Circe Decode to sealed trait extended by multiple case classes

Question

I've seen similar questions before, but none of them have worked. I think that they ask something different so I'm asking here. I have something like this in one file:

sealed trait Thing
case class SomeThing() extends Thing
case class OtherThing() extends Thing

and in another file:

val str = //valid json
val decoded = decode[Thing](str)
println(decoded)

and I get:

Left(DecodingFailure(...))

This works if I did:

val str = //valid json
val decoded = decode[SomeThing](str)
println(decoded)

Answer 1

you can use circe-generic-extras and add type discriminator for your jsons:

In your build.sbt:

libraryDependencies ++= Seq(
  "io.circe" %% "circe-generic-extras" % "0.12.2",
  "io.circe" %% "circe-parser" % "0.12.2"
)

Now let's define our classes and serialize + deserialize them:

sealed trait Thing
case class SomeThing() extends Thing
case class OtherThing() extends Thing

import io.circe.generic.extras.auto._
import io.circe.generic.extras.Configuration
import io.circe.syntax._
import io.circe.parser

implicit val customConfig: Configuration =
  Configuration.default.withSnakeCaseMemberNames.withDiscriminator("type")

val thing: Thing = SomeThing()

// serialize thing to json
val jsString: String = thing.asJson.spaces2
println(s"serialized $thing to:\n$jsString")
/* serialized SomeThing() to:
{
"type" : "SomeThing"
}
*/

// deserialize json to thing
val errorOrMyTrait: Either[io.circe.Error, Thing] = parser.decode[Thing](jsString)

println(errorOrMyTrait) // Right(SomeThing())

Notice that now the serialized json contains the type of the sealed class