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
8
votes
1 answer
Combining `OptionT` and `EitherT` to handle `Future[Either[Error, Option[T]]]`
I want to use Cats EitherT and OptionT to handle the type Future[Either[Error, Option[T]]. Suppose the following methods:
def findTeacher(id: Int): Future[Either[String, Option[Teacher]]]
def findSchool(teacher: Teacher): Future[Either[String,…
Amir Karimi
- 5,401
- 4
- 32
- 51
8
votes
1 answer
cats' NonEmptyList vs scala stdlib ::
I am studying the cats library recently, and I have come across this class called NonEmptyList.
After reading the api, I couldn't help wondering what is it that made the cats authors to create a new class, instead of utilizing something that is…
Haemin Yoo
- 474
- 5
- 11
8
votes
0 answers
Using typelevel/cats flatTraverse for different Functors
I have been trying to use more abstract functional programming concepts like those from typelevel/cats for Scala.
In this specific case I am trying to eliminate the need to call map(_.flatten) after traversing some Futures.
Using the standard…
steinybot
- 5,491
- 6
- 37
- 55
8
votes
1 answer
Using arbitrary trees with Free Monads in Scala+Cats
I am creating a library for grammars that will have 2 different interpretations: 1) parsing strings based on the grammar 2) generating strings in the language defined by the grammar.
The library uses cats to create the AST of the grammar as a free…
Felix
- 8,385
- 10
- 40
- 59
8
votes
1 answer
Why do we need to separate Apply and Applicative type classes?
I read in cats documentation about typeclasses Apply and Applicative. I wonder why the library provides two separate type classes instead of just one type class Applicative, which would extend Functor and add ap ? Does anybody use Apply that is not…
Michael
- 41,026
- 70
- 193
- 341
8
votes
4 answers
Avoiding deeply nested Option cascades in Scala
Say I have three database access functions foo, bar, and baz that can each return Option[A] where A is some model class, and the calls depend on each other.
I would like to call the functions sequentially and in each case, return an appropriate…
Ralph
- 31,584
- 38
- 145
- 282
7
votes
1 answer
Problem with given instances writing MTL style code with Scala cats
I am trying to write some Scala code to have custom behaviour in an mtl style. For example, in order to expose the "write to DB" functionality abstracting over the specific effect I wrote my own type class:
trait CanPersist[M[_]]:
def…
Giacomo Cavalieri
- 375
- 2
- 11
7
votes
1 answer
Using EitherT to evaluate the results of operations using a shared error type inheritance?
I have an error type hierarchy for peeling bananas:
sealed trait PeelBananaError
object PeelBananaError {
case object TooRipe extends PeelBananaError
case object NotRipeEnough extends PeelBananaError
}
And I have some results in EitherT that we…
Cory Klein
- 51,188
- 43
- 183
- 243
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
- 5
- 15
7
votes
0 answers
Unit Testing http4s router websocket endpoints
The idea is to be able to unit test websocket endpoints on a router service. Any other kind of endpoint is fairly easy to test with a Request, but I can't figure out a way of easily testing websocket responses, as hitting the endpoint with a Request…
smetca
- 71
- 4
7
votes
3 answers
Convert List[Either[A, B]] to Either[List[A], List[B]]
How to convert List[Either[String, Int]] to Either[List[String], List[Int]] using a method similar to cats sequence? For example, xs.sequence in the following code
import cats.implicits._
val xs: List[Either[String, Int]] = List(Left("error1"),…
Mario Galic
- 47,285
- 6
- 56
- 98
7
votes
0 answers
FS2 - How to route an element to a specific nested stream/pipe?
I want to run N nested streams/pipes in parallel and send each element to only one of the nested streams. Balance allows me to do this but I want to route elements with the same "key" to the same nested stream or pipe.
I can't see any functions to…
Toby Hobson
- 199
- 7
7
votes
1 answer
How to create Async[Future] from Async[IO]
I am trying to implicitly add Async and Sync in my code for doobie repository. The Sync and Async[F] works fine IO. I want to convert them to Future and facing problem
I have tried to create my own Aync from IO
def futureAsync(implicit F:…
user11034858
- 81
- 1
- 4
7
votes
1 answer
Value withFilter is not a member of Cats IO in for comprehension
I wrote this code and it compiles fine
for {
list : List[Int] <- Future(List(1, 2, 3))
} yield list.size
res7: Future[Int] = Future(Success(3))
But if I convert this code to
for {
list : List[Int] <- IO(List(1, 2, 3))
} yield list.size
I get…
Knows Not Much
- 30,395
- 60
- 197
- 373
7
votes
1 answer
Scala Cats State Monad
I have a class that has a one parameter method that produces a result and returns an object like itself but with updated state for subsequent use.
For example, below contains a simple example of such a class and how I might use it:
case class Foo(x:…
Richard Redding
- 327
- 2
- 11