simple http4s server program for examining Doobie

ServerMain.scala

package withhttp4s

import cats.effect.{IO, IOApp}
import cats.implicits._
import org.http4s.{HttpApp, HttpRoutes}
import org.http4s.implicits._
import org.http4s.server.blaze._
import cats.effect._
import doobie._
import doobie.implicits._
import io.circe.generic.auto._
import io.circe.syntax._
import org.http4s.circe._
import org.http4s.dsl.Http4sDsl

//case class City(id: Int, name: String, district: Double)
case class City(id: Int, name: String, district: String)

class MyService[F[_]: Async: ContextShift] extends Http4sDsl[F] {
  def xa: Transactor[F] = Transactor.fromDriverManager[F](
    "org.postgresql.Driver", "jdbc:postgresql:world", "postgres", "pass"
  )
  def findCitiesByCountry(c: String): doobie.Query0[City] =
    sql"select id, name, district from city where countrycode = $c".query[City]

  object CountryCodeMatcher extends QueryParamDecoderMatcher[String]("countryCode")

  def app: HttpApp[F] = HttpRoutes.of[F] {
    case GET -> Root / "cities" :? CountryCodeMatcher(code) =>
      Ok(findCitiesByCountry(code).stream.map(_.asJson).transact(xa))
  }.orNotFound
}

object ServerMain extends IOApp {
  def run(args: List[String]): IO[ExitCode] =
    BlazeServerBuilder[IO]
      .bindHttp(8080, "localhost")
      .withHttpApp(new MyService[IO].app)
      .serve
      .compile.drain
      .as(ExitCode.Success)
}