combinators.scala

  def optional[A](guard: Codec[Boolean], target: Codec[A]): Codec[Option[A]] =
    either(guard, provide(()), target).xmap[Option[A]]({ _.toOption }, { _ map { \/-(_) } getOrElse -\/(()) })

  def withDefault[A](opt: Codec[Option[A]], default: Codec[A]): Codec[A] = {
    val paired = opt flatZip {
      case Some(a) => provide(a)
      case None => default
    }

    paired.xmap[A]({ _._2 }, { a => (Some(a), a) })
  }

  def recover(codec: Codec[Unit]): Codec[Boolean] = new Codec[Boolean] {
    def encode(a: Boolean): String \/ BitVector = codec.encode(())

    def decode(buffer: BitVector): String \/ (BitVector, Boolean) = {
      codec.decode(buffer).toOption map {
        case (rest, _) => \/-(rest, true)
      } getOrElse \/-(buffer, false)      // backtrack on failure
    }
  }