public void VisitThisExpr(ThisExpr expr)
 {
     if (currentClass == ClassType.None)
     {
         Lox.Error(expr.Keyword, "Cannot use 'this' ouside of class.");
         return;
     }
     ResolveLocal(expr, expr.Keyword);
 }
 public string VisitThisExpr(ThisExpr expr)
 {
     throw new System.NotImplementedException();
 }
 public object VisitThisExpr(ThisExpr expr)
 {
     return(LookUpVariable(expr.Keyword, expr));
 }