entity decode for json that return a list/seq of any val

Question

I am building the back-end of an app using http4s. In the app i receive a json response from an external api (not the one i am working on). The api response is of this pattern below.

json response:

     `{
"datatable" : {
             "data" : [["AAPl", "MRT", "2020-03-20", 123, 123, 12.4, 233, 3234], 
                      ["AAPl", "MRT", "2020-03-20", 123, 123, 12.4, 233, 3234]],
              "meta": {
                      "next_date : null
              }

}`

my question is? can someone show me how to create an entity decoder and entity encoder that would decode the pattern. i can seem to get it to work.

currently i have:

  object Stocks {

   case class tickerInfo(ticker: String, dim:String, date: String, a: Int, b: Int, c: Float, d: Int, e: Int)

   case class data(data: Seq[tickerInfo])

   case class meta(next_date : Option[String])

   case class table(data: data, meta:meta)

   case class stockInfo(datatable:table)


  object stockInfo {
    implicit def stockInfoEntityDecoder[F[_]:Sync]: EntityDecoder [F, stockInfo] = jsonOf
    implicit def stockInfoEntityEncoder[F[_] : Applicative]: EntityEncoder[F, stockInfo] = jsonEncoderOf
     }

     val decodedJson = C.expect[stockInfo](GET(Uri.uri("www.externalApi.com")
     }

But this doesn't work. Please can someone tell me where i am going wrong.

I am getting a run time error *not a compile error). and its an http4s error that says - InvalidMessageBodyFailure.

Thanks

Answer 1

You have a couple of errors in your model, but the main problem is that circe won't be able to automatically decode an array of jsons into a case class.
If you can not modify the source of the data, you would need to create your own custom codec.

object Stocks {
  final case class TickerInfo(ticker: String, dim: String, date: String, a: Int, b: Int, c: Float, d: Int, e: Int)

  final case class Meta(next_date : Option[String])
  final case class Table(data: List[TickerInfo], meta: Meta)
  final case class StockInfo(datatable: Table)
  
  object StockInfo {
    implicit final val TickerInfoDecoder: Decoder[TickerInfo] = Decoder[List[Json]].emap {
      case ticker :: dim :: date :: a :: b :: c :: d :: e :: Nil =>
        (
          ticker.as[String],
          dim.as[String],
          date.as[String],
          a.as[Int],
          b.as[Int],
          c.as[Float],
          d.as[Int],
          e.as[Int]
        ).mapN(TickerInfo).left.map(_.toString)
      
      case list =>
        Left(s"Bad number of fields in: ${list.mkString("[", ", ", "]")}")
    }
    
    implicit final val MetaDecoder: Decoder[Meta] = deriveDecoder
    implicit final val TableDecoder: Decoder[Table] = deriveDecoder
    implicit final val StockInfoDecoder: Decoder[StockInfo] = deriveDecoder
  }
}

(you can see it working here, I leave outside the http4s part as it would be tricky to mock, but it shouldn't matter)

The error reporting of that custom encoder could be improved by providing more useful messages for each field. That is left as an exercise for the reader.