If I could get "launch jvm with main, capture stdout + exitcode" that would be enough to see whether it crashed or locked
and I could have it emit a positive message to stdout once successfully initialized, and check for that with if I had the output
I'm trying to make a very simple looping function with a delay using CATS effects (2.0.0-RC1 due to Scala 13).
import cats.effect._
import scala.concurrent.duration.FiniteDuration
object TaskScheduler {
def loop[F[_] : Sync](delay: FiniteDuration, fun: () => F[Unit])
(implicit timer: Timer[F]): F[Unit] =
for {
_ <- timer.sleep(delay)
_ <- fun()
_ <- loop(delay, fun)
} yield ()
}however, when I try to compile this I get:
sbt:root> tickets/compile
[info] Compiling 15 Scala sources to /home/rohdef/git/smart-tickets/tickets/target/scala-2.13/classes ...
[error] /home/rohdef/git/smart-tickets/tickets/src/main/scala/dk/rohdef/scheduler/TaskScheduler.scala:11:23: value flatMap is not a member of type parameter F[Unit]
[error] _ <- timer.sleep(delay)
[error] ^
[error] /home/rohdef/git/smart-tickets/tickets/src/main/scala/dk/rohdef/scheduler/TaskScheduler.scala:12:15: value flatMap is not a member of type parameter F[Unit]
[error] _ <- fun()
[error] ^
[error] /home/rohdef/git/smart-tickets/tickets/src/main/scala/dk/rohdef/scheduler/TaskScheduler.scala:13:16: value map is not a member of type parameter F[Unit]
[error] _ <- loop(delay, fun)
[error] ^
[error] three errors found
[error] (tickets / Compile / compileIncremental) Compilation failed
[error] Total time: 2 s, completed Aug 10, 2019 1:23:20 PM
sbt:root>which puzzles me thoroughly, since Sync is a Monad as far as I can see, so both map and flatMap should be present to my knowledge. So, what am I doing wrong?
btw if
F is Sync then you don't need () => F[Unit]. It should be equivalent with F[Unit]
@Avasil bingo, thanks a lot :) Now it's giving unrelated error, which I expected :) neat
regarding () => F[Unit] vs F[Unit] I have have a def task[F[_]: Concurrent](): F[Unit] = ??? that I'm currently intending to pass to the loop. Would it be better to do loop(task()) rather than loop(task), or am I completely missing the point
?
You would need something like this with
Future but fortunately that's not necessary with IO implementations
according to https://typelevel.org/cats-effect/tutorial/tutorial.html#polymorphic-cats-effect-code it's recommended to write effects in terms of
F[_] rather than IO, cool I like that. But I ran in to an issue, mainly how to do this with a Futurefrom an external API? I cannot seem to find any general trait having a fromFuture or similar. How is it best to approach this?
If you are still on 1.x
You have to do the transformation manually :D
It has couple days
dates seems to agree with my assumption :)
anyhow, it seems it's not there, thanks for the response @tzimisce012 :)
it is resolving M4, that explains why fromFuture is missing at least
Don't mix cats.syntax with cats.implicits. I always just choose the latter.
And fromFuture is right. You can't just use Future as F because it's strict and therefore can't have a Sync instance.
is there a point to prefer
def foo[F[_]: Sync[F]]() = ??? over def foo[F[_]]()(implicit F: Sync[F]) I kind of like the flexibility by the implicit setup, but I was wondering because the tutorial use the former version. Am I doing it bad, is it taste, or?
If you have some spare time I wrote about the different approaches and its performance implications @rfftrifork.
(shameless plug 😄)
Hi all, i am writing a simple program that access to db via jdbc but i currently i dont want to use doobie. In my code i prepared a small utility like below. Is it enougn to handle local transactions or is there any edge cases that i miss? Thank you in advance:
implicit class richIODatasource[D <: DataSource, F[_]:Sync](val ids: F[D]) extends AnyVal {
import cats.effect.ExitCase.{Completed, Error, Canceled}
def use[T](f: Connection => F[T]): F[T] =
Resource.fromAutoCloseable[F, Connection](ids.map(_.getConnection())).use(c => f(c))
def transaction[T](f:Connection => F[T]):F[T] = use{ c =>
Sync[F].bracketCase(Sync[F].pure(c).map{c =>
c.setAutoCommit(false)
c
})(f){
case (in, Canceled | Error(_)) =>Sync[F].delay(in.rollback())
case (in,Completed) => Sync[F].delay(in.commit())
}
}
}
@capacman I'm fairly new to effects, but to me it looks correct. I'm wondering though if you'd need a begin transaction call. I seem to recall the ExitCase of bracket is a sealed trait, so I think you'll get a compiler warning if you don't cover all cases possible, but if I remember wrong then you might consider a default case where you do a rollback. Also slightly beside your point, I'll strongly urge you to log errors, including the input that cause them ;)