I'm trying to parse json like
{
"element": "string",
"content": "application/json"
}
where element decides which type the json is. But my code fails to parse.
import scalaz._, Scalaz._
import argonaut._, Argonaut._, Shapeless._
case class ArrayAttributes(default: List[StringElement])
sealed trait Element
case class StringElement(content: String) extends Element
case class ArrayElement(attributes: ArrayAttributes, content: List[Element]) extends Element
case class Reference(element: String) extends Element { def content = element }
object Parser {
def kindDecode[T](
kinds: Map[String, DecodeJson[T]],
fail: HCursor => DecodeResult[T] = { c: HCursor => DecodeResult.fail[T]("expected one of ${kind.keys}", c.history) }): DecodeJson[T] = DecodeJson(c =>
(c --\ "element").as[String].flatMap { kind =>
kinds.get(kind).map(_.decode(c)).getOrElse(fail(c))
}
)
implicit def elementDecode: DecodeJson[Element] = kindDecode(
Map(
"string" -> DecodeJson.of[StringElement].map(identity[Element]),
"array" -> arrayDecode.map(identity[Element])
),
{ c => DecodeJson.of[Reference].decode(c).map(identity[Element]) }
)
def arrayDecode: DecodeJson[ArrayElement] = jdecode2L(ArrayElement.apply)("attributes", "content")
}
I'm going to answer with the current milestone of argonaut-shapeless (1.0.0-M1), which has had relevant additions since the 0.3.1 version.
It allows to specify so-called JsonSumCodecs for sum types, that drive how the sub-types are encoded / discriminated.
By defining one for Element in its companion object, like
implicit val jsonSumCodecForElement = derive.JsonSumCodecFor[Element](
derive.JsonSumTypeFieldCodec(
typeField = "element",
toTypeValue = Some(_.stripSuffix("Element").toLowerCase)
)
)
your example just works:
> Parse.decodeEither[Element](member)
res1: (String \/ (String, CursorHistory)) \/ Element =
\/-(StringElement(application/json))
JsonSumCodecFor above is a type class that provides a JsonSumCodec for a given type, here Element. As a JsonSumCodec, we choose JsonSumTypeFieldCodec, that uses a field, "type" by default, to discriminate between sub-types. Here, we choose "element" instead of "type", and we also transform the sub-type names (toTypeValue argument), so that these match the names of the example input.