kotlinlang #arrow

import <http://arrow.fx.IO|arrow.fx.IO>
import arrow.fx.extensions.fx
import kotlinx.coroutines.NonCancellable
import kotlinx.coroutines.delay

val io = IO.fx {
    continueOn(NonCancellable)
    effect { println("STARTED") }.bind()
    effect { delay(10000) }.bind()
    effect { println("FINISHED") }.bind()
}

val disposable = io.unsafeRunAsyncCancellable {  }
disposable()

Fabian

01/20/2020, 3:07 PM

Hello, are there any examples on

monad-comprehensins

StateMonad

? I'm having some code that when implemented with

flatMap

s compiles and run without issues BUT with monad-comprehensions throws

java.lang.NoSuchMethodError: arrow.typeclasses.Monad.getFx()Larrow/typeclasses/MonadFx

So I wanted to check for a reference example

Jannis

01/20/2020, 8:27 PM

I am not sure I understand

raceN

correctly:

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IO.fx {
        dispatchers().io().raceN(timer().sleep(10.seconds).followedBy(effect { println("Hello") }), effect { println("Hello world") })
            .fork().bind().join().bind()
}.suspended()

This code fails with a cancellation exception after one of the effects has occured. The same code without the extra fiber does not.

suspended()

or not also changes nothing, unsafeRunSync will also fail.

kyleg

01/21/2020, 6:44 PM

I’ve been working on one of the issues, updating documentation that advises on using

Try

so that it does not advise this anymore (

Try

is deprecated). In a Prism documentation that uses

Try

, there is an example that uses

pTrySuccess

, which allegedly lives in

arrow.optics

. However, I have cloned the Arrow repo and have searched the entire codebase for this string. It only appears in

arrow-optics/prism/README.me

but in no actual code. I can’t find it in the git history for

Optics.kt

either. Does anyone remember what this function was supposed to do? If I’m already replacing

Try

with

Either

in many places, I need to get this code working so I can make sure my new documentation is functional code.

addamsson

01/21/2020, 6:59 PM

Hi guys, I know I've asked before, but there was no clear path towards a solution...I'd like to use Arrow in my MPP project. Is there a way to get it working? I'm mostly interested in the core datatypes only. 99% of my codebase is in Kotlin common code and I have JVM/JS targets. Little hint?

dnowak

01/21/2020, 10:42 PM

I do not like the intermediate result that is again wrapped inside

EitherT

to perform

mapLeft

Hiosdra

01/22/2020, 11:50 AM

I'm thinking about writing own Retrofit - Arrow IO adapter. Just to clarify - noone written it before?

Mark Fisher

01/22/2020, 12:36 PM

Hi all, I'm trying to follow the Rx2 Docs at https://arrow-kt.io/docs/integrations/rx2/# but am having trouble. The docs setup a function as follows:

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fun <F> getSongUrlAsync(MS: MonadDefer<F>) =
  MS { getSongUrl() }

but this doesn't compile for me, i'm getting (on a similar function) - see below What am I doing wrong, or are the docs wrong? I'm a bit lost tbh The type import for MonadDefer is

arrow.fx.typeclasses.MonadDefer

Jannis

01/22/2020, 2:21 PM

Hi, over the last few days I've been working on improving the prettyprinter I've written a while back (https://github.com/1Jajen1/kotlin-pretty). It is now pretty much stable and comes with documentation! (https://1jajen1.github.io/kotlin-pretty/docs/). For those not familiar with prettyprinters: A prettyprinter allows you to define dynamic (or static) text in a layout agnostic way which can then later be rendered to string with optimal newlines for the specified width constraints. Check out the readme or the doc page to learn more! The project is written with arrow and mainly makes use of

Eval

for stacksafety and optimization and provides primitives to render Documents with applicative effects (for rendering straight to e.g. an output stream). The docs currently include a landing page, a getting started guide and a full api reference. Everything else in the docs is wip, so if you have any questions feel free to ask directly ask me (or open issues). Feedback is much appreciated!

👏 7

arrow 6

Mark Fisher

01/22/2020, 3:02 PM

A follow up to my earlier question on Rx with Arrow. I'm trying to write a comprehension that combines calls to functions that return Rx Single objects. For example:

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class ArrowRxTests {
    @Test
    fun `comprehension with arrow and rx`() {
        val x = SingleK.fx {
            val (count) = getCountSingle       // this is an async call using a SingleK.monadDefer
            val newCount = increment(count)    // this is an async call returning Single<Int>.... how do i use it here?
            newCount
        }.value()

        assertThat(x.blockingGet()).isEqualTo(2)
    }

    private fun getCount() = 1
    private fun <F> getCountAsync(MS: MonadDefer<F>) = MS.later { getCount() }
    private val getCountSingle: SingleKOf<Int> = getCountAsync(SingleK.monadDefer())

    private fun increment(i: Int): Single<Int> = Single.just(i + 1)
}

This works if

increment

just returns an int, but how do I use increment() in the comprehension when it already returns a Single? I want to somehow keep the flow of the code inside

