HtmlWriter type class example

HtmlWriter.scala

trait HtmlWriter[A] {
  def apply(value: A): String
}

object HtmlWriter {
  def apply[A](implicit writer: HtmlWriter[A]): HtmlWriter[A] =
    writer

  def write[A](value: A)(implicit writer: HtmlWriter[A]): String =
    writer.apply(value)

  implicit val intWriter: HtmlWriter[Int] = 
    new HtmlWriter[Int] {
      def apply(value: Int): String =
        value.toString
    }

  implicit val booleanWriter: HtmlWriter[Boolean] = 
    new HtmlWriter[Boolean] {
      def apply(value: Boolean): String =
        (if(value) "yes" else "no")
    }

  implicit def listWriter[A](implicit writer: HtmlWriter[A]): HtmlWriter[List[A]] =
    new HtmlWriter[List[A]] {
      def apply(value: List[A]): String =
        "<ul>" + value.map(value => "<li>" + writer.apply(value) + "</li>").mkString + "</ul>"
    }
}

object Main extends App {
  println(HtmlWriter.write(List(1, 2, 3)))
}