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?

yangzai · Answer 1 · 2021-12-30T11:08:41.397

0

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.

edited Dec 30 '21 at 11:08

answered Dec 30 '21 at 10:54

yangzai

962
5
11

That gives me another error: `Fiber failed. A checked error was not handled. .data[0](invalid disambiguator)` – Aarkon Dec 30 '21 at 11:06
We need to see the data you are parsing, can you provide a small sample that reproduces the errors? – yangzai Dec 30 '21 at 11:10
Of course, I could have thought about that myself: https://cex.io/rest-api#ticker – Aarkon Dec 30 '21 at 11:13
1

You cannot use a `sealed trait` (ADT) for data like this, the paser would not be able to disambiguate between `CexPairWithBidAsk` and `CexPairWithoutBidAsk`. See in the fruit example how it needs the "Banana" and "Apple" keys: https://zio.github.io/zio-json/docs/overview/overview_decoding. I would suggest you use a single case class to represent `CexPair` and for the fields that might be missing use a `Maybe` type. – yangzai Dec 30 '21 at 11:25
I mean `Option` type. `Maybe` is for Haskell. – yangzai Dec 30 '21 at 11:31
I didn't ask if you needed but if you wanted them, trying to be polite. I don't see how I gave you the impression that I'm lazy here though. Never mind, thank you anyway. – Aarkon Dec 30 '21 at 11:55
"I didn't ask if you needed but if you wanted them". So which of your existing comments did you "ask if I wanted them"? Noone is obligated to tell you how they voted. You can make a report if you think the votes aren't fair. – yangzai Dec 30 '21 at 13:16

ZIO Json: Failing Fiber on List in Json

1 Answers1