Using Natchez in IO

[adamjuhasz]

We have some existing http4s services we would like to add tracing to. These were all written in IO though so we can't add the Span typeclass to it.

What's the best way of using Natchez in the concrete IO world?

Example

case class ApplicationRoutes(services: ApplicationServices) {
    private val authMiddleware = AuthMiddleware(services.bar)
  
  private val clientClientRoutes: HttpRoutes[IO] = ClientClientRoutes(services.baz).routes(authMiddleware)

  private val adminClientRoutes: HttpRoutes[IO] = AdminClientRoutes(services.foo).routes(authMiddleware)

  private val newRoutes: HttpRoutes[IO] = adminClientRoutes <+> clientClientRoutes

  val httpApp: HttpApp[IO] = appRoutes.orNotFound
}

object AppMain extends IOApp {
  override def run(): IO[ExitCode] = {
    Resources.make(config).use {
      val httpApp = ApplicationRoutes(ApplicationServices()).appRoutes

      // Would be great to wrap `httpApp` here with Natchez

      BlazeServerBuilder[IO]
        .bindHttp(httpPort, httpHost)
        .withHttpApp(httpApp)
        .withServiceErrorHandler(errorHandler)
        .resource
        .run
    }
  }
}

We've tried making a TracedIO with type TracedIO[A] = Kleisli[IO, Span[IO], A] and lifiting the app but we're not getting the types to match up

import com.ovoenergy.natchez.extras.http4s.server.TraceMiddleware

// <snip>

        val httpApp = ApplicationRoutes(ApplicationServices()).appRoutes

        // lift HttpApp[IO] to HttpApp[TracedIO]
        val liftedApp: Kleisli[TracedIO, Request[TracedIO], Response[TracedIO]] =
          Kleisli.liftF(httpApp)

        val slf4j: EntryPoint[IO] = Slf4j.entryPoint[IO]
        val tracedApp = TraceMiddleware[IO](slf4j, tracingConfig)(liftedApp)

        BlazeServerBuilder[IO]
          .bindHttp(httpPort, httpHost)
          .withHttpApp(tracedApp)
          .withServiceErrorHandler(errorHandler)
          .resource
          .run

I assume I'm missing a conceptual link somewhere