How to decode an ADT with circe without disambiguating objects

Enumerating the ADT constructors

The most straightforward way to get the representation you want is to use generic derivation for the case classes but explicitly defined instances for the ADT type:

import cats.syntax.functor._import io.circe.{ Decoder, Encoder }, io.circe.generic.auto._import io.circe.syntax._sealed trait Eventcase class Foo(i: Int) extends Eventcase class Bar(s: String) extends Eventcase class Baz(c: Char) extends Eventcase class Qux(values: List[String]) extends Eventobject Event {  implicit val encodeEvent: Encoder[Event] = Encoder.instance {    case foo @ Foo(_) => foo.asJson    case bar @ Bar(_) => bar.asJson    case baz @ Baz(_) => baz.asJson    case qux @ Qux(_) => qux.asJson  }  implicit val decodeEvent: Decoder[Event] =    List[Decoder[Event]](      Decoder[Foo].widen,      Decoder[Bar].widen,      Decoder[Baz].widen,      Decoder[Qux].widen    ).reduceLeft(_ or _)}

Note that we have to call widen (which is provided by Cats's Functor syntax, which we bring into scope with the first import) on the decoders because the Decoder type class is not covariant. The invariance of circe's type classes is a matter of some controversy (Argonaut for example has gone from invariant to covariant and back), but it has enough benefits that it's unlikely to change, which means we need workarounds like this occasionally.

It's also worth noting that our explicit Encoder and Decoder instances will take precedence over the generically-derived instances we'd otherwise get from the io.circe.generic.auto._ import (see my slides here for some discussion of how this prioritization works).

We can use these instances like this:

scala> import io.circe.parser.decodeimport io.circe.parser.decodescala> decode[Event]("""{ "i": 1000 }""")res0: Either[io.circe.Error,Event] = Right(Foo(1000))scala> (Foo(100): Event).asJson.noSpacesres1: String = {"i":100}

This works, and if you need to be able to specify the order that the ADT constructors are tried, it's currently the best solution. Having to enumerate the constructors like this is obviously not ideal, though, even if we get the case class instances for free.

A more generic solution

As I note on Gitter, we can avoid the fuss of writing out all the cases by using the circe-shapes module:

import io.circe.{ Decoder, Encoder }, io.circe.generic.auto._import io.circe.shapesimport shapeless.{ Coproduct, Generic }implicit def encodeAdtNoDiscr[A, Repr <: Coproduct](implicit  gen: Generic.Aux[A, Repr],  encodeRepr: Encoder[Repr]): Encoder[A] = encodeRepr.contramap(gen.to)implicit def decodeAdtNoDiscr[A, Repr <: Coproduct](implicit  gen: Generic.Aux[A, Repr],  decodeRepr: Decoder[Repr]): Decoder[A] = decodeRepr.map(gen.from)sealed trait Eventcase class Foo(i: Int) extends Eventcase class Bar(s: String) extends Eventcase class Baz(c: Char) extends Eventcase class Qux(values: List[String]) extends Event

And then:

scala> import io.circe.parser.decode, io.circe.syntax._import io.circe.parser.decodeimport io.circe.syntax._scala> decode[Event]("""{ "i": 1000 }""")res0: Either[io.circe.Error,Event] = Right(Foo(1000))scala> (Foo(100): Event).asJson.noSpacesres1: String = {"i":100}

This will work for any ADT anywhere that encodeAdtNoDiscr and decodeAdtNoDiscr are in scope. If we wanted it to be more limited, we could replace the generic A with our ADT types in those definitions, or we could make the definitions non-implicit and define implicit instances explicitly for the ADTs we want encoded this way.

The main drawback of this approach (apart from the extra circe-shapes dependency) is that the constructors will be tried in alphabetical order, which may not be what we want if we have ambiguous case classes (where the member names and types are the same).

The future

The generic-extras module provides a little more configurability in this respect. We can write the following, for example:

import io.circe.generic.extras.auto._import io.circe.generic.extras.Configurationimplicit val genDevConfig: Configuration =  Configuration.default.withDiscriminator("what_am_i")sealed trait Eventcase class Foo(i: Int) extends Eventcase class Bar(s: String) extends Eventcase class Baz(c: Char) extends Eventcase class Qux(values: List[String]) extends Event

And then:

scala> import io.circe.parser.decode, io.circe.syntax._import io.circe.parser.decodeimport io.circe.syntax._scala> (Foo(100): Event).asJson.noSpacesres0: String = {"i":100,"what_am_i":"Foo"}scala> decode[Event]("""{ "i": 1000, "what_am_i": "Foo" }""")res1: Either[io.circe.Error,Event] = Right(Foo(1000))

Instead of a wrapper object in the JSON we have an extra field that indicates the constructor. This isn't the default behavior since it has some weird corner cases (e.g. if one of our case classes had a member named what_am_i), but in many cases it's reasonable and it's been supported in generic-extras since that module was introduced.

This still doesn't get us exactly what we want, but it's closer than the default behavior. I've also been considering changing withDiscriminator to take an Option[String] instead of a String, with None indicating that we don't want an extra field indicating the constructor, giving us the same behavior as our circe-shapes instances in the previous section.

If you're interested in seeing this happen, please open an issue, or (even better) a pull request. :)

I have to deal a lot ADT to JSON lately, so decide to maintain my own extension library, which provides a bit different way to solve it using annotations and a macro:

ADT definitions:

import org.latestbit.circe.adt.codec._sealed trait TestEvent@JsonAdt("my-event-1") case class MyEvent1(anyYourField : String /*, ...*/) extends TestEvent@JsonAdt("my-event-2")case class MyEvent2(anyOtherField : Long /*, ...*/) extends TestEvent

Usage:

import io.circe._import io.circe.parser._import io.circe.syntax._// This example uses auto coding for case classes. // You decide here if you need auto/semi/custom coders for your case classes.import io.circe.generic.auto._ // One import for this ADT/JSON codecimport org.latestbit.circe.adt.codec._// Encodingimplicit val encoder : Encoder[TestEvent] =   JsonTaggedAdtCodec.createEncoder[TestEvent]("type")val testEvent : TestEvent = TestEvent1("test")val testJsonString : String = testEvent.asJson.dropNullValues.noSpaces// Decodingimplicit val decoder : Decoder[TestEvent] =   JsonTaggedAdtCodec.createDecoder[TestEvent] ("type")decode[TestEvent] (testJsonString) match {   case Right(model : TestEvent) => // ...}

Details: https://github.com/abdolence/circe-tagged-adt-codec