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.

Peter Neyens Peter Neyens · Accepted Answer · 2017-01-01T23:35:39

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)

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

1 Answers