+b] = OptionT[Id, b] })#m, a] })#l]]
  implicitly[Functor[({ type l[+a] = OptionT[ ({ type m[+b] = Writer[String, b] })#m, a] })#l]]

  // implicitly[Functor[({ type l[+a] = OptionT[WWW, a] })#l]]
/*
  blah.scala:19: error: could not find implicit value for parameter e: blah.Functor[[+a]blah.OptionT[[+A]blah.Writer[java.lang.String,A],a]]

  implicitly[Functor[({ type l[+a] = OptionT[WWW, a] })#l]]
*/
}