Comfy Cats Effect room. Ask questions. Rant about config file formats. Be nice!
if I were writing this code
since you know that that piece of code is very CPU-intensive
I don't think that's that big of a deal though, as long as control gets yielded back at some point
in the differences between
IO and Task
what's surprising here is the behaviour of
Task
Will start with the code from master
Thanks for your help!
that makes sense
Hi.
Is this behavior of bracket correct?
import cats.data.{Chain, WriterT}
import cats.effect.{ExitCode, IO, IOApp, Sync}
import cats.effect.implicits._
import cats.implicits._
import cats.mtl.FunctorTell
import cats.mtl.implicits._
object Test extends IOApp {
def bracketWithTell[F[_], A](a: A)(implicit F: Sync[F], W: FunctorTell[F, Chain[String]]): F[ExitCode] =
a.pure[F].bracket { x =>
F.delay(println(s"used $x")) *> W.tell(Chain.one(s"used $x")) as ExitCode.Success
} { x =>
F.delay(println(s"released $x")) *> W.tell(Chain.one(s"released $x"))
}
override def run(args: List[String]): IO[ExitCode] =
bracketWithTell[WriterT[IO, Chain[String], ?], Int](1).run.flatMap {
case (log, code) => IO(println(s"writer log: $log")) as code
}
}Output:
used 1
released 1
writer log: Chain(used 1) We lose the writer effect in the release part. The same holds for all transformers.
If it's correct, doesn't it need clarification in docs?
@LukaJCB I've read that old thread, "impossible to write ... with current signature" doesn't look as an appropriate excuse ;) @oleg-py raised a concern I agree with (https://github.com/typelevel/cats-effect/pull/113#issuecomment-363461345) and you seemed to agree but then you crossed it out (https://github.com/typelevel/cats-effect/pull/113#issuecomment-374628912) and @alexandru said it's fine. But I still don't get why it's ok to ignore last change in
WriterT or StateT.
While learning cats-effect IO- https://typelevel.org/cats-effect/datatypes/io.html#parmapn, I found an
parMapN example missing ContextShift[IO]. I thought it could be helpful to add one way to provide contextShift helpful noobs like me. So created a pull request https://github.com/typelevel/cats-effect/pull/372/files
Learning Question: I'm trying to run the cats.effect.Resource example from https://typelevel.org/cats-effect/datatypes/resource.html#example. I get value map is not a member of cats.effect.Resource[cats.effect.IO,String]. Wondering why is it looking for map not flatMap,
// example taken from https://typelevel.org/cats-effect/datatypes/resource.html#example
test("cats Resource") {
import cats.effect.{IO, Resource}
import cats.implicits._
def mkResource(data: String): Resource[IO, String] = {
val acquire = IO(println(s"Acquiring $data")) *> IO.pure(data)
def release(res: String) = IO(println(s"Releasing $res"))
Resource.make(acquire)(release)
}
val out: Resource[IO, String] = mkResource("outer")
val in: Resource[IO, String] = mkResource("inner")
// val rr = out.flatMap { a =>
// in.flatMap { b =>
// Resource.pure((a, b))
// }
// }
// out.map { o =>
// in.map { i =>
// (o, i)
// }
// }
val r = for {
outer <- out
inner <- in
} yield (outer, inner)
r.use { case (x, y) => IO(println(s"Using $x and $y")) }.unsafeRunSync
}Error
you probably don't have partial unification on
scalacOptions += "-Ypartial-unification"
that option fixes a bug in the type system that occurs when you want to make a type of more than one type parameter fit into a one type parameter shape
for example
Resource[IO, String] (G[_, _]) needs to fit into F[_] so that you can fetch Monad[F] to get flatMap
which it can by having
F[_] be Resource[IO, ?]
so by partially applying
Resource so it can unify with F[_]