Flattening Future[EitherT[Future, A, B]] - scala

Question

As the title mentions.
Having many operations done using EitherT[Future, A, B]. Sometimes I want map left or right through another operation having signature A => Future[C]. Other scenario is that EitherT[Future, A, B] the result of a mapping over a future resulting Future[EitherT[Future, A, B]].
How can I elegantly flatten types like:
EitherT[Future, Future[A], Future[B]] and Future[EitherT[Future, A, B]]
Thank you in advance.

Answer 1

In all your cases you can use EitherT#flatMap (or EitherT#flatMapF), in combination with lifting some value to EitherT (or disjunction (\/) with flatMapF).
Mapping a B => F[C] over an EitherT[F, A, B] :
flatMap + lift
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
import scalaz._, Scalaz._
def f(i: Int): Future[Double] = Future.successful(i.toDouble)
val r = EitherT.right[Future, String, Int](Future.successful(1))
r.flatMap(i => EitherT.right(f(i)))
// or
r.flatMapF(i => f(i).map(_.right))
Mapping a A => F[C] over an EitherT[F, A, B] :
swap + flatMap + lift
def g(s: String): Future[Int] = Future.successful(s.length)
val l = EitherT.left[Future, String, Int](Future.successful("error"))
l.swap.flatMap(s => EitherT.right(g(s))).swap
// or
l.swap.flatMap(s => EitherT.left[Future, Int, Int](g(s)))
// or
l.swap.flatMapF(s => g(s).map(_.left))
Mapping an A => Either[F, B, C] to an F[A] :
lift + flatMap
def h(i: Int): EitherT[Future, String, Int] =
EitherT.right(Future.successful(i + 1))
val fut = Future.successful(1)
// mapping gives us Future[EitherT[Future, String, Int]]
fut.map(h)
// lifting to EitherT and flatMap gives us EitherT[Future, String, Int]
EitherT.right(fut).flatMap(h)