alternative_decoders.scala

implicit val alternativeForRowDecoder =
  new Alternative[RowDecoder] {
    // Kind of like true, same as in our Monoidal above
    def pure[A](x: A): RowDecoder[A] =
      Monoidal[RowDecoder].pure(x)

    // Kind of like AND, same as in our Monoidal abaove
    def product[A, B](fa: RowDecoder[A], fb: RowDecoder[B]): RowDecoder[(A, B)] =
      Monoidal[RowDecoder].product(fa, fb)

    // Kind of like false but for decoders
    def empty[A]: F[A] =
      new RowDecoder[A] {
        def apply(s: String) = Left(new RuntimeError("no dice"))
      }

    // Kind of like XOR but for decoders
    def combineK[A](x: F[A], y: F[A]): F[A] =
      RowDecoder.from(z =>
        x.decode(z) match {
         case Left(_) => y.decode(z)
         case Right(v) => Right(v)
       })
}