binop.scala

trait Expression

trait BinaryPredicate extends Expression {
  def left: Expression
  def right: Expression
}

case class Literal(value: Int) extends Expression

case class Lt(left: Expression, right: Expression) extends BinaryPredicate
case class Gt(left: Expression, right: Expression) extends BinaryPredicate
case class Eq(left: Expression, right: Expression) extends BinaryPredicate
case class And(left: Expression, right: Expression) extends BinaryPredicate
case class Or(left: Expression, right: Expression) extends BinaryPredicate

object BinaryPredicate {
  def unapply(e: Expression): Option[(Expression, Expression)] = e match {
    case e: BinaryPredicate => Some((e.left, e.right))
    case _ => None
  }

  // Or even simpler:
  //
  // def unapply(e: BinaryPredicate): Option[(Expression, Expression)] = Some((e.left, e.right))
}

val lt = Lt(Literal(1), Literal(2))
val gt = Gt(Literal(3), Literal(2))
val and = And(lt, gt)

lt match {
  case BinaryPredicate(lhs, rhs) =>
    println(s"left=$lhs right=$rhs")
}

gt match {
  case BinaryPredicate(lhs, rhs) =>
    println(s"left=$lhs right=$rhs")
}

and match {
  case BinaryPredicate(lhs, rhs) =>
    println(s"left=$lhs right=$rhs")
}