A Visitor pattern example, which solved "Expression Problem" related to Visitor.

VisitorMain.scala

trait VisitorsBase {self =>
  trait Node {
    def accept(v: V)
  }
  case class Add(l: Node, r: Node) extends Node {
    def accept(v: V) {
      v.visit(this)
    }
  }
  case class Sub(l: Node, r: Node) extends Node {
    def accept(v: V) {
      v.visit(this)
    }
  }
  case class Value(value: Int) extends Node {
    def accept(v: V) {
      v.visit(this)
    }
  }
  type V <: Visitor
  trait Visitor {self: V =>
    def visit(node: Add) { 
      println("Add!") 
      node.l.accept(this)
      node.r.accept(this)
    }
    def visit(node: Sub) {
      println("Sub!") 
      node.l.accept(this)
      node.r.accept(this)
    }
    def visit(node: Value) {
      println("Value: " + node.value) 
    }
  }
}

object VisitorsPlus extends VisitorsBase {
  type V = Visitor
  case class Mul(l: Node, r: Node) extends Node {
    def accept(v: V) {
      v.visit(this)
    }
  }
  case class Div(l: Node, r: Node) extends Node {
    def accept(v: V) {
      v.visit(this)
    }
  }
  class Visitor extends super.Visitor {
    def visit(node: Mul) {
      println("Mul") 
      node.l.accept(this)
      node.r.accept(this)
    }
    def visit(node: Div) {
      println("Div") 
      node.l.accept(this)
      node.r.accept(this)
    }
  }
}

object VisitorMain {
  import VisitorsPlus._
  def main(args: Array[String]) {
    val v = new Visitor
    Add(Value(1), Value(2)).accept(v)
    Sub(Value(1), Value(2)).accept(v)
    Mul(Value(1), Value(2)).accept(v)
    Div(Value(1), Value(2)).accept(v)
  }
}