How to set initial value for foldLeft When Using EitherT[Future, Failure, Option[B]]

Question

I am using EitherT[Future, Failure, Option[B]] in the following function which does foldLeft. How to set Initial value for foldLeft in the code below ?

    def doWork[A, B](seq: Set[A])(f: A => EitherT[Future, Failure,  Option[B]]): EitherT[Future, Failure, Set[Option[B]]] = 
        seq.foldLeft(????? how to set this initial value here ????) {
          case (acc, nxt) => acc.flatMap(bs => f(nxt).map(b => bs :+ b))
        }

Using Right(Set.empty), acc does not have flatMap function to invoke — Tom, Jul 16 '19 at 05:17
If using EitherT.right(Future(Set.empty)), then found : cats.data.EitherT[Future,Failure,Seq[Option[B]]] [error] required: cats.data.EitherT[scala.concurrent.Future,Nothing,Seq[Nothing]] — Tom, Jul 16 '19 at 05:29
it seems that you mixed `Either` and `Try` monads. What type do you expect to have on left param of `Either` instead of `Failure`? Maybe `Throwable`? — Bogdan Vakulenko, Jul 16 '19 at 05:54
or `Failure` is just your custom type implemented somewhere? — Bogdan Vakulenko, Jul 16 '19 at 05:59
use `EitherT[Future, Failure, Set[Option[B]]](Future.successful(Right(Set())))` but change `bs :+ b` to `bs + b` — Bogdan Vakulenko, Jul 16 '19 at 06:54

Mario Galic · Answer 1 · 2019-07-16T09:10:59.543

1

Try

EitherT.rightT[Future, Failure](Set.empty[Option[B]])

like so

def doWork[A, B](seq: Set[A])(f: A => EitherT[Future, Failure,  Option[B]]): EitherT[Future, Failure, Set[Option[B]]] =
  seq.foldLeft(EitherT.rightT[Future, Failure](Set.empty[Option[B]])) {
    case (acc, nxt) => acc.flatMap(bs => f(nxt).map(b => bs + b))
  }

This is a slightly shorter version of Bogdan's suggestion

EitherT[Future, Failure, Set[Option[B]]](Future.successful(Right(Set())))
EitherT.rightT[Future, Failure](Set.empty[Option[B]])

edited Jul 16 '19 at 09:10

answered Jul 16 '19 at 06:43

Mario Galic

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Which package defined EitherT.rightT ? I use import cats.data.EitherT, but does not have rightT – Tom Jul 16 '19 at 06:50
Yes that should be the correct import: https://typelevel.org/cats/datatypes/eithert.html#from-a-or-b-to-eithertf-a-b You will also have to follow Bogdan’s suggestion and change the operator. – Mario Galic Jul 16 '19 at 07:40

How to set initial value for foldLeft When Using EitherT[Future, Failure, Option[B]]

1 Answers1