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  • Sep 05 2019 14:43
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    codecov-io commented #460
Adam Rosien
@AlexITC there is a slight amount of overhead, but it's ok. you can also use the Clock#timed method
Tomasz Bekas
Hi, is there a way to express via the type system in cats-effect that certain function is non-suspendable? I would like to prevent people from implementing the function that would call stuff like Temporal[F].sleep(1.second) and force them to schedule any suspendable action in the background or on a different fiber than the one that function was executed on.
Alexandr Oshlakov
Hello. Anyone have problem after update ce to new versions with messages like this?
"Non-daemon threads currently preventing JVM termination: - 107: Thread[search-backend-elastic-interaction-dispatcher-32,5,main]"

Hello, am new to cats-effects and am confused about something

trait CampaignOpsResult {
  def message: String
case class CampaignOperationError(message: String) extends CampaignOpsResult
case class CampaignCreated(message: String = "Campaign Successfully created") extends CampaignOpsResult

val rr = for {
  db <- client
  coll <- db.getCollectionWithCodec[Campaign]("docs")
  existing: Option[Campaign] <- coll.find.filter(Filter.eq("campaignName", campaign.campaignName)).first
  writeResult: F[CampaignOpsResult] = if(existing.nonEmpty) CampaignOperationError("Campaign with that name exists before").pure
  else {
    for {
      res <- coll.insertOne(campaign)
      resp: CampaignOpsResult  = if(res.wasAcknowledged()) CampaignCreated() else CampaignOperationError("Unable to create campaign")
    } yield resp
  res <- writeResult
} yield res

now i expect the writeResult variable to be a valid F[CampaignOpsResult] but everytime i tried get this error

  polymorphic expression cannot be instantiated to expected type;
  found   : [F[_]]F[CampaignOperationError]
  required: F[CampaignOpsResult]
  writeResult: F[CampaignOpsResult] = if(existing.nonEmpty) CampaignOperationError("Campaign with that name exists before").pure

Pls any advice is appreciated

Luis Miguel Mejía Suárez
@horlahlekhon this chat is mostly dead, you may want asking in the discord server instead: https://discord.gg/vgn8sEV3
Laurent Valdes
I guys, nice to know you
I would like to introspect an IO, so that I can have access to its execution graph
is it possible to do it ?
Or let's rephrase the intent maybe: I need a Show instance or an interpreter that let people see the contents of an IO instance.
Luis Miguel Mejía Suárez
@valdo404 you may have better luck in the discord server: https://discord.gg/JDQSt72z
Hi all, I want to control the bulb using stream of signals. Each signal from the stream just toggle the bulb. I have no idea how to connect the pieces togheter. I have been trying to use for-comprehension but without any success. I have something like this at the moment.
  val signal1 =  Stream.repeatEval(producer("A", 100.millis))
  val signal2 =  Stream.repeatEval(producer("B", 200.millis))
  val signal: Stream[IO, String] = signal1.merge(signal2)

  class Bulb(ref: Ref[IO, Int]) {
    def toggle(): IO[Unit] = IO.println(("toggle")) >> IO.unit //todo

  def pipes(bulb: Bulb): Pipe[IO, String, String] = in =>
    in.evalMap(signal => bulb.toggle() >> IO("Toggled"))

  val ioBulb: IO[Bulb] = for {
    r <- Ref[IO].of(0)
    b = new Bulb(r)
  } yield b

ayeo (ayeo) assuming every toggle sets the internal Ref to either 1 or 0, you can do something like this.

object Main extends IOApp.Simple:
  val signal1 = Stream.emit[IO, String]("A").metered(100.millis)
  val signal2 = Stream.emit[IO, String]("B").metered(200.millis)

  def run: IO[Unit] = 
    Bulb.make[IO].flatMap { bulb =>
        .evalMap(_ => IO.println("toggle") *> bulb.toggle)

trait Bulb[F[_]]:
  def toggle: F[Unit]

object Bulb:
  def make[F[_]: Functor: Ref.Make]: F[Bulb[F]] =
    Ref[F].of(0).map { ref => 
        def toggle: F[Unit] = ref.update(x => if x == 0 then 1 else 0)

Sleeps and timeouts don't work so well in the browser but you can see the compiling version here: https://scastie.scala-lang.org/NXdBGJLZTwyJeFh3tsgIwg

