An Implementation of a Safe Multi-value Race

raceN.scala

object RaceN {
  import cats.implicits._
  import cats.effect._
  import cats.effect.concurrent._
  import cats.data.NonEmptyList

  // Use with atleast 3, otherwise you should prefer Concurrent.race
  def raceN[F[_]: Concurrent, A](x1: F[A], x2: F[A], x3: F[A], xs: F[A]*): F[Either[NonEmptyList[Throwable], A]] = {
    for {
      deferred <- Deferred[F, A]
      out <- Bracket[F, Throwable].bracket(
        NonEmptyList.of(x1, x2 +: x3 +: xs:_*).traverse(fa =>
          Concurrent[F].start(fa.flatMap(a => deferred.complete(a).as(a)).attempt)
        )
      ){
        fibers: NonEmptyList[Fiber[F, Either[Throwable, A]]] => 
          Concurrent[F].race(
            fibers.traverse(_.join).map(
                _.traverse(_.swap).swap
            ),
            deferred.get
          )
      }(
        fibers => fibers.traverse_(_.cancel.attempt)
      )
    } yield out.fold(identity, Either.right)
  }
}