In Scala, how to apply implicit conversion to subtypes of a collection?

Question

I am trying to convert a list of objects to json using argonaut. The list contains a list of validation errors of varying type. For example it can contain an instance of 'MissingParameter' or, 'InvalidParameter' or any other type. I have defined EncodeJson (argonaut) codecs for both classes mentioned above. Is there any way I can convert a list of the above types to json using argonaut? I mean, can I achieve the following?

List(new MissingParameter("name"), new InvalidParameter("email")).asJson

Answer 1

I haven't used argonaut, but I guess the problem here is a general issue. The type of the list here is List[Product with Serializable]. And the compiler won't know how to serialize it to Json.

I would suggest you to create a Parameter trait, extend the classes with it, and write a serializer for Parameter that checks all the types with pattern matching:

Note: I'm using play-json library, you should adapt it to argonaut.

import play.api.libs.json.{Writes, JsValue, Json}
import play.api.libs.json.Writes._

trait Parameter

case class MissingParameter (paramName: String) extends Parameter

case class InvalidParameter (paramName: String) extends Parameter

implicit val writes1 = Json.writes[MissingParameter]
implicit val writes2 = Json.writes[InvalidParameter]

implicit val implicitParamWrites = new Writes[Parameter] {
  def writes(param: Parameter): JsValue = {
    param match {
      case missing: MissingParameter => Json.toJson(missing)
      case invalid: InvalidParameter => Json.toJson(invalid)
    }
  }
}

val list: List[Parameter] = List( MissingParameter("p1"), InvalidParameter("i1") )

Json.toJson(list)