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Rafał Sumisławski
@RafalSumislawski
My team ins currently on the implicit side. Before there was Blocker in cats-effect we used to create our own wrappers around diffrent ExecutionContext, to be able to pass them around implicitly without the risk of using the wrong implicit instance.
Fabio Labella
@SystemFw
you (@danielkarch) can look at various tests for stack-safety in cats-effect for these various corners
also, you will notice that in my code there is a comment saying "no double execution check" on Async, so my async there is not idempotent (which it has to be)
all that being said, I'm planning to expand on that IO for blog post series and/or a playground for the new cancelation semantics (unfortunately the optimisations in cats-effect proper make the code quite convoluted and hard to experiment with, at least for me)
Rafał Sumisławski
@RafalSumislawski
@djspiewak @paul-snively @SystemFw thanks sharing your opinions on the topic of implicit Blockers :+1:
Daniel Karch
@danielkarch
@SystemFw thank you, I will look at the tests.
Nik Gushchin
@Dr-Nikson

Hello guys!)
I’ve started exploring world of FP recently, and now I’m trying tu build my first http-service with http4s.
And I get these debug messages in my console (on every request that consul does):

Request headers: Headers(Host: docker.for.mac.localhost:7000, User-Agent: Consul Health Check, Accept: text/plain, text/*, */*, Accept-Encoding: gzip, Connection: close)
[21:31:24.850] [scala-execution-context-global-40               ] DEBUG - closeConnection()
[21:31:24.850] [scala-execution-context-global-40               ] DEBUG - Shutting down HttpPipeline
[21:31:24.850] [scala-execution-context-global-40               ] DEBUG - Canceled request
[21:31:24.850] [scala-execution-context-global-40               ] DEBUG - Shutting down.
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Shutting down HttpPipeline
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Canceled request
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Shutting down.
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Shutting down idle timeout stage
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Shutting down.
[21:31:24.851] [scala-execution-context-global-40               ] DEBUG - Shutting down.
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Stage NIO1HeadStage sending inbound command: Disconnected
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Shutting down idle timeout stage
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Shutting down.
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Stage IdleTimeoutStage sending inbound command: Disconnected
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Shutting down HttpPipeline
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Canceled request
[21:31:24.851] [blaze-selector-7                                ] DEBUG - Shutting down.

My code looks like:

 def apply[F[_]: Sync: Async](port: Int)(
    implicit cs: ConcurrentEffect[F],
    timer: Timer[F],
  ): F[Http4sPrometheusExporter] = {

    for {
      prometheusService <- Http4sPrometheusService.build[F]
      httpApp = Router[F]("/" -> prometheusService.routes).orNotFound
      server <- BlazeServerBuilder[F]
        .bindHttp(port, "0.0.0.0")
        .withHttpApp(httpApp)
        .resource
        .use(_ => Async[F].never[Unit])
        .start
    } yield new Http4sPrometheusExporter()
  }

Is it ok? Or I’m duing something wrong?
Appreciate any help :)

BTW api-endpoint seems to be working properly
Rafał Sumisławski
@RafalSumislawski
@Dr-Nikson https://gitter.im/http4s/http4s would be a better place to ask this question.
Nik Gushchin
@Dr-Nikson
@RafalSumislawski ok, thanks)
vimalaguti
@vimalaguti
Hi, what if I want to learn how to use the ConcurrentEffect type class? I'm not sure how it relates with IO.
Gavin Bisesi
@Daenyth
@vimalaguti tldr version: You need Effect when you want to call .unsafeToFuture() or .unsafeRunSync() and Concurrent when you need concurrent execution, racing, timeouts, etc. ConcurrentEffect is both together
vimalaguti
@vimalaguti
Thanks, but somewhat cryptic. Maybe I just need to refresh the difference between type classes and data types, any clue?
Gavin Bisesi
@Daenyth
A typeclass describes a set of behaviors and laws that establish how those behaviors relate to each other
for example the Functor typeclass describes the behavior of map: F[A] => (A => B) => F[B] and laws like composition; fa.map(f).map(g) == fa.map(f andThen g)
vimalaguti
@vimalaguti
but the same is valid for IO, isn't it?
Gavin Bisesi
@Daenyth
"the same" as in?
Ryan Peters
@sloshy
IO extends implements Functor (indirectly, it's a distant parent in the hierarchy)
Oleg Pyzhcov
@oleg-py
It doesn't. IO has an instance of a Functor, and it's not done via inheritance.
Ryan Peters
@sloshy
Err - right, poor choice of words on my part (was thinking of the typeclass hierarchy and got a little mixed up when the words came out)
Oleg Pyzhcov
@oleg-py
There are things you can't have with inheritance nicely like Monoid.zero and Applicative.pure
Oleg Pyzhcov
@oleg-py

@vimalaguti you want to use ConcurrentEffect, like any other typeclasses, when you don't want to be tied to a concrete IO in your signatures.
Specifically, Effect is used when you need to interop with some library that calls YOUR code, such as APIs that let you register a callback (almost everything in Scala.JS) and Concurrent is used where you want flow control with fibers and .start, or for data structures with asynchronous blocking, like Semaphore, Deferred, fs2's Queue, monix ConcurrentChannel, etc.

ConcurrentEffect bundles the two to avoid ambiguity issues (they both extend Async which would break extension methods), and also gives you a run, but give me a cancel token on top of Effect.

IO has some intricacies - you can't do io.start without implicit ContextShift[IO] in scope, but you can always unsafeRunAsync it. For instance, Monix Task requires a Scheduler for unsafeRunAsync but you can always do task.start. ConcurrentEffect lets you write code that is independent of these details and works for all effect types in the same way.

