如何制作由前缀驱动的Codec [Option [T]]？

Question

I'm trying to implement codec for PSQL COPY BINARY format. Details are at Tuple section

Int, Bool, String fields are encoded as <4 byte length><var length payload>

I implemented them like this

val psqlUtf8: Codec[String] = variableSizeBytes(int32, utf8)
val psqlBool: Codec[Boolean] = variableSizeBytes(int32, byte).xmap[Boolean](_ == 1, v ⇒ if (v) 1 else 0)
val psqlInt: Codec[Int] = variableSizeBytes(int32, int32)

But to encode NULL they use -1 in length field.

Could you please suggest how I can implement Codec[Option[T]] for such situation ?

Answer 1

The best I could come up with is

def psqlNullable[T](codec: Codec[T]): Codec[Option[T]] =
  fallback(constant(-1),  codec).xmap[Option[T]]({
    case Left(_) ⇒ None
    case Right(v) ⇒ Some(v)
  }, {
    case None ⇒ Left(())
    case Some(v) ⇒ Right(v)
  })