Stackoverflow when implicitly creating an EntityEncoder for an implicit circe Encoder

I brought this up in the gitter channel and received great help from @SystemFw.

I’m adding it as an issue so that other people who might stumble over it are warned and also for tracking a possible fix for it.

If you create an implicit EntityEncoder from an implicit Encoder like so:

implicit def entityEncoder[A: Encoder]: EntityEncoder[F, A] = jsonEncoderOf[F, A]

You will get a StackOverflowError.

The solution (thanks @SystemFw) is to define the implicit like this:

  implicit def entityEncoder[A](implicit enc:  Encoder[A], ev: EntityEncoder[F, String]): EntityEncoder[F, A] = jsonEncoderOf[F, A](ev, implicitly[Applicative[F]], enc)

[SystemFw]

@kevinmeredith sure. Let’s look at the signature of jsonEncoderOf: def jsonEncoderOf[F[_]: EntityEncoder[?[_], String]: Applicative, A: Encoder]: EntityEncoder[F, A]

It wants an Encoder for A, and an EntityEncoder[F, String]. So first of all your signature above is making the mistake of not propagating the EntityEncoder[F, String] constraint. Normally forgetting to propagate an implicit will cause a compilation error, since the method call needing it will not find it, however in this specific case, to define entitytEncoder you call jsonEncoder, which looks for an EntityEncoder[F, String] and finds…entityEncoder (by applying A to String), and therefore the code compiles.

At runtime, calling entityEncoder will evaluate jsonEncoder, which will evaluate entityEncoder, which will evaluate jsonEncoder, which will evaluate… and so on.

This infinite recursion causes a stack overflow.

[gvolpe]

Normally I define my sealed traits that are ADT’s by extending Product with Serializable to help the type inference system. Not sure when one would try to get an EntityEncoder for an object though but it’s good to keep this in mind.