How to get Circe to skip certain values from the e

问题:

We have an "enumeration" like so:

sealed abstract class StatusEnumeration(val value: String)
case object Status {
  case object Mine extends StatusEnumeration("mine")
  case object Someone extends StatusEnumeration("someone")
  case object Neighbor extends StatusEnumeration("neighbor")
  case object Unknown extends StatusEnumeration("unknown")
}

This structure is dictated by our domain/needs. However, I'd like to convert this StatusEnumeration into an output JSON like so:

case class Root(status: StatusEnumeration)

I've set the following implicit encoder and it works as expected:

implicit val encoder = deriveEnumerationEncoder[StatusEnumeration]

However, if the StatusEnumeration is set to Status.Unknown I'd like for the value generation to be skipped Or just output a null instead (which I can then drop via dropNullValues). How can I do something like this? Is it even possible or do I need to redefine another set of enums?

So far this is what I was trying when @Alec also commented to try something similar:

implicit val encoder = deriveEnumerationEncoder[StatusEnumeration].mapJson(x=>{

    x.asString match {
      case Some("Unknown") => Json.Null
      case Some(s) => Json.fromString(s.toLowerCase) // output expect lowercase - not sure if the consumer is case-sensitive
      case None => Json.Null
    }

  })

This does seem to work as expected. Is this the recommended way or is there a better one?

回答1:

Haven't tried to compile this, but based on the Scaladocs you should be able to do:

implicit val encoder = Encoder.encodeOption(deriveEnumerationEncoder[StatusEnumeration])
  .contramap {
    case Unknown => None
    case known => Some(known)
  }

Basically, construct an encoder Option[StatusEnumeration], then map StatusEnumeration onto that accordingly (I'm pretty sure the encoder for Option puts None as null and Some(x) as x encoded).