Convert EitherT[Future, A, B] to Option[B]

Question

I have a method

def getInfoFromService(reqParams: Map[String, String]): Future[Either[CustomException, A]] 

and I have another function

import cats.implicits._
def processInfoAndModelResponse(input: Future[Either[CustomException, A]]): Option[A] = {
    for {
         either <- input
         resultA <- either
     } yield resultA.some
}

So basically, I am trying to convert Future[Either[CustomException, A]]) to Option[A] . The resultA <- either in above code would not work as map is not happy with type.

Answer 1

What you try to do is basically something like

def convert[A](future: Future[Either[CustomException, A]]): Option[A] =
  Try(Await.result(future, timeout)).toEither.flatten.toOption

This:

• looses information about error, which prevents any debugging (though you could log after .flatten )
• blocks async operation which kills any benefit of using Future in the first place

Answer 2

Monads do not work like that. Basically you can handle several monadic calculations with for for the same monad, but Future and Either are different monads.

Try

import scala.concurrent.{Await, Future}
import scala.concurrent.duration._

def processInfoAndModelResponse(input: Future[Either[CustomException, A]]): Option[A] = {
  Await.result(input, 1.minute).toOption
}

@MateuszKubuszok 's answer is different in that it doesn't throw.

Answer 3

You can't sensibly convert a Future[Either[CustomException, A]] to Option[A] , as you don't know what the result will be until the Future has completed.

Future[Either[CustomException, A]] might just bet kept that way, or put in IO as IO[Either[CustomException, A]] . If you don't care about CustomException , you can also capture it Future (or IO ) error handling mechanisms, or just discard it entirely.

Some options:

absorb the CustomException in Future's error handling:

val fa: Future[A] = fut.transform(_.map(_.toTry))

ignore the CustomException an make the Future return with None

val fa: Future[Option[A]] = fut.map(_.toOption)

if you really want to block a thread and wait for the result,

Await.result(input, Duration.Inf).toOption

but all the caveats of awaiting apply. See the documentaion , reproduced here:

WARNING: It is strongly discouraged to supply lengthy timeouts since the progress of the calling thread will be suspended—blocked—until either the Awaitable has a result or the timeout expires.