Http4s: decide response type based on stream head

demo.scala

import cats.effect.IO
import fs2.Stream
import org.http4s.{HttpRoutes, Request, Response}
import org.http4s.dsl.Http4sDsl

import scala.concurrent.ExecutionContext
import scala.util.control.NoStackTrace

val ec = ExecutionContext.global
implicit val cs = IO.contextShift(ec)

object Service extends Http4sDsl[IO] {
  def streamResponse: Stream[IO, String] => IO[Response[IO]] = s =>
    s.head.compile.lastOrError.flatMap(_ => Ok(s)).handleErrorWith(e => BadRequest(e.getMessage))
  
  val routes = HttpRoutes.of[IO]{
    case GET -> Root / "fail" =>
      val body = Stream.bracket(IO(println("acquire...")))(_ => IO(println("release..."))).drain ++
        Stream.raiseError[IO](new RuntimeException("boom!!!") with NoStackTrace).covaryOutput[String]
      streamResponse(body)

    case GET -> Root / "succeed" =>
      val body = Stream.bracket(IO(println("acquire...")))(_ => IO(println("release..."))).drain ++
        Stream[IO, String]("content")
      streamResponse(body)
  }
}

import org.http4s.implicits._


(for {
  fail <- Service.routes.orNotFound.run(Request(uri = uri"/fail"))
  content <- fail.bodyText.compile.string
} yield fail -> content).unsafeRunSync()

// acquire...
// release...
// res0: (org.http4s.Response[[+A]cats.effect.IO[A]], String) = (Response(status=400, headers=Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 7)),boom!!!)


(for {
  success <- Service.routes.orNotFound.run(Request(uri = uri"/succeed"))
  content <- success.bodyText.compile.string
} yield success -> content).unsafeRunSync()

// acquire...
// release...
// acquire...
// release...
// res1: (org.http4s.Response[[+A]cats.effect.IO[A]], String) = (Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Transfer-Encoding: chunked)),content)