field based circe decoder

example.sc

import cats.syntax.functor._
import io.circe.Decoder
import io.circe.generic.auto._
import io.circe.parser.decode

trait Event

case class Foo(x: Int) extends Event
case class Bar(y: String) extends Event

val decoderFoo: Decoder[Foo] = Decoder[Foo].prepare(_.downField("foo-event"))
val decoderBar: Decoder[Bar] = Decoder[Bar].prepare(_.downField("bar-event"))

implicit val decodeFooBar: Decoder[Event] = decoderFoo.or(decoderBar.widen[Event])

decode[Event]("""{"foo-event": {"x": 1}}""")
decode[Event]("""{"bar-event": {"y": "1"}}""")