Part of the Scala Cats ecosystem for an IO type and associated effects.
Questions tagged [cats-effect]
191 questions
27
votes
1 answer
Making sense of Scala FP Libraries
Just for the sake of quick clarity for someone who wants to start working with Scala FP library, on a journey to become better at pure FP.
Would someone clarify the difference/relation between Cats and Cats-Effect, Cats-Effects IO? On top of that,…
MaatDeamon
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26
votes
1 answer
Cats-effect and asynchronous IO specifics
For few days I have been wrapping my head around cats-effect and IO. And I feel I have some misconceptions about this effect or simply I missed its point.
First of all - if IO can replace Scala's Future, how can we create an async IO task? Using…
ukulele
- 317
- 4
- 9
18
votes
1 answer
How to write tail-recursive functions when working inside monads
In general I have problems figuring out how to write tailrecursive functions when working 'inside' monads. Here is a quick example:
This is from a small example application that I am writing to better understand FP in Scala. First of all the user is…
Florian Baierl
- 2,378
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13
votes
2 answers
How to make virtual time pass in a test in Cats Effect 3?
I am trying to convert Mules to Cats Effect 3 (CE3). Since it is a caching library, in its tests it needs (virtual) time to pass to test whether items will be expired. It is currently making fairly heavy use of cats.effect.laws.util.TestContext,…
Keith Pinson
- 7,835
- 7
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13
votes
1 answer
Cats effect - parallel composition of independent effects
I want to combine multiple IO values that should run independently in parallel.
val io1: IO[Int] = ???
val io2: IO[Int] = ???
As I see it, I have to options:
Use cats-effect's fibers with a fork-join pattern
val parallelSum1: IO[Int] = for {
…
amitayh
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12
votes
1 answer
scala cats ambiguous implicit values
import cats._
import cats.implicits._
trait Console[F[_]]{
def readInput() : F[Int]
def print(msg: String) : F[Unit]
}
class Foo {
def doFoo[F[_]: Monad](number: Int)(implicit C: Console[F]) : F[Unit] = {
C.readInput().flatMap{input =>…
Knows Not Much
- 30,395
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12
votes
2 answers
cats-effect:How to transform Map[x,IO[y]] to IO[Map[x,y]]
I have a map of string to IO like this Map[String, IO[String]], I want to transform it into IO[Map[String, String]]. How to do it?
Kumar Waghmode
- 509
- 2
- 18
9
votes
2 answers
Doobie - lifting arbitrary effect into ConnectionIO CE3
I am trying to migrate project from cats-effect 2 to cats-effect 3, i am using doobie for interacting with database. Previously i could lift ConnectionIO to IO as it was described, but with the upgrade i didn't find any implementation of…
poweright
- 93
- 5
9
votes
2 answers
Should constructing stateful objects be modeled with an effect type?
When using a functional environment like Scala and cats-effect, should the construction of stateful objects be modeled with an effect type?
// not a value/case class
class Service(s: name)
def withoutEffect(name: String): Service =
new…
Mark Canlas
- 9,385
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8
votes
1 answer
Expected behavior in cats-effect for exceptions thrown in `.delay` or `.map`
During a PR review, I was asked to replace Sync[F].delay with Sync[F].catchNonFatal because an exception might be thrown.
This does work:
scala> Sync[IO].delay(throw new Exception).recover{ case t: Throwable => 42 }.unsafeRunSync
res10: Int =…
betehess
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- 19
8
votes
1 answer
How to add proper error handling to cats-effect's Resource
I am trying to get some basic file IO (write/read) in a purely functional way using cats-effect. After following this tutorial, here is what I ended up with for reading a file:
private def readFile(): IO[String] = for {
lines <- …
Florian Baierl
- 2,378
- 3
- 25
- 50
7
votes
1 answer
Is it okay to use "unsafeRunSync()" in Cats-Effects?
I am using Doobie and in the examples that I found, it uses unsafeRunSync, like:
sql"select name from country"
.query[String] // Query0[String]
.to[List] // ConnectionIO[List[String]]
.transact(xa) // IO[List[String]]
…
ClassNotFoundException
- 409
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7
votes
1 answer
Why is Deferred factory method has return value in the context of F
I'm looking at cats.effect.concurrent.Deferred and noticed that all pure factory methods inside its companion object return the F[Deferred[F, A]], not just Deferred[F, A] like
def apply[F[_], A](implicit F: Concurrent[F]): F[Deferred[F, A]] =
…
Some Name
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6
votes
0 answers
What is the current cats-effects IO Error handling state of arts?
I have been browsing a lot around the subject.
Just from stackoverflow, the best comprehensive post i found were
Try[Result], IO[Result], Either[Error,Result], which should I use in the end
How to add proper error handling to cats-effect's…
MaatDeamon
- 9,532
- 9
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- 127
6
votes
1 answer
Improving performance of fs2 stream involving file transformation
I've got something like this (it's an example from https://github.com/typelevel/fs2, with my additions, which I marked with comments):
import cats.effect.{Blocker, ExitCode, IO, IOApp, Resource}
import fs2.{io, text, Stream}
import…
Dawid Łakomy
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