smithy4s-precompilation.scala

import org.http4s._
import org.http4s.syntax.all._
import smithy4s.kinds._
import smithy4s.Service
import smithy4s.http4s.SimpleRestJsonBuilder
import cats.data.OptionT
import cats.effect._
import cats.effect.syntax.all._

def precompileRoutes[Alg[_[_, _, _, _, _]]](service: Service[Alg])(
    f: service.Impl[IO] => Resource[IO, HttpRoutes[IO]]
): Resource[IO, service.Impl[IO] => HttpRoutes[IO]] = {
  IOLocal[Option[service.FunctorInterpreter[IO]]](None).toResource.flatMap {
    local =>
      val delegatingImpl =
        service.fromPolyFunction(new service.FunctorInterpreter[IO] {
          def apply[I, E, O, SI, SO](
              operation: service.Operation[I, E, O, SI, SO]
          ): IO[O] = local.get.flatMap {
            case None =>
              IO.raiseError(
                new IllegalAccessError(
                  s"Cannot call ${service.id.name} methods here"
                )
              )
            case Some(interpret) => interpret(operation)
          }
        })
      f(delegatingImpl).map { hotswappingRoutes => (impl: service.Impl[IO]) =>
        HttpRoutes[IO] { request =>
          OptionT.liftF {
            local.set(Some(service.toPolyFunction(impl)))
          } *> hotswappingRoutes(request)
        }
      }
  }
}

// Here's the usage : 
val deferredRouter: Resource[IO, HelloWorldService[IO] => HttpRoutes[IO]] =
  precompileRoutes(HelloWorldService) { impl: HelloWorldService[IO] =>
    // if you call a method of `HelloWorldService` in this lambda,
    // you'll get the `IllegalAccessError`
    SimpleRestJsonBuilder.routes(impl).resource
  }