SingleK.fx {}

but using functions that return Rx types

kyleg

01/22/2020, 5:52 PM

Since

Try

is deprecated, is the preferred replacement for (non-suspending)

Try { … }

: 1. an explicit

try { Either.right(…) } catch(e) { Either.left(…) }

or 2.

runBlocking { Either.catch { … } }

(which is a suspend fun)

Attila Domokos

01/22/2020, 6:14 PM

I created an example app for my upcoming Arrow talks leveraging

EitherT<ForIO>

extensively. I really like the code I have, however, some people are lost with monad transformers. I tried to dummy down the example to use

IO<Either<A, B>

, however, the code is not very attractive. Check out the functions

run

using EitherT and

run2

using IO<Either> types. Why do I need to call

unsafeRunSync

in function

run2

to call that code? Does

flatMap

invalidate the

fx

context? https://github.com/adomokos/kotlin-sandbox/blob/ad/io-either/src/main/kotlin/sandbox/explorer/App.kt#L65-L98

Luke Rohde

01/22/2020, 7:41 PM

does arrow provide any facilities for typeclass instance derivation, a la scala shapeless macros?

Mark Fisher

01/23/2020, 10:48 AM

Hi All, Continuing my understanding of arrow and concurrency, how do I write a comprehension that makes concurrent calls within it. My naive first attempt is this, but the "network" calls happen sequentially.

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@Test
    fun `concurrent arrow and rx`() {

        val x = SingleK.fx {
            val requiredValue = !getCountSingle
            println("network call A")
            val networkCallAValue = !pauseAndIncrement(requiredValue).k()
            println("network call B")
            val networkCallBValue = !pauseAndIncrement(requiredValue).k()

            networkCallAValue + networkCallBValue
        }.value()

        assertThat(x.blockingGet()).isEqualTo(4)
    }

    private fun pauseAndIncrement(i: Int): Single<Int> {
        println("Entering pause...")
        Thread.sleep(2000)
        println("... returning single")
        return Single.just(i + 1)
    }

getCountSingle

in this context should block the subsequent assignments until it is available (this is fine) but its value should be usable by the 2 subsequent calls which I'd like to happen concurrently as they are independent, and I just need to block on the result of them both. Is there a nice way of doing this without it becoming spagetti code?

Satyam Agarwal

01/23/2020, 2:15 PM

Hi. I am in the process of moving from Try to IO in my web app. but the whole stack is built upon that. Basically Try wraps my network requests. Since its a big project moving to IO will not be easy. So I want to do it in steps, And thinking this is how I will do the migration. 1. Replace

Try { … }.toEither()

with

IO { … }.attempt().unsafeRunSync()

in the clients/repositories 2. Start Replacing Either from services with IO and remove

unsafeRunSync()

and bring it to services. 3. Eventually remove

unsafeRunSync()

and bring them finally to controllers. 4. And finally get rid of springboot ❤️ I have worked with

unsafeRunSync

before in production for very small things (mostly one time things like migrations, imports/exports where I couldn’t do without springboot 💢), so I am comfortable and have not seen any problems, but with above, my apis can get affected, consequently my user-base. I am basically wondering about this line from doc

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**NOTE** this function is intended for testing, it should never appear in your mainline production code!

Can you guys help me with deciding if this makes sense and is sane thing to do, and what kind of problems I can see with this approach.

aballano

01/24/2020, 11:22 AM

Hi there folks, I'm looking into a ticket for Arrow to try to reduce the duplicated extension functions projected over types (e.g. ListK has around ~5 map functions). One question I'm wondering, does it make sense to provide instances of different typeclasses when they're related? Example: ListK provides an instance for Monad (which has map defined) and another for Applicative (which also has map), but Monad is already an Applicative, so shouldn't be sufficient by providing the former?

☝️ 1

Attila Domokos

01/24/2020, 5:24 PM

This sentence in the Arrow guides is misleading: "Complementing the Kotlin Coroutines library, Arrow Fx adds an extra layer of safety to concurrent and asynchronous programming so you’re well aware of where effects are localized in your apps." TIL - Arrow is not using JetBrain's coroutine library (https://github.com/Kotlin/kotlinx.coroutines), the wording should be "Competing" or "As an alternative to the Kotlin Coroutines library". Thoughts? (Thanks to @simon.vergauwen for chatting with me about it.)

