expressions-current.scala

package examples.expressions

class Ref[a](var elem:a) {}

abstract class Lang {
  trait Visitor {
    def caseNum(n: int): unit
  }

  abstract class Exp {
    def visit(v: visitor): unit
  }

  type visitor <: Visitor

  class Num(n: int) extends Exp {
    def visit(v: visitor): unit = v.caseNum(n)
  }

  class Eval(result: Ref[int]) requires visitor extends Visitor {
    def caseNum(n: int) = result.elem = n
  }
}

abstract class Lang2 extends Lang {
  trait Visitor2 extends Visitor {
    def casePlus(left: Exp, right: Exp): unit
  }

  type visitor <: Visitor2

  class Plus(l: Exp, r: Exp) extends Exp {
    def visit(v: visitor): unit = v.casePlus(l, r)
  }

  //  class Eval2(result: Ref[int]): visitor extends Eval(result) with Visitor2 {
  class Eval2(result: Ref[int]) requires visitor extends Eval(result) with Visitor2 {
    def casePlus(l: Exp, r: Exp) =
      result.elem = { l.visit(this); result.elem } + { r.visit(this); result.elem }
  }

  class Show2(result: Ref[String]) requires visitor extends Visitor2 {
    def caseNum(n: int) = result.elem = n.toString()
    def casePlus(l: Exp, r: Exp) =
      result.elem =
        "(" + { l.visit(this); result.elem } +
        "+" + { r.visit(this); result.elem } + ")"
  }
}

object Main {
  def main(args: Array[String]): unit = {
    //val l1 = new Lang { type visitor = Visitor } // not yet implemented
    object l1 extends Lang { type visitor = Visitor } // workaround
    val e1: l1.Exp = new l1.Num(42)
    val iref = new Ref(0)
    Console.println("eval: " + { e1.visit(new l1.Eval(iref)); iref.elem })

    //val l2 = new Lang2 { type visitor = Visitor2 } // not yet implemented
    object l2 extends Lang2 { type visitor = Visitor2 } // workaround
    val e2: l2.Exp = new l2.Plus(new l2.Num(5), new l2.Num(37))
    val sref = new Ref("")
    Console.println("eval: " + { e2.visit(new l2.Eval2(iref)); iref.elem })
    Console.println("show: " + { e2.visit(new l2.Show2(sref)); sref.elem })
    e1.visit(new l1.Eval(iref))
    e2.visit(new l2.Show2(sref))
  }
}