json - Scala Circe with generics

Question

I am trying to use the scala json library Circe, wrapping it in a simple trait to provide conversion to/from json for which I have the following:

import io.circe.generic.auto._
import io.circe.parser._
import io.circe.syntax._

trait JsonConverter {
  def toJson[T](t : T) : String
  def fromJson[T](s: String) : T
}

case class CirceJsonConverter() extends JsonConverter{
  override def toJson[T](t: T): String = t.asJson.noSpaces
  override def fromJson[T](s: String): T = decode[T](s).getOrElse(null).asInstanceOf[T]
}

The aim of this is to simply be able to call JsonConverter with any object and have it convert it to/from json as such jsonConverter.toJson(0) must equalTo("0") , however when I try to compile it I get the following:

[error] could not find implicit value for parameter encoder: io.circe.Encoder[T]
[error]   override def toJson[T](t: T): String = t.asJson.noSpaces
[error]                                            ^
[error] could not find implicit value for parameter decoder: io.circe.Decoder[T]
[error]   override def fromJson[T](s: String): T = decode[T](s).getOrElse(null).asInstanceOf[T]
[error]                                                     ^
[error] two errors found

I can of course have a class that everything I intend to put through the converter inherit from, but I had the impression that circe could auto generate the encoders/decoders?

Answer 1

What you want is not going to work unless you can implement a strategy for turning any object into Json... which seems unlikely. Circe (and many other libs) instead choose to use a common pattern called Type Classes to make it convenient to define how you want to do something, in this case Encoder/Decoder, for a specific type.

I recommend researching Type Classes if you are unfamiliar with them. And then take a look at the Circe docs to see how you can implement Encoders/Decoders specifically.