Hei, does arrow have support for

java.util.concurrent.Future

?/ Can I convert java’s future to IO ? I know I can alway do

IO.invoke {  sendFuture().get() }

. I am not exactly able to explain, what I want, But I am looking for something like coroutines/reactor support in arrow-fx, for java’s futures

Hello. I have a sequence of Option<T>. How do I get the first non-None item or None if there is none? I can do that by mapping the Option<T> to T?, working with the nullable and remapping it to Option<T> at the end, of course. What is the Λrrow idiom for this operation?

In actual programming, you can always define

fold

on any Functor, but not all operations can be called on it (there needs to be a default value). So it sounds like

fold

is defined neither on a category nor on a morphism but on the pair of both of them?

if it can form one you can fold without providing an accumulator or empty value because you can use the monoid empty identity as the empty value and the associative operation as the accumulator

Hello, I am giving a talk about FP and Arrow at a meetup this week and at Voxxed in March, is it ok if I use in my presentation arrow logos?

Hello all. Can i build an entire Rest API with arrow + ktor without using OO (classes, inheritance ...)?

thanks! - Ill give that a go - appreciate the response - love Arrow btw 🙂

I have a situation. I have a repo which has a dependency

DBClient

, which is not in my hands (say a lib impl) and it returns me a concrete type, like

IO<String>

. Now I wish to have a generic repo and only commit to a concrete type at the edge of my application, like

main()

here. But I get a compiler error in repo as commented below, which is expected. Is there any idiomatic workaround to achieve what I desire for? Thanks.

open class Repo<F>(
        private val dbClient: DBClient,
        M: Monad<F>
) : Monad<F> by M {
    fun get(): Kind<F, String> = fx.monad { 
        dbClient.get() // vvv Compiler error. This needs to be converted to generic Higher kind
    }
}

fun main() {
    val repo = Repo(DBClient(), IO.async())
    print(repo.get().fix().unsafeRunSync())
}

class DBClient {
    fun get() = IO { "abc" }
}

Is there a standard set of imports for Arrow that gets me (roughly) everything, kind of analogous to Scala cats:

import cats._
import cats.data._
import cats.implicits._
import cats.syntax._
import cats.effect._

?

As we go into programming we would frequently represent things like a TwitterHandle with the type String. The ugly truth is that Twitter is already telling us String covers a lot more values than what a TwitterHandle should be. It does not have to be this way. We can get better types in Kotlin that guarantee some constraints at compile time with a flavor of type refinements. I based this example in the talk https://kwark.github.io/refined-in-practice/#1 by @kwark to show how its example around TwitterHandle could be modeled in the upcoming type proofs April release with Arrow Meta. We have an initial draft with some tests that show we can bring expressions that serve both for constant and runtime validation for more precise and safe types. The user does not need to learn any type-level predicates or a specialized DSL since predicates can be expressed as receiver functions whose expressions are evaluated at compile-time without contextual dependencies. The user just provides the rules to describe the type predicates and uses the Apis available in the std lib or other user-supplied libs that can run alongside the compiler class-path in a compiler plugin.

hmm, I really don't like declaring something like this as an extension method on F:

fun <F> F.updateTimezone(ctx: DocumentContext): Kind<F, DocumentContext> where F: MonadThrow<F> { ... }

(with arrow 0.10.x), but IIRC I am waiting for Arrow 1.0 to have the equivalent of Scala's

(implicit MT: MonadThrow[F])

, right?

.let

is

map

on an

Option

?.

notation is

flatMap

on an

Option

Your thoughts?

Have you all tried disabling the Machine Learning plugin in IntelliJ to see if that improves editor performance? I have that turned off right now and IJ seems to be .. "better", if not good

Does Arrow have a Monoid instance for endofunctions?

I use version 0.10.5.-SNAPSHOT. Any ideas what I have done wrong?

Hello. I'm trying to write my first comprehension like this:

Either.monad<Exception>().fx {

}

but the IDE says there is no

fx

. If I go to the definition of

Either.monad()

, I see that it returns a

EitherMonad<L>

, which has a definition for

fx

. What's happening? That is an attempt to adapt the code from your comprehension tutorial. If, instead, I'm supposed to use

Either.monad<Any>().binding {

}

then the problem is that I don't thave any

binding

method around.

Hi, is the major (or only) application of HKT is to code effect agnostic libraries? I am not able to realize any benefit of using them in an application Domain logic, as the implementation of any application is hardly going to change (like from Rx2 to Coroutines or something) and even if I forsee a migration in future (like from blocking to non-blocking)), filling the domain code with HKTs, fx blocks and monad transformers might seem an overkill. Please help me understand if I missing anything. P.S: Also I recently referred to this talk by @raulraja [KotlinConf 2018 - Functional Programming in Kotlin with Λrrow by Raúl Raja Martínez]() and he starts with let’s build a library!

Hi, I'm just getting started with arrow and I hope this is the right place to ask. First of all I'm using Iatest IntellIJ 2019.3.3 Whenver I add any arrow dependencies via gradle auto completion within IntelliJ becomes unusable and will be stuck in a constant loop of reloading ( see attached gif). Removing everything related to arrow from gradle will allow autocomplete to work normally again. Tried the obvious stuff in invalidating cache and checking the logs for anything out of the ordinary. Any ideas what might be the issue here?

