Hi, it seems today's SNAPSHOT build didn't upload correctly to jfrog.

Could not find arrow-fx-0.11.0-SNAPSHOT.jar (io.arrow-kt:arrow-fx:0.11.0-SNAPSHOT:20200821.081149-114).
Searched in the following locations:
    <https://oss.jfrog.org/artifactory/oss-snapshot-local/io/arrow-kt/arrow-fx/0.11.0-SNAPSHOT/arrow-fx-0.11.0-20200821.081149-114.jar>

Looking here that seems to be correct. Last snapshot has only pom, sources and javadoc files available.

For any docs, are both these URL patterns synonymous? - next.arrow-kt.io/docs and arrow-kt.io/docs/next... For example:

<https://next.arrow-kt.io/docs/fx/async/>

<https://arrow-kt.io/docs/next/fx/async/>

I hope

parTraverse

uses fibers underneath, how can I see that effect apart from measuring time (like by printing Fiber-id or something). I tried this example:

@Test
    fun `parTraverse + blocking`() {
        val time = measureTimeMillis {
            runBlocking {
                (1..20).parTraverse {
                    blockingWork(it)
                }
            }
        }
        // 3s, multiple blocked bg threads
        println("Done in $time ms [${Thread.currentThread().name}]")
    }

Hi! Probably a basic question, about

either{}

DSL. The return type of this is

Either<E,A>

, so I understand one way the composition trips to

left

is -- one of the functions inside returns an either in

left

I am unable to figure out how to explicitly return an either in

left

from within the block. For example:

val result: Either<String, String> = either { // This casts the side-effect function into a suspend function.
            val id = !getSomethingFromDb(true)
            val data = !getSomethingElseFromNetwork(id)
           val either = if (data.isEmpty()) {
               // vvv how to return either on left
               !"SOME_CUSTOM_ERROR".raiseError()
           } else data.right()
           either
        }

I understand for a case like this where I have this requirement on the last step, I can return from outside the block, but just curious how to short-circuit from the middle of the block?

With

Option

map2

, one provides a lambda that handles the case where both values are

Some

. But what if one wants to provide a lambda for all four possible cases? I can do this from scratch. But I wondered if there's a higher order function in Arrow that's already designed for this.

Hi! can someone pls help me if I am using

IOPool

in conjunction with

parTraverse

in the right way? I couldn't find any documentation about it.

@Test
    fun `IODispatcher + parTraverse + blocking`() {
        val time = measureTimeMillis {
            runBlocking {
                evalOn(IODispatchers.IOPool) {
                    (1..20).parTraverse {
                        blockingWork(it)
                    }
                }
            }
        }
        println("Done in $time ms [${Thread.currentThread().name}]")
    }

 private fun blockingWork(i: Int) {
        Thread.sleep(1000)
        println("Work $i done [${Thread.currentThread().name}]")
    }

Good Morning! 🙂 Would somebody please be so kind to explain to me what the idea is behind Invariant's imap function? I'm more than a little bit puzzled about it's sginature. Thanks!

Hi, just playing with 0.11.0-SNAPSHOT... I see that Either.fx() is marked as deprecated. I know you are working in order to use the continuations solution not only for IO but for every monad, but is it going to be included in 0.11? If not, what will be the right way to manage comprehension-like syntax for either in 0.11?

`Apply`'s

mapN

takes in at most 10 params (0.10.5), any tips how to workaround a case where there's more than 10

I was reading source for

State

and saw this:

fun <S, T, P1, R> State<S, T>.map(sx: State<S, P1>, f: (T, P1) -> R): State<S, R> =
  flatMap(IdBimonad) { t -> sx.map { x -> f(t, x) } }.fix()

What is

IdBimonad

doing? (I read the docs available for

BiMonad

, but I'm still mystified.)

I'm building a service with a retrofit. Is there an adapter to use

Either

as return type? Or is there some different suggested way of handling retrofit calls?

What is the correct way of logging when using arrow? Would one do: IO { logger.info("Hallo world") } ?

When I check out

arrow-core

and run

./gradlew build

, I get this error:

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/bayo/dev/arrow-core/build.gradle' line: 2

* What went wrong:
A problem occurred evaluating root project 'arrow-core-lib'.
> Could not read '<https://raw.githubusercontent.com/arrow-kt/arrow/master/setup.gradle>' as it does not exist.

What am I doing wrong? I just want to be able to open and explore the source with all the features of IntelliJ that assist that to be available.

How to solve such as scenario with arrow?

fun Program.handleRead(id: String?): UserMapper =
    id?.let {
        UUID.fromString(it)?.let { uuid ->
            queries.read(uuid)?.toMapper()
        } ?: throw UUIDWrongFormatException()
    } ?: throw UserNotFoundException()

Hi all I am using Kotlin script in gradle and would like to enable arrow-core. On the website, it describes https://arrow-kt.io/docs/quickstart/setup/#arrow-core but how to enable it in Kotlin script

plugins {
    application
    kotlin("jvm") version "1.4.0"
    ?????
}

Would be nice if someone could help.

