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.

From memory you don't have your `MyError` trait extend `Throwable` but instead match the `Throwable` to one of your `MyError`s. Then you just need to encode the error's case class — randbw, Jan 16 '20 at 14:48
You mean extend Error instead of Throwable? What if functions I use need Throwable in signature? — Developus, Jan 16 '20 at 14:49
At the "end of the world" you have a function of `Throwable => MyError` and then circe encodes the result of that — randbw, Jan 16 '20 at 14:50
Here is an article which deals with this: https://typelevel.org/blog/2018/08/25/http4s-error-handling-mtl.html — randbw, Jan 16 '20 at 14:52
I have encoders for my custom errors. The problem is only with Throwable — Developus, Jan 16 '20 at 14:57
What type is `InternalServerError` exactly: `org.http4s.dsl.impl.InternalServerError`? — pme, Jan 16 '20 at 18:50

pme · Accepted Answer · 2020-01-16T19:36:10.797

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:

Just use the Message of the error:

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

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(_)
  )

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(_)
  )

edited Jan 16 '20 at 19:36

answered Jan 16 '20 at 18:58

pme

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The problem here is that my functions `things.all` return `ZIO[Any, Throwable, Data]`, so the second solution is not possible to use. I'll try with first one. – Developus Jan 16 '20 at 19:02
Yet another possibility;) check my answer – pme Jan 16 '20 at 19:08
Thanks, I have done some magic and also remove Throwable from extend and it work. – Developus Jan 17 '20 at 09:00

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

1 Answers1