https://gitter.im/http4s/http4s?at=5bbdb915600c5f6423829ebf

collect-successful.hs

collectSuccessful :: [IO a] -> IO [a]
collectSuccessful xs = do
  ref <- newIORef []
  parTraverse (f ref) xs `onError` readIORef ref
 where
  f ref fa = fromEitherIO $ try fa >>= \a -> action ref a $> a
  action ref a = case a of
   (Right x) -> void $ atomicModifyIORef ref (\y -> (x : y, x))
   (Left _)  -> pure ()

traverse-until-first-failure-generic.scala

def generic[F[_]: MonadError[?[_], Throwable], G[_]: Traverse, A](
  gfa: G[F[A]],
  append: G[A] => A => G[A],
  ref: Ref[F, G[A]]
): F[G[A]] =
  gfa.traverse(_.attempt.flatTap {
    case Right(x) => ref.update(append(_)(x))
    case Left(_)  => Applicative[F].unit
  }.rethrow)
   .handleErrorWith(_ => ref.get)

 generic[IOIO, , ListList, , StringString](list2, g => x => g :+ x, ref)](list2, g => x => g :+ x, ref)

traverse-until-first-failure.scala

val ioa = IO.sleep(1.second) *> IO.pure("a")
val iob = IO.pure("b")
val ioc = IO.pure("c")
val iod = IO.sleep(3.seconds) *> IO.pure("d")
val ioe = IO.sleep(2.seconds) *> IO.pure("e")

val list1 = List(ioa, iob, ioc, iod, ioe)
val list2 = List(ioa, iob, IO.raiseError(new Exception("boom")), iod, ioe)

def collectSuccessful(list: List[IO[String]])(ref: Ref[IO, List[String]]): IO[List[String]] =
  list
    .traverse(_.attempt.flatTap {
      case Right(x) => ref.update(_ :+ x)
      case Left(_)  => IO.unit
    }.rethrow)
    .handleErrorWith(_ => ref.get)

val program = Ref.of[IO, List[String]](List.empty).flatMap { ref =>
  collectSuccessful(list2)(ref).flatMap(x => IO(println(x)))
}