JsonFormat for case objects inheriting a sealed abstract class

[Josh Lemer]

So I have, for instance

sealed abstract class TemperatureUnit

object TemperatureUnits {
  case object Celsius extends TemperatureUnit
  case object Fahrenheit extends TemperatureUnit
}

object TemperatureUnitJsonProtocol extends DefaultJsonProtocol {
  implicit object TemperatureUnitFormat extends RootJsonFormat[TemperatureUnit] {
    override def read(json: JsValue) = ???
    override def write(temperatureUnit: TemperatureUnit) = temperatureUnit match {
      case TemperatureUnits.Celsius => JsString("Celsius")
      case TemperatureUnits.FahrenHeit => JsString("Fahrenheit")
    }
  }
}

However, when I go and try to use this

TemperatureUnits.Celsius.toJson

I get error:

Error:(15, 22) Cannot find JsonWriter or JsonFormat type class for com.joshlemer.TemperatureUnits.Celsius.type

I also can't seem to be able to explicitly write a CelsiusFormat:

implicit object CelsiusFormat extends RootJsonFormat[TemperatureUnits.Celsius]

doesn't compile, as `TemperatureUnits.Celsius` cannot be found. However I can fix this and probably get what I want if I use 0-parameter case classes, but those are deprecated and so I don't want to use them.

[Age Mooij]

The type of your format is RootJsonFormat[TemperatureUnit] so it will only be able to read/write instances that are explicitly typed as such. 

So this will work:

val c: TemperatureUnit = Celcius

val f: TemperatureUnit = Fahrenheit

c.toJson

f.toJson

Hope this helps

Age

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