Scalaz extensions


FutureEither is a specialised version of scalaz.EitherT[Future, A, B] which allows simple composition of Future[A \/ B] operations.


Include the dependency from maven central: com.beamly:beamly-core-scalaz

import scala.concurrent.Future
import beamly.core.scalaz.future.{FutureEither, \?/}
import com.zeebox.core.http.common.HttpStatus.{NotFound, Ok}
import com.zeebox.core.http.server.http.HttpResponse

case class UserNotFound(userId: String)

class Example {
  def findFriends(userId: UserId): UserNotFound \?/ Seq[User] = ???

  def handleUserNotFound: UserNotFound => HttpResponse[Nothing] = {
    userNotFoundError => HttpResponse.failure(NotFound, s"User $userId not found")

  def friendsResponse(userId: UserId): Future[HttpResponse[Seq[User]]] = {
    val response: FutureEither[HttpResponse[Nothing], HttpResponse[Seq[User]]] =
      for {
        friends <- findFriends(userId) leftMap handleUserNotFound
      } yield HttpResponse(Ok, data = Some(friends))

The steps above when calling friendsResponse are:

  • call findFriends which returns either a user not found error or a list of friends
  • the call to leftMap maps the left-hand result (ie: UserNotFound) to a 404 HttpResponse
  • the yield returns the right-hand result which is the list of your friends, but wrapped in a 200 HttpResponse
  • the entire for-comprehension thus returns FutureEither[HttpResponse[Nothing], HttpResponse[Seq[User]]]
  • the union method flattens the FutureEither[HttpResponse[Nothing], HttpResponse[Seq[User]]] to a Future[HttpResponse[Seq[User]]]

Converting a regular Future to a FutureEither

def findUser(userId: UserId): Future[Option[User]]

findUser.toEither.leftMap {
    case ex: UserNotFoundException =>

findUser.toEither returns FutureEither[Throwable, Option[User]] and the leftMap call simply converts Throwable to UserNotFound.