VisitorExample.kt

sealed class Expression {
  class Add(val lhs: Expression, val rhs: Expression) : Expression() {
    override fun accept(visitor: IExpressionVisitor){
      visitor.visit(this)
    }
  }
  class Number(val num: Int): Expression() {
    override fun accept(visitor: IExpressionVisitor){
      visitor.visit(this)
    }
  }

  abstract fun accept(visitor: IExpressionVisitor)
}


interface IExpressionVisitor{
  fun visit(add: Expression.Add)
  fun visit(num: Expression.Number)
}

class PrettyPrintExpressionVisitor : IExpressionVisitor {
    override fun visit(add: Expression.Add){
      add.lhs.accept(this)
      print(" + ")
      add.rhs.accept(this)
    }

    override fun visit(num: Expression.Number){
      print(num.num)
    }
}

class JVMExpressionVisitor : IExpressionVisitor {
  override fun visit(add: Expression.Add){
    add.lhs.accept(this)
    add.rhs.accept(this)
    println("iadd")
  }

  override fun visit(num: Expression.Number){
    println("iload ${num.num}")
  }
}