ScalaEnabledVisitorPattern.scala

  abstract class Element(text: String) {
    def accept(visitor: Element => Unit): Unit =
      visitor(this)
  }

  case class Title(text: String) extends Element(text)
  case class Text(text: String) extends Element(text)
  case class Hyperlink(text: String, val url: String) extends Element(text)

  class Document(parts: List[Element]) {
    def accept(visitor: Element => Unit): Unit =
      parts.foreach(p => p.accept(visitor))
  }

  object VisitorExample {
    val line = System.getProperty("line.separator")

    def htmlExporterVisitor(builder: StringBuilder): Element => Unit = element => element match {
      case Title(text) => builder.append(s"<h1>${text}</h1>").append(line)
      case Text(text) => builder.append(s"<p>${text}</p>").append(line)
      case Hyperlink(text, url) => builder.append(s"""<a href=\"${url}\">${text}</a>""").append(line)
    }

    def plainTextExporterVisitor(builder: StringBuilder): Element => Unit = element => element match {
      case Title(text) => builder.append(text).append(line)
      case Text(text) => builder.append(text).append(line)
      case Hyperlink(text, url) => builder.append(s"${text} (${url})").append(line)
    }

    def main(args: Array[String]): Unit = {
      val document = new Document(
        List(
          Title("The Visitor Pattern Example"),
          Text("The visitor pattern helps us add extra functionality without changing the classes."),
          Hyperlink("Go check it online!", "https://www.google.com/"),
          Text("Thanks!")
        )
      )
      val html = new StringBuilder
      println(s"Export to html:")
      document.accept(htmlExporterVisitor(html))
      println(html.toString())

      val plain = new StringBuilder
      println(s"Export to plain:")
      document.accept(plainTextExporterVisitor(plain))
      println(plain.toString())
    }
  }