Cats is a library that provides abstractions for functional programming in Scala.
Cats is a library that provides abstractions for functional programming in Scala.
The name is a playful shortening of the word category.
Questions tagged [scala-cats]
929 questions
5
votes
1 answer
flatMap ignoring the result
I was wondering if there exists a function (in scala or cats) which omits the result within flatMap. E.g.
Some("ignore this").ignoreArgumentFlatMap(Some("result"))
which would be the same as
Some("ignore this").flatMap(_ => Some("result"))
Nicolas Heimann
- 2,561
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5
votes
2 answers
Scala Cats Accumulating Errors and Successes with Ior
I am trying to use Cats datatype Ior to accumulate both errors and successes of using a service (which can return an error).
def find(key: String): F[Ior[NonEmptyList[Error], A]] = {
(for {
b <- service.findByKey(key)
} yield…
Chris Johnston
- 335
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5
votes
1 answer
When exactly I have to use State Monad or .copy?
I saw an example in a book of haskell in which is created a simple stack (push, pop) and return monad states all the time they update the stack using .put(..) or .pop().
As a reminder the example looks similar to this:
pop :: State Stack Int
pop…
Francisco Albert
- 1,577
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5
votes
1 answer
Elegant way to change either to error with tagless final
I often do things like:
import cats.effect.Sync
import cats.implicits._
case class User(name: String)
case object Error extends Exception
def validate[F[_]: Sync](name: String): F[Either[Error, User]] = Sync[F].pure(User(name).asRight)
def…
Stu Redman
- 262
- 2
- 9
5
votes
2 answers
EitherT: Call function returning Either only if a certain condition is true (otherwise return right)
So I have a certain function which I need to call only if a certain condition is true. If it's false, I consider it as Right.
I would use EitherT.cond, but the thing is my function's return type is Future[Either[ErrorType, Unit]], so it's not…
Anastasia
- 53
- 5
5
votes
2 answers
How to write for comprehension in cats with IO monad
I have the following code:
import cats.effect.IO
import cats.data.State
import cats.data.StateT
import cats.implicits._
import cats.effect.LiftIO
abstract class Example {
object implicits {
implicit def myEffectLiftIO:…
Mike
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- 9
- 25
5
votes
1 answer
Reduce / fold over list of monoid but reducer returns Either
Ive found myself in the situation a couple of times where i have a reducer / combine fn like so:
def combiner(a: String, b: String): Either[String, String] = {
(a + b).asRight[String]
}
Its a dummy implementation but the fn can fail so it…
Luke De Feo
- 2,025
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5
votes
0 answers
Scala-cats: Memoization support
Scalaz provides Memo for memoization. I hope scala-cats also has a similar solution. In general I have a trait with different methods:
trait t {
def f1:Int = ???
def f2(i:Int):Int = ???
def f3(i:Int, s:String):String = ???
}
If such solution…
Alexandr
- 9,213
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5
votes
2 answers
Implement while(true) in cats
How do you implement the following loops in cats?
First (normal while(true) loop):
while(true) { doSomething() }
Second (while(true) loop with increment):
var i = 1
while(true) { i +=1; doSomething() }
Third (while(true) with several independent…
Piu130
- 1,288
- 3
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- 28
5
votes
2 answers
Scala, cats, http4s - does not recognize <+> symbol from Http4s
I have created two Http4s routes:
class FirstRoutes[F[_] : Async](service: FirstService[F]) extends Http4sDsl[F] {
def routes: HttpRoutes[F] = HttpRoutes.of[F] {
//... some code
}
}
class SecondRoutes[F[_] : Async] extends…
Developus
- 1,400
- 2
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5
votes
1 answer
Cats: mapping over tuples with the same applicative
Let's say I have:
val x1: Either[String, Int] = Right(1)
val x2: Either[String, Float] = Left("Nope")
val x3: Either[String, Double] = Left("Not Today")
I want to combine these together and get a Either[NonEmptyList[String], (Int, Float, Double)].…
Maths noob
- 1,684
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5
votes
1 answer
Is there a library in F# that is similar to scalaz, cats and arrow (Kotlin)?
What is a recommended approach in F# to use "high level" functional patterns? For Scala there are scalaz and cats, even there is Arrow for Kotlin, though I have not found anything similar for F#.
sampai
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5
votes
1 answer
Semigroup typeclass (Either) with slightly altered combine
Using cats.Semigroup one can write this:
import cats.Semigroup
import cats.implicits._
val l1: String Either Int = Left("error")
val r1: String Either Int = Right(1)
val r2: String Either Int = Right(2)
l1 |+| r1 // Left("error")
r1 |+| r2 //…
Florian Baierl
- 2,378
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5
votes
1 answer
ConcurrentHashMap with functional programming. Is suspending unsafeRun safe?
Question: Is it safe to suspend unsafeRunSync with IO? E.g.
val io: IO[Unit] = //...
val io2: IO[Unit] = IO(io.unsafeRunSync)
The reason I would do so is that I have some class parameterized with F[_]: Effect which is like a cache:
import…
Some Name
- 8,555
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- 77
5
votes
2 answers
Unit-testing with cats-effect's IO monad
The Scenario
In an application I am currently writing I am using cats-effect's IO monad in an IOApp.
If started with a command line argument 'debug', I am delegeting my program flow into a debug loop that waits for user input and executes all kinds…
Florian Baierl
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