argonaut union type example

argonaut.scala

package argonaut.example

import argonaut._, Argonaut._
import scalaz._, Scalaz._

object UnionExample extends {

  sealed trait Thing
  final case class One(n: String, i: Int) extends Thing
  final case class Two(n: String) extends Thing
  case object Three extends Thing

  implicit def ThingEncodeJson: EncodeJson[Thing] =
    EncodeJson(_ match {
      case One(n, i) => Json("one" := Json("name" := n, "age" := i))
      case Two(n)    => Json("two" := Json("name" := n))
      case Three     => Json("three" := ())
    })

  implicit def ThingDecodeJson: DecodeJson[Thing] =
    DecodeJson(c =>
      tagged("one", c, jdecode2L(One.apply)("name", "age")) |||
      tagged("two", c, jdecode1L(Two.apply)("name")) |||
      tagged("three", c, implicitly[DecodeJson[Unit]].map(_ => Three)))

  def tagged[A](tag: String, c: HCursor, decoder: DecodeJson[A]): DecodeResult[A] =
    (c --\ tag).hcursor.fold(DecodeResult.fail[A]("Invalid tagged type", c.history))(decoder.decode)
}

Mark, is there an update to this example? I get:

UnionExample.scala:21: type mismatch;
[error] found : argonaut.DecodeResult[UnionExample.Two]
[error] required: argonaut.DecodeResult[Product with Serializable with UnionExample.Thing]
[error] Note: UnionExample.Two <: Product with Serializable with UnionExample.Thing, but class DecodeResult is invariant in type A.
[error] You may wish to define A as +A instead. (SLS 4.5)
[error] tagged("two", c, jdecode1L(Two.apply)("name")) |||
[error] ^

Never mind, it's possible to just widen the type to the common supertype, using another map[T], scalaz widen, or in my case, simple ascription of the supertype to a case object or case class constructor.

(h/t @tpolecat for leading me to the finish line on this one)

Hi @toby5box... do you happen to have an updated example on how you got the gist working?