TraditionalVisitorPattern.scala

trait Visitor {
  def visit(title: Title)
  def visit(text: Text)
  def visit(hyperlink: Hyperlink)
}

abstract class Element(val text: String) {
  def accept(visitor: Visitor)
}

class Title(text: String) extends Element(text) {
  override def accept(visitor: Visitor): Unit = 
    visitor.visit(this)
}

class Text(text: String) extends Element(text) {
  override def accept(visitor: Visitor): Unit = 
    visitor.visit(this)
}

class Hyperlink(text: String, val url: String) extends Element(text) {
  override def accept(visitor: Visitor): Unit = 
    visitor.visit(this)
}

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

class HtmlExporterVisitor extends Visitor {
  val line = System.getProperty("line.separator")
  val builder = new StringBuilder

  def getHtml(): String = builder.toString

  override def visit(title: Title): Unit = 
    builder.append(s"<h1>${title.text}</h1>").append(line)

  override def visit(text: Text): Unit = 
    builder.append(s"<p>${text.text}</p>").append(line)

  override def visit(hyperlink: Hyperlink): Unit = 
    builder.append(s"""<a href=\"${hyperlink.url}\">${hyperlink.text}</a>""").append(line)
}

class PlainTextExporterVisitor extends Visitor {
  val line = System.getProperty("line.separator")
  val builder = new StringBuilder

  def getText(): String = builder.toString

  override def visit(title: Title): Unit = 
    builder.append(title.text).append(line)

  override def visit(text: Text): Unit = 
      builder.append(text.text).append(line)

    override def visit(hyperlink: Hyperlink): Unit = 
      builder.append(s"${hyperlink.text} (${hyperlink.url})").append(line)
  }

  object VisitorExample {
    def main(args: Array[String]): Unit = {
      val document = new Document(
        List(
          new Title("The Visitor Pattern Example"),
          new Text("The visitor pattern helps us add extra functionality without changing the classes."),
          new Hyperlink("Go check it online!", "https://www.google.com/"),
          new Text("Thanks!")
        )
      )
      val htmlExporter = new HtmlExporterVisitor
      val plainTextExporter = new PlainTextExporterVisitor

      println(s"Export to html:")
      document.accept(htmlExporter)
      println(htmlExporter.getHtml())

      println(s"Export to plain:")
      document.accept(plainTextExporter)
      println(plainTextExporter.getText())
    }
  }