http4s-error-handling-mtl.scala

import cats.{ ApplicativeError, MonadError }
import cats.data.{ Kleisli, OptionT }
import cats.effect.Sync
import cats.effect.concurrent.Ref
import cats.syntax.all._
import io.circe.generic.auto._
import io.circe.syntax._
import org.http4s._
import org.http4s.circe.CirceEntityDecoder._
import org.http4s.circe._
import org.http4s.dsl.Http4sDsl

case class User(username: String, age: Int)
case class UserUpdateAge(age: Int)

sealed trait UserError extends Exception
case class UserAlreadyExists(username: String) extends UserError
case class UserNotFound(username: String) extends UserError
case class InvalidUserAge(age: Int) extends UserError

trait UserAlgebra[F[_]] {
  def find(username: String): F[Option[User]]
  def save(user: User): F[Unit]
  def updateAge(username: String, age: Int): F[Unit]
}

object UserInterpreter {

  def create[F[_]](implicit F: Sync[F]): F[UserAlgebra[F]] =
    Ref.of[F, Map[String, User]](Map.empty).map { state =>
      new UserAlgebra[F] {
        private def validateAge(age: Int): F[Unit] =
          if (age <= 0) F.raiseError(InvalidUserAge(age)) else F.unit

        override def find(username: String): F[Option[User]] =
          state.get.map(_.get(username))

        override def save(user: User): F[Unit] =
          validateAge(user.age) *>
            find(user.username).flatMap {
              case Some(_) =>
                F.raiseError(UserAlreadyExists(user.username))
              case None =>
                state.update(_.updated(user.username, user))
            }

        override def updateAge(username: String, age: Int): F[Unit] =
          validateAge(age) *>
            find(username).flatMap {
              case Some(user) =>
                state.update(_.updated(username, user.copy(age = age)))
              case None =>
                F.raiseError(UserNotFound(username))
            }
      }
    }

}

class UserRoutes[F[_]: Sync](userAlgebra: UserAlgebra[F]) extends Http4sDsl[F] {

  val routes: HttpRoutes[F] = HttpRoutes.of[F] {

    case GET -> Root / "users" / username =>
      userAlgebra.find(username).flatMap {
        case Some(user) => Ok(user.asJson)
        case None => NotFound(username.asJson)
      }

    case req @ POST -> Root / "users" =>
      req.as[User].flatMap { user =>
        userAlgebra.save(user) *> Created(user.username.asJson)
      }

    case req @ PUT -> Root / "users" / username =>
      req.as[UserUpdateAge].flatMap { userUpdate =>
        userAlgebra.updateAge(username, userUpdate.age) *> Ok(username.asJson)
      }
  }

}

class UserRoutesAlt[F[_]: Sync](userAlgebra: UserAlgebra[F]) extends Http4sDsl[F] {

  val routes: HttpRoutes[F] = HttpRoutes.of[F] {

    case GET -> Root / "users" / username =>
      userAlgebra.find(username).flatMap {
        case Some(user) => Ok(user.asJson)
        case None => NotFound(username.asJson)
      }

    case req @ POST -> Root / "users" =>
      req.as[User].flatMap { user =>
        userAlgebra.save(user) *> Created(user.username.asJson)
      }.handleErrorWith { // compiles without giving you "match non-exhaustive" error
        case UserAlreadyExists(username) => Conflict(username.asJson)
      }

    case req @ PUT -> Root / "users" / username =>
      req.as[UserUpdateAge].flatMap { userUpdate =>
        userAlgebra.updateAge(username, userUpdate.age) *> Ok(username.asJson)
      }.handleErrorWith { // compiles without giving you "match non-exhaustive" error
        case InvalidUserAge(age) => BadRequest(s"Invalid age $age".asJson)
      }
  }

}

trait HttpErrorHandler[F[_], E <: Throwable] {
  def handle(routes: HttpRoutes[F]): HttpRoutes[F]
}

object RoutesHttpErrorHandler {
  def apply[F[_]: ApplicativeError[?[_], E], E <: Throwable](routes: HttpRoutes[F])(handler: E => F[Response[F]]): HttpRoutes[F] = 
    Kleisli { req =>
      OptionT {
        routes.run(req).value.handleErrorWith(e => handler(e).map(Option(_)))
      }
    }
  }

object HttpErrorHandler {
  def apply[F[_], E <: Throwable](implicit ev: HttpErrorHandler[F, E]) = ev
}

class UserRoutesMTL[F[_]: Sync](userAlgebra: UserAlgebra[F])
                               (implicit H: HttpErrorHandler[F, UserError]) extends Http4sDsl[F] {

  private val httpRoutes: HttpRoutes[F] = HttpRoutes.of[F] {

    case GET -> Root / "users" / username =>
      userAlgebra.find(username).flatMap {
        case Some(user) => Ok(user.asJson)
        case None => NotFound(username.asJson)
      } 

    case req @ POST -> Root / "users" =>
      req.as[User].flatMap { user =>
        userAlgebra.save(user) *> Created(user.username.asJson)
      } 

    case req @ PUT -> Root / "users" / username =>
      req.as[UserUpdateAge].flatMap { userUpdate =>
        userAlgebra.updateAge(username, userUpdate.age) *> Created(username.asJson)
      } 
  }

  val routes: HttpRoutes[F] = H.handle(httpRoutes)

}

class UserHttpErrorHandler[F[_]: MonadError[?[_], UserError]] extends HttpErrorHandler[F, UserError] with Http4sDsl[F] {
  private val handler: UserError => F[Response[F]] = {
    case InvalidUserAge(age) => BadRequest(s"Invalid age $age".asJson)
    case UserAlreadyExists(username) => Conflict(username.asJson)
    case UserNotFound(username) => NotFound(username.asJson)
  }

  override def handle(routes: HttpRoutes[F]): HttpRoutes[F] =
    RoutesHttpErrorHandler(routes)(handler)
}