How to transform Either[Future[A], Future[B]] to Future[Either[A, B]]

问题

I have an instance of Either[Future[A], Future[B]] and I would like to transform it to Future[Either[A, B]].

Since my previous question, cats 0.8.1 has been released, changing the structure of the library and dropping Xor in favor of Either, which is right-biased in 2.12.

Thus the method described in the previous accepted answer does not work anymore. I have tried to find the appropriate imports but failed.

cats.instances.either._
cats.implicits._ 
cats.syntax.bitraverse._

looked plausible but unfortunately does not work. Compilation still fails with value bisequence is not a member of Either[scala.concurrent.Future[A],scala.concurrent.Future[‌​B]]

Widening the imports to *.all._ did not change the compiler error.

I am on scala 2.11.8 as not all libraries the project depends upon have released a version for 2.12

回答1:

have compiled the answers from the comments for reader convenience:

1: you can do it in plain scala (using cats.Functor for the definition here), courtesy of @wheaties & @jean-philippe-pellet

def eitherFmap[F[_], A, B](either: Either[F[A], F[B])(implicit F: Functor[F]): F[Either[A, B]] =   
either.fold(f => F.map(f)(Left(_)), f => F.map(f)(Right(_)))

2: cats bisequence refused to compile because of conflicting imports: a single import is enough here - courtesy of @kolmar

import cats.instances._
...
val futureOfEither = eitherOfFutures.bisequence

3: read the cats import guide to avoid the issue in the future - courtesy @peter-neyens

来源：https://stackoverflow.com/questions/41188301/how-to-transform-eitherfuturea-futureb-to-futureeithera-b