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

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)