TypeLambda #Scala

TypeLambdaDemo.scala

// Example HList with type member Hd
trait HList{
  type Hd
}

class IntList extends HList {
  type Hd = Int
}


// Example of TypeLambda
trait Functor[A, +M[_]]{
  def map[B](f: A => B): M[B]
}

case class ListFunctor[A](seq: Seq[A]) extends Functor[A, Seq]{
  override def map[B](f: (A) => B): Seq[B] = seq.map(f)
}

case class MapFunctor[K,V](mapKV: Map[K,V]) extends Functor[V, ({type L[a] = Map[K,a]})#L]{
  override def map[V2](f: V => V2): Map[K, V2] = mapKV map{
    case (k,v) => (k, f(v))
  }
}


case class ReadableMapFunctor[K,V](mapKV: Map[K,V]){
  def mapFunctor[V2] = {
    type `Map[K]`[V2] = Map[K, V2]
    new Functor[V, `Map[K]`] {
      override def map[V2](f: (V) => V2): `Map[K]`[V2] = mapKV map{
        case (k,v) => (k, f(v))
      }
    }
  }


object TypeLambdaDemo extends App{
  //type projection with # 
  implicitly[Int =:= IntList#Hd]
  val x: IntList#Hd = 10


  //list functor
  val lst = ListFunctor(List(1,2,3)).map(_ * 10)
  println(lst)
  
  //map functor
  val mapKV = MapFunctor(Map(1->1, 2->2, 3->3)).map(_ * 10)
  println(mapKV)

  val mapKV2 = ReadableMapFunctor(Map(1->1, 2->2, 3->3)).mapFunctor.map(_*10)
  println(mapKV2)

}