Scala, Circe, Http4s - is it any way to encode Throwable in Circe?

Question

I have created hierarchy of errors:

sealed trait MyError extends Throwable
final case class SecondError(msg: String) extends MyError

Now I could get this kind of error in my http4s routes:

case GET -> Root / "things" => for {
        response <- things.all.foldM(
          error => error match {
            case SecondError(_) => InternalServerError(error)            
          }
...

But I get compiled error:

could not find implicit value for parameter encoder: io.circe.Encoder[Throwable]

Is it possible to encode Throwable with circe and http4s? I tried to do it this way:

implicit def encoderHttpThrowable: EntityEncoder[Env, Throwable] = jsonEncoderOf[Env, Throwable]

But it did not solve a problem.

Answer 1

It is not possible to encode automatically a Java Hierarchy with Circe (or any other library) (I am pretty sure of this).

So you have 3 possibilities:

1. Just use the Message of the error:

   case GET -> Root / "things"  =>
       things.all.foldM (
         error => InternalServerError(error.getMessage()),
         Ok(_)
       )
   

2. Or get rid of the Throwable:

     sealed trait MyError
     final case class SecondError(msg: String) extends MyError
   

   And now you can encode the error

   ...
   case GET -> Root / "things"  =>
     things.all.foldM (
       error => InternalServerError(error),
       Ok(_)
     )
   

3. Map your Throwable to your own Error (Sealed Trait without Throwable):

   ...
   case GET -> Root / "things"  =>
     things.all
       .mapError{
         case ex: IllegalArgumentException => SecondError(ex.getMessage)
         case ex => FirstError(ex.getMessage)
      }
       .foldM (
       error => InternalServerError(error),
       Ok(_)
     )