Weird EntityResponseGenerator behaviour since 0.20.0-M7

package repro

import cats.implicits._
import cats.effect._
import org.http4s.implicits._
import org.http4s.HttpRoutes
import org.http4s.server.blaze.BlazeServerBuilder

object App extends IOApp {
  override def run(args: List[String]): IO[ExitCode] =
    server[IO].compile.drain.as(ExitCode.Success)


  def server[F[_]: ConcurrentEffect: Timer] =
    BlazeServerBuilder[F]
      .bindHttp(8080, "0.0.0.0")
      .withHttpApp(routes[F].orNotFound)
      .serve

  def routes[F[_]: Sync]: HttpRoutes[F] = {
    val dsl = org.http4s.dsl.Http4sDsl[F]
    import dsl._
    HttpRoutes.of {
      // this results in empty response, used to result in 500 
      case _ => Ok(prg)
    }

    // this works as expected, returns 500
//    HttpRoutes.of {
//      case _ => for {
//        x <- prg
//        resp <- Ok(x)
//      } yield resp
//    }
  }

  def prg[F[_]: Sync]: F[String] = Sync[F].raiseError(new RuntimeException("oh no"))

}

I think the cause is that overload of EntityResponseGenerator.apply used to just mean flatMap (up to 0.20.0-M6), but now it’s deferring evaluating the F[A]-program in a Stream.eval of the body.

[SystemFw]

basically here https://github.com/http4s/http4s/blob/b3ea4b22fee858310ab88e9d5186457f3ec4482a/dsl/src/main/scala/org/http4s/dsl/impl/ResponseGenerator.scala#L50

you have this

 def apply[A](body: G[A])(implicit F: Applicative[F], w: EntityEncoder[G, A]): F[Response[G]] = {
    val entity = Entity(fs2.Stream.eval(body).flatMap(w.toEntity(_).body))
    val headers = addEntityLength(entity, w.headers)
    F.pure(Response[G](status = status, headers = headers, body = entity.body))
  }

F.pure(eval(body).flatMap(_.toEntity.body)) seems weird compared to body.map(_.toEntity.body) is there a reason for that that I don’t know?

(there is also a broader discussion about having such overloads, not a massive fan)

EDIT: Ah, it’s related to the introduction of the new type parameter here ofc 00cd51e ,that flatMap won’t compile. Interesting tradeoff since the reported behaviour is certainly a bit weird.

[rossabaker]

Alright, I’ll leave it to you to decide if it works just as well there.