Circe: decoding a container type with different possible content types

Question

My goal is to transform JSON into the following model:

case class Container(typeId: Int, timestamp: Long, content: Content)

sealed trait Content
case class ContentType1(...) extends Content
case class ContentType2(...) extends Content
case class ContentType3(...) extends Content

There is a container type whose structure always looks the same.
The content of the container is represented by types that look totally different (regarding the amount and type of attributes). However, all content types are known at compile time and implement a sealed trait.
the typeId attribute of the container indicates the content type. E.g. a value of N means that content is of type ContentTypeNand so on.
The JSON structure looks exactly like you would expect it to do and maps direcly to the Scala types shown above.
(Btw: I'm open to change the container type to Container[A <: Content] if that's a more elegant solution).

What would be a good way to decode that with circe? I guess automatic decoding does not work in this case.

edit: the documentation of the json structure describes the content field as ?mixed (object, integer, bool), so it can also be a simple Int or Boolean instead of a case class object. But for now it would be ok to ignore these two types (although it would be nice to have a solution for that).

Reactormonk Reactormonk · Accepted Answer · 2017-06-25T15:43:57

I'd use semiauto (with deriveDecoder), validate and or.

case class Container[+C](typeId: Int, timestamp: Long, content: C)

sealed trait Content
@JsonCodec case class ContentType1(...) extends Content
@JsonCodec case class ContentType2(...) extends Content

object Content {
  import shapeless._
  import io.circe._
  import io.circe.generic.semiauto._

  def decodeContentWithGuard[T: Decoder](number: Int): Decoder[Content[T]] = {
    deriveDecoder[Content[T]].validate(_.downField("typeId") == Right(number), s"wrong type number, expected $number")
  }

  implicit val contentDecoder: Decoder[Container[Content]] = {
    decodeWithGuard[ContentType1](1) or
    decodeWithGuard[ContentType2](2)
  }
}

If you don't want the covariant annotation, you'll have to map and upcast the inner ContentTypeX to Content.

There's probably also a solution without the generic, but then you can't use semiauto.

Might not compile, didn't test it.

Circe: decoding a container type with different possible content types

1 Answers