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
10
votes
3 answers
Scala / Cats: How to unzip an NonEmptyList
The standard library offers the unzip method on List:
scala>val l = List((1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five"))
scala> l.unzip
// res13: (List[Int], List[String]) = (
// List(1, 2, 3, 4, 5),
// List("one", "two",…
Valy Dia
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10
votes
1 answer
How to separate out parsing from validation in case of versioned config using scala?
Background
I have a set of configuration JSON files that look like the following:
{
"version" : 1.0,
"startDate": 1548419535,
"endDate": 1558419535,
"sourceData" : [...] // nested json inside the List.
"destData" : [...] // nested json…
advocateofnone
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10
votes
1 answer
Scala Cats Effects - IO Async Shift - How Does it Work?
Here is some Scala cats code using the IO Monad:
import java.util.concurrent.{ExecutorService, Executors}
import cats.effect.IO
import scala.concurrent.{ExecutionContext, ExecutionContextExecutor}
import scala.util.control.NonFatal
object Program…
Michael Lafayette
- 2,972
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10
votes
2 answers
The object-functional impedance mismatch
In OOP it is good practice to talk to interfaces not to implementations. So, e.g., you write something like this (by Seq I mean scala.collection.immutable.Seq :)):
// talk to the interface - good OOP practice
doSomething[A](xs: Seq[A]) = ???
not…
lambdista
- 1,850
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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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9
votes
3 answers
Recovering underlying Future into Cats' EitherT's Left?
If I have a Future[Either[String, Int]] that represents either a possible error message (String) or a successful computation (Int), it is simple to move the Future's potential failure into the left side as an error message:
def handleFailure(fe:…
Cory Klein
- 51,188
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9
votes
1 answer
Doobie transact over a list of ConnectionIO programs
Let's say I have a list of Doobie programs (all with Unit type parameters, fwiw):
val progList: List[ConnectionIO[Unit]] = prog1 :: prog2 :: ... :: Nil
Is there any way I can run them in one transaction? A for-comprehension won't work here, because…
Lasf
- 2,536
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9
votes
1 answer
Intersperse is missing in cats?
I only found several methods which were all unaccessible - intersperseList.
Why isn't it accessible? Or is there somewhere a function which does the intersperse operation?
Example of functionality from ScalaZ:
scala> intersperse(List(1, 2, 3),…
monnef
- 3,903
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9
votes
1 answer
How to transform Either[Future[A], Future[B]] to Future[Either[A, B]]
I have an instance of Either[Future[A], Future[B]] and I would like to transform it to Future[Either[A, B]].
Since my previous question, cats 0.8.1 has been released, changing the structure of the library and dropping Xor in favor of Either, which…
kostja
- 60,521
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9
votes
1 answer
Use Unapply to extract identical type classes
I have the following situation, given two types MA and MB, I would like to be able to prove that they both not only have an Applicative but also that they both have the same underlying shape. I tried doing the following:
type UnapplyM[TC[_[_]], MA,…
wheaties
- 35,646
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- 131
8
votes
4 answers
List[Try[T]] to Try[List[T]] in Scala
I would like to know how to convert a List[Try[T]] into Try[List[T]] in Scala?
I have tried using an accumulator and folding right but it doesn't seem ideal.
Valahu
- 763
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8
votes
2 answers
Conditional state monad expressions
I'm using the State monad from the Scala Cats library to compose imperative sequences of state transitions in a functional manner.
My actual use-case is quite complicated, so to simplify matters, consider the following minimal problem: there is a…
Mike Allen
- 8,139
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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
- 839
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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
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8
votes
1 answer
Convert Set to cats.data.NonEmptySet?
Is there an extension method in cats for the standard Set which converts it to Option[cats.data.NonEmptySet]?
Midiparse
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