I am working on creating some kafka extension functions and I want to take a

KStream<Either<Error, Result>>

and create a

Pair

of

KStream<Error>

and

KStream<Result>

. I was not sure how to accomplish this using Either from arrow. We have some helpers in scala that do this now, and for my learning I wanted to do the same in kotlin - in scala it looks something like this -

def forkValuesEither: (KStream[K, LV], KStream[K, RV]) = (
      kStream.flatMapValues(_.left.toSeq),
      kStream.flatMapValues(_.right.toSeq)
    )

but I was not sure how to get to the value in arrow/kotlin of the either to return.

Hi, I get the following error when running tests, although it works correctly on standard runtime:

java.lang.NoClassDefFoundError: arrow/typeclasses/MonadContinuation
	at arrow.typeclasses.MonadFx$DefaultImpls.monad(Monad.kt:107)
	at arrow.core.extensions.OptionFxMonad.monad(option.kt:223)
	at arrow.core.extensions.OptionFxMonad.monad(option.kt:220)
	at arrow.core.extensions.OptionKt.fx(option.kt:324)

The incriminated code block:

private fun Option<Duration>.moreThan5Minutes(compared: Option<Duration>) = (isEmpty() != compared.isEmpty()) || Option.fx {
        val first = bind()
        val second = compared.bind()
        abs(abs(first.toMinutes()) - abs(second.toMinutes())) >= FIVE_MINUTES
    }.getOrElse { false }

In my gradle.kts:

implementation(io.arrow-kt:arrow-fx:0.10.4)
implementation(io.arrow-kt:arrow-syntax:0.10.4)

 Oh, and the test work when run through Android Studio 🤔 Any idea what might be causing this?

okay, so your problem is that you want to move from something implementation-specific like Rx to a generic Kind

doing an

fx

block with a single line

!

is the same as just pulling the thing you’re binding outside

Hi, I see

Either.fx<L,R>{}

works with only one type of

Either<L,R>

. Is there any way to work with different types of

Either

to avoid nested fold?

val a: Either<Int, String> = "a".right()
    val b: Either<String, Int> = 1.right()
    
    Either.fx<String, Int> {
        val bind = a.bind() // ^^^ Compiler Error
        b.bind()
    }

Hi, How to we deal with coroutines'

Flow

and Arrow's

IO

? Here is an example of what I'd like to achieve. But it doesn't compilbe because

IO.fx

restrict suspension:

val flow: Flow<Int> =
    flowOf(1, 2, 3)

fun putStrLn(value: Any?): IO<Unit> =
    IO { println(value) }

val program = IO.fx {
  flow.collect {
    !putStrLn(it)
  }
}

fun main() {
  program.unsafeRunSync()
}

Hi, I wish to short-circuit

fx.async{}

block below:

fun <F> Async<F>.something(): Kind<F, Either<String, String>> = fx.async {
    val left = "1-left".left().just()
    // I wish to short-circuit here if either above is left
    "2-right".right()
}

This is what I could come up with:

fun <F> Async<F>.something2(): Kind<F, Either<String, String>> = fx.async {
    Either.fx {
        !"1-left".left()
        "2-right".right().bind()
    }
}

Is there a better way than to use nested blocks?

Hey Guys, need some help. I have multiple Either’s and I want to collect the right from all of them to build an object. Something like this, but I’m not able to make this work:

@Test
    fun combineEithers() {
        val name = randomEither(null, "Alfredo Lambda")
        val phone = randomEither(null, 55555555)
        val address = randomEither(null, listOf("1 Main Street", "11130", "NYC"))

        val result = Either.applicative<Profile>()
            .tupled(name, phone, address)
            .fix()
            .map { Profile(it.a, it.b, it.c) }

        println(result)
    }

    fun <K> randomEither(left: ResponseError?, right: K): Either<ResponseError, K> {
        return if (left != null) {
            Either.left(left)
        } else {
            Either.right(right)
        }
    }

    data class Profile(val name: String, val phone: Int, val address: List<String>)

Hi, I am referring to this [Error Handling Pattern using ApplicativeError](https://next.arrow-kt.io/docs/patterns/error_handling/#example--alternative-validation-strategies-using-applicativeerror) How can I extend for other datatypes (apart from

FormField

used in this example)? I could come-up with something like below, using inheritance, but I feel that’s not the ideal way, as I hv to touch the Framework code (

Rules<F>

) every time I extend this for a new type. Cc: @raulraja

interface FormFieldValidation<F> : ApplicativeError<F, Nel<ValidationError>> {
...// This has contain FormField.contains, FormField.maxLength, FormField.validateWithEmailRules() 
}

interface FormFieldValidation2<F> : ApplicativeError<F, Nel<ValidationError>> {
...// This has contain FormField2.contains, FormField2.maxLength, FormField2.validateWithEmailRules()
}

sealed class Rules<F>(A: ApplicativeError<F, Nel<ValidationError>>) : 
        ApplicativeError<F, Nel<ValidationError>> by A,
        FormFieldValidation2<F>,
        FormFieldValidation<F> {
.....
}

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