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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))
}
}
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