How to implement union types / List[Either[A,B]] in Scala 2.13 ?

[domdorn]

Hello!

I have a endpoint that streams a json-array (via akka json-streaming) (array, starting with [, then the elements, then ])
Usually, the stream returns elements of Type A (happy path). But the stream could also fail at one point and then return a single B as the last element and then the closing ].

I'm using Scala 2.13.
In Scala 3 I could use a union type to express this as List[A | B] ..
However, I'm currently stuck with scala 2.13, so as I cannot make A and B have the same supertype,
I thought about encoding this as a List[Either[A,B]] but this expects that the elements then
have a type field, e.g. its serializing the Either and I'm getting this error:

      Exception in thread "zio-fiber-120" com.github.plokhotnyuk.jsoniter_scala.core.JsonReaderException: expected key: "type", offset: 0x00000012, buf:
      +----------+-------------------------------------------------+------------------+
      |          |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f | 0123456789abcdef |
      +----------+-------------------------------------------------+------------------+
      | 00000000 | 5b 0a 7b 22 63 6f 72 72 65 6c 61 74 69 6f 6e 49 | [.{"correlationI |
      | 00000010 | 64 22 3a 22 30 30 30 30 30 30 30 30 2d 30 30 30 | d":"00000000-000 |
      | 00000020 | 30 2d 30 30 30 30 2d 30 30 30 30 2d 30 30 30 30 | 0-0000-0000-0000 |
      | 00000030 | 30 30 30 30 30 30 30 30 22 2c 22 65 72 72 6f 72 | 00000000","error |
      +----------+-------------------------------------------------+------------------+

I created a small sample which would encode this and expectedly its encoding the either like this:

val responseFailure: Failure = ScenarioValidationFailedFailure(
          correlationId,
          scenario1,
          List(
            JSR303ValidationFailedScenarioValidationFailure(
              "some failure",
              assetId,
              "some-name",
              AssetType.of("entities.speicher.Speicher"),
              Some(AssetAttributeId.of(2)),
              "STAUZIEL"))
        )

        val expectedError = VioptError(
          correlationId,
          ConcreteVioptError(
            VioptErrorCode.UnexpectedServerFailure,
            responseFailure.toString,
            masterdataServiceName,
            None
          ))
        val r: List[Either[VioptError, PublishScenario.PublishScenarioEvent]] = List(Right(PublishScenario.ScenarioPublished(correlationId, scenario1)), Left(expectedError))
        val json = com.github.plokhotnyuk.jsoniter_scala.core.writeToString(r)
        println(json)

returns this json

[{"type":"Right","value":{"type":"ScenarioPublished","correlationId":"00000000-0000-0000-0000-000000000000","scenarioId":1}},{"type":"Left","value":{"correlationId":"00000000-0000-0000-0000-000000000000","error":{"code":{"httpStatusCode":500,"name":"UnexpectedServerFailure"},"message":"ScenarioValidationFailedFailure(\n  correlationId: CorrelationId(00000000-0000-0000-0000-000000000000),\n  scenarioId: ScenarioId(1),\n  validationFailures: \nJSR303ValidationFailedScenarioValidationFailure(some failure,AssetId(10),some-name,AssetTypeImpl(entities.speicher.Speicher,false,false,List()),Some(AssetAttributeId(2)),STAUZIEL)\n)","service":"masterdata-service"}}}]

Is there any way to get the union type functionality in scala 2.13 ? e.g. a codec that first tries the JsonValueCodec[A] and if that fails tries JsonValueCodec[B] ?

Thanks!

[plokhotnyuk]

Hi Dominik,

Unfortunately, the current version of jsoniter-scala-macros doesn't support union types for Scala 3 and their simulation with Either[A, B] for Scala 2.

But you can write a custom codec that would be suitable for you case. The main challenge here is how to distinguish different types of A and B during JSON parsing efficiently. There are could be a couple of approaches with own pros and cons:

1. If A types have the same required field that should not exists in B types then we can mark current position with in.setMark(), scan for that discriminating field using in.skipToKey("field-name"), rollback to the marked position with in.rollbackToMark() and then decode with provided codecs for A or B type depending on the result of the skipToKey call.

2. If A types are less frequent than B then you can grab a raw values to a byte array with in.readRawValAsBytes() and then try to parse them as B in a try block catching a parsing error with subsequent re-parsing of that byte array as A type.

3. If JSON values of A and B types start from different tokens like " and { for for String and some case class accordingly then the 1st value token could be used as a discriminator to parse with the custom codec in one pass, like here.