https://groups.google.com/d/topic/scalaz/BS0JfZGbJn4/discussion

build.sbt

libraryDependencies += "org.scalaz" %% "scalaz-effect" % "7.1.0-M4"

Main.scala

trait MyStuff { def foobar: String }

object Main{
  import scalaz._,Scalaz._
  import effect.IO

  final class MyFunctorOps[F[_], A](self: F[A], F: Functor[F]){
    def foobar(implicit A: A =:= MyStuff): F[String] =
      F.map(self)(_.foobar) 
  }

  implicit def myFunctorOps[FA](fa: FA)(implicit F: Unapply[Functor, FA]) =
    new MyFunctorOps[F.M, F.A](F(fa), F.TC)

  val a = OptionT[IO, MyStuff](IO(None))
  val b = EitherT[IO, Throwable, MyStuff](IO(-\/(new Error)))

  a.foobar
  b.foobar
}