This question has come up a few times recently, so I'm FAQ-ing it here. Suppose I've got some case classes like this:
import io.circe._, io.circe.generic.semiauto._
object model {
case class A(a: String)
case class B(a: String, i: Int)
case class C(i: Int, b: Boolean)
implicit val encodeA: Encoder[A] = deriveEncoder
implicit val encodeB: Encoder[B] = deriveEncoder
implicit val encodeC: Encoder[C] = deriveEncoder
implicit val decodeA: Decoder[A] = deriveDecoder
implicit val decodeB: Decoder[B] = deriveDecoder
implicit val decodeC: Decoder[C] = deriveDecoder
}
And I want to encode a value that could be any one of these as JSON using circe and Shapeless coproducts.
import io.circe.shapes._, io.circe.syntax._
import shapeless._
import model._
type ABC = A :+: B :+: C :+: CNil
val c: ABC = Coproduct[ABC](C(123, false))
This looks fine at first:
scala> c.asJson
res0: io.circe.Json =
{
"i" : 123,
"b" : false
}
But the problem is that I can never decode a coproduct containing a B
element, since any valid JSON document that could be decoded as B
can also be decoded as A
, and the coproduct decoders provided by circe-shapes try the elements in the order they appear in the coproduct.
scala> val b: ABC = Coproduct[ABC](B("xyz", 123))
b: ABC = Inr(Inl(B(xyz,123)))
scala> val json = b.asJson
json: io.circe.Json =
{
"a" : "xyz",
"i" : 123
}
scala> io.circe.jawn.decode[ABC](json.noSpaces)
res1: Either[io.circe.Error,ABC] = Right(Inl(A(xyz)))
How can I disambiguate the elements of my coproduct in my encoding?