Hello everyone,
Sorry if noob question, but I don't understand why using an instance of Async[F] where F is an effect monad, basically gives us access to the methods in MonadError too.
Async doesn't seem to extends MonadError...
Does Async have MonadError instances available regardless of what F is?
Luis Miguel Mejía Suárez
Hi @yanly007, first this channel is mostly dead, switch to discord.
Second, Async is a subtype of MonadError both in CE2 and in CE3.
Thanks for the answer Luis!
Hey, would anyone be able to help me understand some better ways to compose IO actions with cats?
Currently I've got something very akin to javascripts "callback hell", and there has to be a better way
I tried for example to use Either and map through assuming that the right bias will help, but it seems map keeps the value within the IO context
basically any usage examples that have 3-4 io actions that depend on each being composed would be amazingly helpful
Luis Miguel Mejía Suárez
for? Or just plain flatMap?
flatMap still propagates Either/Option etc though, so if I have something returning an IO[Either[A,B]], flatMap will pass Either[A,B] to the callback
so I've still got to do the standard unwrapping
let me show you the terrible code I've managed to cobble together
  def registerIntegration(req: Request[IO], auth: String) = {
    // First check that we have the correct payload
        case None => BadRequest("Malformed request body")
        case Some(payload) => {
          // Check that the user referenced exists
              case None => BadRequest("No user found")
              case Some(_) => {
                // Unwrap the lightning auth header
                authStringToLightningAuth(auth) match {
                  case Left(st) => BadRequest(st)
                  case Right((user, pass)) => {
                    // Make sure we have the correct authentication
                      .getAccount(user, pass)
                        case Left(error) =>
                            s"Lightning ad server failed: $error"
                        case Right(accountName) => {
                          // Mark that the user is lightning registered
                            .flatMap(registered =>
                              if (registered)
                                Ok("Registered for lightning integration")
                                  "Failed to register for lightning integration"
it seems to me that I should be able to avoid this amount of identation in idiomatic scala/cats
but, I've no idea how :/
Luis Miguel Mejía Suárez
@CameronDiver So two things that may help.
Use EitherT or OptionT
(make sure to just use one, like convert all Eithers into Option or vice versa)
And then remember that foo.flatMap(x => x.bar.flatMap(y => ???)) is the same as foo.flatMap(x => x.bar).flatMap(y => ???)
i.e. you can always unnest flatMap calls.
The for syntax may help with that.
The unnested flatMap calls is huge, I didn't notice that! I'll check out how to use EitherT/OptionT too. Really appreciate the help!
Naftoli Gugenheim
You cannot always unnest flatMaps easily. For instance if y => ??? depended on x then to unnest you'd have to return a tuple to pass x along with the result of x.bar
Hi folks! Would it be possible to convert A => F[B] to F[A => B]? I'm definitely missing something.
Naftoli Gugenheim
@tpetillot try discord
@nafg done, thanks!
Hi guys, I'm playing with Cats Fibers and having hard time to find some information regarding the integration of Fibers and JVM NIO; specifically do Fibers block when calling NIO operations or are they somehow integrated with such calls and automatically poll/select on file/socket descriptors?
Or is such construct orthogonal to Fibers and user needs to provide such functionality?
湿垃圾 == 菜狗
Hi guys, for the past two days, I've iterated my code by times. I find a simple way to do something like macwire in cats use IO or Resource. First it only works in Scala3. Finally I choose cats-effect-cps, It becomes more maintainable and intuitive. And than we can do the same thing in zio.RIO, zio.ZManaged, Future because the support of cats-effect-cps. I organized my code on macwire issue Further in Scala3.2 it can be
Mario Siegenthaler
Hey, I'm having some weird issues with my application just completely locking up after some time. It's all cats effect 3.3.13 (http4s server that makes calls to some old-school blocking apis [keycloak client wrapped with IO.interuptibleMany]). What I observed is that I seem to loose all cats-effect compute threads (io-compute). At the beginning I have two, then some compute-blocker in addition, then when it works I consistently see only the io-scheduler and no other io-* threads..
1 reply
Tomas Klubal
Hi all, why is the WorkStealingThreadPool#shutdown() calling Thread.currentThread.interrupt()? It causes the calling thread to die, which might not be desirable.
Abdulkhamid Muminov
Hi all. Is there any resource expect official documentation to learn the cats-effect library and cats data structures? Somehow the official docs are quite hard to grasp it for the functional newbee, or I'd call imperative rookie :D Thanks in advance!
Luis Miguel Mejía Suárez
@homycdev hey, you may get more help in Discord but check the resources and examples I have here: https://github.com/pslcorp/programs-as-values
What are the correct imports that I should use to be able to use the Id Monad?
I have cats-core version 2.8.0 as a dependency in my project, and have the import cats._
but my IDE still could not find the Id Monad

I'm trying to use parReduceMapA on a List[Set[MyType]] where MyType has a Semigroup instance defined for it, but I'm getting the error:
could not find implicit value for parameter T: cats.Reducible[List]

The code is using tagless final, so it looks kinda like this

object MyService {
  def impl[F[_]: Spawn: Parallel]: MyService[F] = new MyService[F] {
    private def otherFunc(param1: String, param2: Set[MyType]): List[Set[MyType]] = ???

    def myFunc(param1: String, param2: Set[MyType]): F[List[Set[MyType]]] =
      for {
        myVal <- otherFunc(param1, param2).parReduceMapA(Applicative[F].pure)
        ... etc
Okay, figured this out: List can be empty, so requires a Monoid, and parFoldMapA operates on the Monoid instance, which is why that works and parReduceMapA does not work because it requires a non-empty container since it operates on Semigroup