Hi all Coming to Scala and would like to know, how to define the following tagless interface in Kotlin with Arrow:

trait GenderQuery[F[_]] {

  def findAll: F[Genders]

}

Is there a wiki article that explains how to do: “I have two different classes that have the same function, but they were written by two different people and therefore don't implement a common interface. How can I write an extension interface so that I can then use polymorphism on these classes” From reading the wiki and discussing here, I understand that this is pretty much spot on what typeclasses are used for, but I'm not really sure what the syntax would look like (especially with Arrow Meta). I think it could be very convenient if there was a wiki page with small self-contained use-cases like this one, what do you think?

Hi all I have this function:

typealias GetCustomer = (config: Config, domain: Domain) -> IO<Either<ApiError, Option<CustomerDetails>>>

which will do a rest call to get CustomerDetails Reading the arrow documentation i managed to get this:

val io = getCustomer(cloudConfig, domain).repeat(IO.concurrent(), Schedule.doWhile(IO.monad()) { it.isEmpty()})

val maybeCustomer = EitherT(io).bind()

How can i add exponential backoff with a max retries of 10 starting with a 1 second delay?

Are there

suspend

versions of

map

and

flatMap

for

Either

?

Is this a known issue?

Hello everyone. I'm now reading "Category Theory for Programmers" by Bartosz Milewski and need some mentoring in this, in part of "Challenges" (after each chapter there are some tasks, need help in treating/coding them correct in Kotlin language)

Hi all, where can i get an example of "IO.repeatOrElse" ?

Hi, since Option got deprecated, is there an alternative for FunctorFilters filterMap? What is the preferred way to use optics with nullables now?

This is more of a general fp-question, but this seems like a good place to ask it. Why does Applicative exist instead of just apply? This might sound weird, but can't

just

be implemented for any Functor by using an instance of that functor and calling map on it with a function that disregards it's results and just returns whatever value was passed in. So something like this

inline fun <reified F,A,B> Kind<F,A>.Companion.just(value: B): Kind<F,B> {
    return getSomeStaticValueForFunctor<F>().map { B }
}

So instead of having Applicative, couldn't there be just a

FunctorWithStaticInstance

that then

Apply

inherits from (and by extension

Monad

also inherits from). Additonally, the docs mention that some classes can implement

Apply

but not

Applicative

, but isn't it guaranteed that every typeclass in general will have an instance? so then with a pretty simple implementation you could probably produce a

Functor<Unit>

that will be a

val

on

FunctorWithStaticInstance

. Or maybe even every functor could just have a static instance by default. Maybe then you could even have

just

on

Functor

itself. I'm sure that there's definitely something wrong with my reasoning, but I honestly cannot figure it out.

Hi, for using arrow-core-retrofit integration on 0.11.0-SNAPSHOT I have: (left is Unit bec. I will never have body there)

@GET("foo")
  suspend fun getFoo(
    @Query("param") param: String
  ): Either<Unit, FooDto>

and helper (ideally unnecessary) method:

fun notGonnaHappenError(): Throwable = TODO()

How to return

Either<DomainError, FooDto>

? I have this method:

private suspend fun getFoo(param: Bar): Either<DomainError, FooDto> =
    Either.catch { client.getFoo(param.value).mapLeft { notGonnaHappenError() } }
      .flatten()
      .mapLeft { DomainError(it) }

But I think it’s not as beautiful as it can be…

Hey Team, I have seen kotlin and Arrow courses on 47degree website. But those are not available for individuals. Will it available anytime soon? Thanks

Hi, I try to serialize

Either

to json using Jackson, and I faced with issue: when I try to serialize

Either

instance - everything is ok, but when I try to serialize some wrapper-class with

Either

I’ve got an exception. For Gson behavior changes: it’s returning null.

println(toJson("x".right()))
// { "value" : "x" }

data class Wrapper(val value: Either<Throwable, String>)
println(toJson(Wrapper("x".right())))
// java.lang.RuntimeException: com.fasterxml.jackson.databind.JsonMappingException: Index: 0 (through reference chain: Test$test$Wrapper["value"])
// null (for Gson)

I’m using custom serializer which converts

Either.Right

to

{ "value" : … }

and

Either.Left

to

{ "error": … }

, but when I’m removing it, behavior doesn’t change.

🎉 switching from

IOs parTraverse

to

fx-coroutines parTraverse

seem to gives me about 20 to 26 percent performance gain ! ...to authors & experienced users of fx-coroutines: is that your observation too ? (and for sure it is much more concise and elegant 🙂 )

Hi! After learning a lot about this, I'm considering picking up Haskell. Do you have any recommendations on how to go with that for someone that is used to JetBrains IDEs and such tooling?