typelevel/cats-effect

Most discussion is on Typelevel Discord: https://discord.gg/bSQBZA3Ced

Ross A. Baker

@rossabaker

No, it will work on Scala 2.

chet khatri

@chetkhatri

Nicolas Francois

@Koisell

If it's release before Scala 3 which is probable

Daniel Spiewak

@djspiewak

I'll set up a highlight, which will cause Twitch to keep the video forever

it'll be interesting because I had to split the stream a bit, but should be fine

Ross A. Baker

@rossabaker

Wonderful talk. Thanks for doing that.

Daniel Spiewak

@djspiewak

Thanks to everyone who came! I’d love to do more of that sort of thing, especially now that the technical stuff is sorted.

Christopher Davenport

@ChristopherDavenport

Can't wait to see it once I'm back from strangeloop, thanks for saving it.

Ryan Peters

@sloshy

+1 really excited to listen in! Maybe I'll play it on the drive home. Is it mostly audio-friendly?

Daniel Spiewak

@djspiewak

Should be pretty audio friendly

davidnadeau

@davidnadeau

So i'm using akka, and this bindAnHandle function forks off and returns immediately, which means my main IO program just ends. So I'm using IO.never to prevent the program from ending. Is this a valid usuage off IO.never, or will this cause issues?

    for {
      _ <- IO
        .fromFuture(IO(Http().bindAndHandle(routes, settings.httpInterface, settings.httpPort)))
        .bracket { serverBinding =>
          actorSystem.log.info("API listens on {}", serverBinding.localAddress)
          IO.never
        }(serverBinding => IO(serverBinding.terminate(1.minute)))
    } yield ()

Christopher Davenport

@ChristopherDavenport

Can't promise it won't cause issues, but it's a fairly common pattern when servers return immediately.

Daniel Spiewak

@djspiewak

I usually create an async around it and have the callback complete when the server closes

Or if the server has a separate shutdown function, I would use a Resource

davidnadeau

@davidnadeau

Resource would make this much nicer to read, i'll definitely be using that.

davidnadeau

@davidnadeau

something like this i guess

_ <- akkaHttpResource(routes).use { serverBinding =>
        actorSystem.log.info("API listens on {}", serverBinding.localAddress)
        Async[IO].never
    }

davidnadeau

@davidnadeau

@ChristopherDavenport what sort of issues could you forsee this pattern causing?

Jakub Kozłowski

@kubukoz

I do this in production everywhere. No issues so far

With akka.

davidnadeau

@davidnadeau

good to know

chet khatri

@chetkhatri

@kubukoz could you share sample with akka on that. Thanks

chet khatri

@chetkhatri

@rossabaker @djspiewak great documentation - https://typelevel.org/cats-effect/datatypes/io.html I have gone through this. well explained.

Jakub Kozłowski

@kubukoz

@chetkhatri for akka http servers I do this:

def httpServer(routes: Routes): Resource[IO, ServerBinding] = {
   val serverF = IO.deferFuture {
    IO{
        Http().bindAndHandle(
          routes,
          config.http.host,
          config.http.port
        )
    }
  }
  Resource.make(serverF)(server => IO.fromFuture(IO(server.unbind())).void)
}

Then use it like this:

def run(args: List[String]): IO[ExitCode] = {
  val appResource: Resource[F, Unit] = for {
    service1 <- ...
    service2 <- ...
    _ <- httpServer(service1.routes ~ service2.routes)
  } yield ()

  appResource.use(_ => IO.never)
}

minus formatting because I'm too lazy

Ryan Zeigler

@rzeigler

API design question. Does it make sense to include a close action in my trait algebras if the expectation is they should always be used as resources anyway?

Oleg Pyzhcov

@oleg-py

My preference is no - exiting from .use is enough, and it only buys you an ability to crash something else that is using same object.

Jakub Kozłowski

@kubukoz

same here - close the resource in a Resource'sclose and that's it

Piotr Gawryś

@Avasil

Agreed, if you use Resource then close can be dangerous

Jakub Kozłowski

@kubukoz

so the above snippets should actually return an immutable clone of ServerBinding that doesn't include unbind.

or make it a Resource[F, Unit], which is actually fine :)

Ryan Zeigler

@rzeigler

:thumbsup:

Jakub Kozłowski

@kubukoz

a bit harder to do if your resource is ActorSystem.

Ryan Zeigler

@rzeigler

heh

i'm writing a wire protocol client that is derived from fs2.Socket which does expose a close but none of my higher things do

so the question occured to me

Jakub Kozłowski

@kubukoz

yeah it still feels strange to me that it's there

Ross A. Baker

@rossabaker

We ripped out shutdown functions on http4s resources when they became Resources. No regrets. I think it makes things simpler.

bifunctor

@bifunctor_gitlab

Hi all
I am trying to understand https://typelevel.org/cats-effect/datatypes/io.html#deferred-execution--iosuspend

what does it mean So it is useful for suspending effects

What does suspending effects mean?

Gavin Bisesi

@Daenyth

it means that you can construct an IO which wraps a side effect but doesn't execute it until the IO executes

val hi: IO[Unit] = IO(println("hi")) <- prints nothing when constructing hi

bifunctor

@bifunctor_gitlab

Thanks a loty

lot

What does Asynchronous boundaries mean in context with https://typelevel.org/cats-effect/datatypes/io.html#concurrency-and-cancellation

Daniel Spiewak

@djspiewak

an asynchronous boundary is usually something like a thread pool yield

like really anything that's going to take a callback, then later on invoke it

bifunctor

@bifunctor_gitlab

@djspiewak thanks a lot

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