Example of using a type lambda for implementing Functor over Maps

tl1.scala

  // Functor
  trait JFunctor[F[_]] {

    def map[A,B](fa: F[A])(f: A => B) : F[B]

  } 

  trait JMapFunctor[K] extends JFunctor[({type LT[V] = Map[K, V]})#LT] {

    def map[A, B](fa: Map[K,A])(f: A => B): Map[K,B] = {

      fa.foldLeft(Map.empty[K,B]) {

        case (acc, (k,a)) =>
            acc updated (k, f(a))
      }

    }

  }

  object JLongMap extends JMapFunctor[Long]

  LongMap.map(Map(1L -> "Hello", 2L -> "Goodbye")){_.length} 

// Map[Long, Int] = Map(1L -> 5, 2L -> 7)