instance_for_decoders.scala

implicit val monoidalForRowDecoder = new Monoidal[RowDecoder] {
  // This always succeeds returning the given value!
  def pure[A](x: A): RowDecoder[A] =
    RowDecoder.from(Function.const(Right(x)))

  // This processes the output of the given decode with the given function
  def map[A, B](fa: RowDecoder[A])(f: A => B): RowDecoder[B] =
    RowDecoder.from(x => fa.decode(x).map(f))

  // Combine the output of two into one!
  def product[A, B](fa: RowDecoder[A], fb: RowDecoder[B]): RowDecoder[(A, B)] =
    RowDecoder.from(x => fa.decode(x).flatMap(a => f.decode(x).map(b => (a, b))))
}