Parsing recursive JSON structure with Lists in Scala

Question

I've got the following JSON structure and I can't find a good way to parse it in Scala (I'm using circe BTW):

{
  "name": "xx",
  "args": [
    {"name":"xy", "args": []},
    [
      {"name":"xy", "args": []},
      {"name":"xy", "args": [[]]}
    ],
    [
      [
        {"name":"xy", "args": [{"name":"xy", "args": []}]}
      ]
    ]
  ]
}

Basically, it's a recursive structure that can contain either object, or list of objects, or list of lists, or list of lists of lists... or objects and lists.

How can I handle that? I'm thinking about some recursive types but I'm not sure about that.

@GaëlJ yes it is. - Well, it has a trailing comma at line 13 but other than that is valid. — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Nov 30, 2021 at 18:45

Luis Miguel Mejía Suárez · Accepted Answer · 2021-11-30 18:45:48Z

You need to model that as an ADT and derive a custom Decoder like this:

import io.circe.{Decoder, DecodingFailure, parser}
import io.circe.generic.semiauto.deriveDecoder

sealed trait Arg
object Arg {
  final case class OneArg(name: String, args: List[Arg]) extends Arg
  final case class MultipleArgs(args: List[Arg]) extends Arg
  
  private implicit final val OneArgDecoder: Decoder[OneArg] = deriveDecoder
  
  implicit final val ArgDecoer: Decoder[Arg] =
    Decoder.instance[Arg] { cursor =>
      cursor.focus match {
        case Some(json) =>
          // Here you may also ask if it is an array before to attempt to get it as a List[arg], and if not then provide a custom failure.
          if (json.isObject) json.as[OneArg]
          else json.as[List[Arg]].map(MultipleArgs)
        
        case None =>
          Left(DecodingFailure("Empty cursor", cursor.history))
      }
    }
}

Which can be used to decode your JSON: https://scastie.scala-lang.org/BalmungSan/W0lLBYRzTIS3PC4E5i0wXA/26

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