Currency Quick Evaluation

ConcurrentSuccess.scala

import cats.effect._
import cats.effect.implicits._
import cats.effect.concurrent._
import cats.implicits._
import cats._
import cats.temp.par._

object ConcurrentSuccess{

  def parAttempt[G[_]: Traverse, F[_]: Par: MonadError[?[_], Throwable], A](xs: G[F[A]]): F[G[Either[Throwable, A]]] = 
    xs.parTraverse(_.attempt)

  def parAttempt2[G[_]: Traverse, F[_]: Concurrent, A](l: G[F[A]]): F[G[Either[Throwable, A]]] = {
    val intermediate: F[G[Fiber[F, Either[Throwable, A]]]] = l.traverse(_.attempt.start)
    intermediate.flatMap(_.traverse(_.join))
  }

  def firstComplete[G[_]: Traverse, F[_]: Concurrent, A](l: G[F[A]]): F[Option[A]] = for {
    deferred <- Deferred[F, Option[A]]
    gFibers <- l.traverse(_.flatMap(a => deferred.complete(a.some).attempt).start)
    completeEmpty <- (gFibers.traverse_(_.join) *> deferred.complete(None).attempt).start
    value <- deferred.get
      .guarantee(
        gFibers.traverse(_.cancel.attempt.start).start.void *>
        completeEmpty.cancel.start.void
      )
  } yield value

}