gistfile1.scala

trait Functor {
  type M <: { type T }
  def fmap[A, B](fa: M { type T = A })(f: A => B): M { type T = B }
}

implicit class EitherRightFunctor extends Functor { self =>
  type L
  type M = Either { type A = self.L ; type T = B  } //doesn't this specify a subtype of Either, rather than Either itself?
  
  def fmap[A0, B0](fa: M { type A = self.L ; type B = A0 })(f: A0 => B0): Either { type A = self.L ; type B = B0 } = 
    fa match {
      case Right(a) => Right(f(a))
      case Left(l) => Left(l)
    }
}

// current Scala

trait Functor[M[_]] {
  def fmap[A, B](fa: M[A])(f: A => B): M[B]
}

implicit def EitherRightFunctor[L,a] = new Functor[Either[L, a]] {
  def fmap[A, B](fa: Either[L, A])(f: A => B): Either[L, B] = 
    fa match {
      case Right(a) => Right(f(a))
      case Left(l) => Left(l)
    }
}