Simple example of using Circe Decoder with prepare to preprocesses Json (JSonObjects in this case)

CirceDecodorWithPrepare.scala

  import io.circe._

    // Generalized in real-life
    def fieldNameChangeEx(in: JsonObject): JsonObject = {
      val fieldName = "type"
      val newName = "tipe"

      // Not sure what happens on missing fieldName or fieldName = null
      val updated: Option[JsonObject] = in(fieldName)
                                        .map(oldVal ⇒ in.add(newName, oldVal))
                                        .map(jo ⇒ jo.remove(fieldName))
      updated.getOrElse(in)
    }

    def prepareByMungingJsonObject(fn: JsonObject => JsonObject)(in: ACursor): ACursor = {
      in.withFocus(json ⇒ json.mapObject(jobj ⇒ fn(jobj)))
    }

    val myFunc         : JsonObject ⇒ JsonObject = fieldNameChangeEx
    val myPrepareStyle1: ACursor ⇒ ACursor       = prepareByMungingJsonObject(myFunc)
    val myPrepareStyle2: ACursor ⇒ ACursor       = prepareByMungingJsonObject(fieldNameChangeEx)


    val decoder: Decoder[FieldType] = deriveDecoder[FieldType].prepare(myPrepareStyle1)

    implicit val decodeFieldType: Decoder[FieldType] =
      Decoder.forProduct5("nth", "isVLEncoded", "isSerialized", "isSigningField", "type")(FieldType.apply)


  }