ZIO Json: Failing Fiber on List in Json

Question

I've got some data structures which I'm trying to hydrate from JSON. The short version (or the full version here):

sealed trait CexPair:
    val timestamp: String
    // ... (cut for brevity)
  implicit val decoderCexPair: JsonDecoder[CexPair] = DeriveJsonDecoder.gen

case class CexPairWithBidAsk(
      timestamp: String,
      // snip
      bid: Double,
      ask: Double
  ) extends CexPair
  implicit val decoderWithBidAsk: JsonDecoder[CexPairWithBidAsk] = DeriveJsonDecoder.gen

case class CexPairWithoutBidAsk(
    timestamp: String,
    // snip
  ) extends CexPair
  implicit val decoderWithoutBidAsk: JsonDecoder[CexPairWithoutBidAsk] = DeriveJsonDecoder.gen

case class Tickers(data: Seq[CexPair])
implicit val decoderTickers: JsonDecoder[Tickers] = DeriveJsonDecoder.gen

implicit val decoderPairs: JsonDecoder[Seq[CexPair]] =
  decoderTickers.map(_.data)

But when I try to run this, I get:

Assertion failed:
  Fiber failed.
  A checked error was not handled.
  .data(expected '{' got '[')

This tells me that deserializing Tickers already goes haywire. Is this because a sealed trait is not the ideal basis for unmarshalling JSON or am I missing something entirely different?

Answer 1

For you test case that is trying to parse tickers <- ZIO.fromEither(str.fromJson[Tickers]), he custom JsonDecoder decodePairs expects the data field of Tickers to also be parsed as Tickers (before mapping in a Seq) instead of a List \ Seq hence it is expecting { instead of [.

Just remove decoderPairs and it will fallback to the default decoder for Seq and hopefully it should parse fine if there are no other issues.