It isn't, however, a typeclass you want to use often, it literally says "here be dragons, anything can happen", and also you can't use it with anything less powerful than IO+ContextShift, or a Task+Scheduler. Something that only requires Sync, for example, can be used with SyncIO or OptionT[IO, ?]

But if your goal isn't to write effect-agnostic code, it's possible to just use IO directly. And if it is, ConcurrentEffect is a typeclass of last resort.
vimalaguti
@vimalaguti
thanks so much to everyone for the explanation. I just need to wrap a concurrent effect (a memory copy) and suspend it until a global slot gets available - eg not used by other memory-copies. As far as I can understand, the non blocking class is the Async, so that I can run many of them, and use Deferred as data type?
vimalaguti
@vimalaguti
Like I have 2 slots, and I want to precharge the next once the slot is free again. But than I need to Await - ah too many overlapping type classes
Oleg Pyzhcov
@oleg-py
Deferred sounds like a good approach, and if you want to be able to concurrently cancel the acquisition somehow, you'd need Concurrent.cancelable builder
The Async doesn't let you "run many of them" out of the box - if your underlying API is thread-blocking, you can't get around with thread management, but you should be able to only use Sync[F] + ContextShift[F] + Blocker
You might need to have Concurrent on the outside, to manage two slots independently, but that is irrelevant
Oleg Pyzhcov
@oleg-py
Async[F] only means "I can lift anything that is calling a callback once, that also doesn't care much about cancelability, into F".
So, it depends on the API you're wrapping - and if you're making your own, Concurrent is probably better since you get cancelable version of Deferred when you have it
vimalaguti
@vimalaguti
Thanks!
I'll try with Concurrent and Deferred - although I don't mind about cancelability, just concurrency.
vimalaguti
@vimalaguti
Another question: in the Ref page, they use Ref.of[IO, Int]. Is IO replaceable with Concurrent or it requires a similar class like SyncIO or Eval?
Ryan Peters
@sloshy
IO is replaceable with any type that implements Sync, so you can write:
def giveMeRef[F[_]: Sync] = Ref.of[F, Int](1)
Gabriel Volpe
@gvolpe
Concurrent is a typeclass. Ref.of takes any data type that has an instance of Sync, so SyncIO is included.
Ryan Peters
@sloshy
Ah right sync (edited my previous comment)
Gabriel Volpe
@gvolpe
:+1: :)
Ryan Peters
@sloshy
I need to slow down I'm answering questions faster than my brain works this morning
vimalaguti
@vimalaguti
thanks!
Rolando Manrique
@rolandomanrique
Hi, I'm trying to use tuple parMapN syntax but I can't make it work with parameterized effect, this is a very simple representation of what I'm trying to do:
scala> :paste
// Entering paste mode (ctrl-D to finish)

  import cats.effect._
  import cats.implicits._
  import cats.effect.implicits._
  import scala.concurrent.ExecutionContext
  val cs = IO.contextShift(ExecutionContext.global)

  def parMapIO(implicit cs: ContextShift[IO]) = {
    val ioA = IO("A")
    val ioB = IO(10)
    val ioC = IO(false)
    (ioA, ioB, ioC).parMapN { (a, b, c) => println(s"done: $a $b $c") }
  }

scala> parMapIO(cs).unsafeRunSync
done: A 10 false

scala> :paste
// Entering paste mode (ctrl-D to finish)


  def parMapF[F[_]](implicit F: ConcurrentEffect[F], cs: ContextShift[F]) = {
    val ioA = F.delay("A")
    val ioB = F.delay(10)
    val ioC = F.delay(false)
    (ioA, ioB, ioC).parMapN { (a, b, c) => println(s"$a > $b > $c") }
  }

// Exiting paste mode, now interpreting.

<pastie>:25: error: could not find implicit value for parameter p: cats.NonEmptyParallel[F,F]
    (ioA, ioB, ioC).parMapN { (a, b, c) => println(s"$a > $b > $c") }
                            ^
<pastie>:21: warning: parameter value F in method parMapF is never used
def parMapF[F[_]](implicit F: ConcurrentEffect[F], cs: ContextShift[F]) = {
                           ^
<pastie>:21: warning: parameter value cs in method parMapF is never used
def parMapF[F[_]](implicit F: ConcurrentEffect[F], cs: ContextShift[F]) = {
                                                   ^
Oleg Pyzhcov
@oleg-py
@rolandomanrique you need Parallel constraint on your F to use .par methods
Loránd Szakács
@lorandszakacs
did someone write a cats-effect wrapper over better-files? :smile:
while I love fs2-io, sometimes I need more :sweat:
Piotr Gawryś
@Avasil
haha, the same topic just came up in few different places
Fabio Labella
@SystemFw
Piotr Gawryś
@Avasil
it would be nice to have common files library on top of cats-effect and then specialized modules for streaming
Wojtek Pituła
@Krever
totally agree with Piotr as much as it matters :D
Piotr Gawryś
@Avasil
Having it in fs2 is not that bad considering it's easy to have this dependency because of doobie/http4s but it can be harder to discover/use for Monix/ZIO users. Also many utilities are not related to streaming at all. I would be happy to contribute to something like it and bring fresh contributors but I would count on someone else (TM) to kickstart it :D
Fabio Labella
@SystemFw
the same could be said for monix-catnap though
Piotr Gawryś
@Avasil
monix-catnap doesn't have a dependency on Task / Observable / Iterant but that's still a good point