tc.scala

/* Data definition */
trait DocumentInfo {
  val title: String
}

case class D1(title: String, content: String) extends DocumentInfo
case class D2(title: String, count: Int) extends DocumentInfo
/* -------------- */

/* Type Class */
trait DocumentWriter[A] {
  def write(document: A): String  
}

object WriterInstances {
  implicit val d1Writer = new DocumentWriter[D1] {
    def write(document: D1): String = s"DocumentInfo D1 -> ..."
  }
  
  implicit val d2Writer = new DocumentWriter[D2] {
    def write(document: D2): String = s"DocumentInfo D2 -> ..."
  }
}

object DocumentSe {
  def write[A](document: A)(implicit w: DocumentWriter[A]): String =
    w.write(document)
}

/* --------------- */

object Test {
  import WriterInstances._
  def generalF(d: DocumentInfo): String = DocumentSe.write(d)
  
  val d1 = D1("title1", "asd")
  val d2 = D2("title2", 2)
  
  println(generalF(d1))
  println(generalF(d2))
}