Scala、Circe、Http4s - 有没有办法在 Circe 中对 Throwable 进行编码？

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(_) )