Bruno

01/24/2020, 9:20 PM

I want to translate something from F# to Arrow: F#

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[1..10]
|> List.map (add 2)
|> List.map (mult 3)

My attempts: #1

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(1..10)
    .pipe { it.map { add(2) } }
    .pipe { it.map { mult(3) } }

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(1..10)
    .map { add(2) }
    .map { mult(3) }

What am I missing? I'd expect a list like:

[9,12,15,18,21,24,27,30,33,36]

, but instead get a

List<(Int) -> Int>

mbrandonw

01/27/2020, 2:25 PM

It seems that it is impossible to use

@optics

to generate optics for generic data classes and sealed classes. The generated code has compiler errors. 1) is this right? and 2) are there alternatives to this besides writing the optics by hand?

👍 1

pakoito

01/27/2020, 4:46 PM

// TODO which we need to have access to the implementation of map in Option

yes! it's also a fun exercise not to call

fix()

and see a stackoverflow exception because the function calls itself

😂 1

Satyam Agarwal

01/27/2020, 6:11 PM

So this program fills up ~8G and does not print anything, after some time it either throws error for thread starvation or exits due to heap space unavailable (Because I am providing 8G of heap). But this shouldn’t take this much. Can you guys help me on what am I doing wrong here. Actual use case could be, I want to make 100000 network request and semaphore limit is my http client pool connections, say 10. Using 0.10.4 version

Untitled

kyleg

01/27/2020, 7:56 PM

Has anyone given any decent thought about implementing an Android app using MVI architecture, but using Arrow/FP? I have been using an app I wrote as a personal project in MVVM style to learn other things, and first I learned MVI by refactoring it that way, and now I’ve been thinking about how one might use Arrow thoroughly. (my thoughts in thread)

Bruno

01/28/2020, 9:27 AM

Hello everyone, I got a simple console program, that reads to numbers from console and prints the sum. I have learned that the FP way of effects is not to use IO<Unit> for everything, because that does not give us control of which types of effects we want to allow for certain parts of our code. Thus I want to allow only console reading and printing or console IO for this program, how would I do that with arrow-fx?

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fun add(a: Int, b: Int): Int = a + b

suspend fun main1(args: Array<String>) =
    IO.fx {

        val sc = Scanner(System.`in`)

        val nextInt = effect { sc.nextInt() }

        val added = IO.applicative()
            .map(nextInt, nextInt) { (a, b) -> add(a, b) }.bind()

        effect { println(added) }.bind()

    }.run { suspended() }

aeruhxi

01/29/2020, 8:52 AM

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Either.fx() {
  val (x) = someLeftValue()
  10
} // Left propagates

Either.fx() {
  someLeftValue()
  10
} // Either.Right(10)

rcd27

01/29/2020, 10:47 AM

Hello, mates. Learning Scala, correct me if I'm not right: if

Kotlin

compiler had such feature like

implicit

Scala

, there would be less work for people who do

Arrow Meta

? This

Scala

feature looks very useful.

Hiosdra

01/29/2020, 8:38 PM

Hi, in arrow 0.11.0-snapshot (testing it, because of BIO) in this code:

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fun validatePhotoMatch(login: Login, photo: ByteArray): IO<Error, Unit> =
    IO.monad<Error>().fx.monad {
      val user = !userService.getUserByLogin(login)
      val modelVectors = !extractFaceVectors(user)
      val unknownVectors = !recognitionService.calculatePhoto(photo)
      val resultDto = !recognitionService.calculateDifference(unknownVectors, modelVectors)
      !failureOrUnit(resultDto.result)
    }.fix()

I cannot make second line: as

IO.fx {

. Idea cannot decide if it should be:

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IO.Companion.fx(c: suspend IOSyntax<E>.() -> A): IO<E, A>

Or:

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IO.Companion.fx(c: suspend ConcurrentSyntax<IOPartialOf<Nothing>>.() -> A): IO<Nothing, A>

Is it some kind of bug with new IO being BIO? Or I do something bad? Type inference should see that there is Error on left side, not Nothing. (I use my own Error class in ADT) Every method used in for-comprehension is returning

IO<Error, x>

Arrow Gradle deps are:

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val arrowVersion = "0.11.0-SNAPSHOT"
	implementation("io.arrow-kt:arrow-fx:$arrowVersion")
	implementation("io.arrow-kt:arrow-fx-reactor:$arrowVersion")
	implementation("io.arrow-kt:arrow-mtl:$arrowVersion")
	implementation("io.arrow-kt:arrow-mtl-data:$arrowVersion")
	implementation("io.arrow-kt:arrow-syntax:$arrowVersion")
	implementation("io.arrow-kt:arrow-integration-retrofit-adapter:$arrowVersion")
	kapt("io.arrow-kt:arrow-meta:$arrowVersion")

And second: Maybe add function

Either<E, A>.toIO<E, A>()

? There are already functions that map left side of Either to “below” side of IO, but I couldn’t see any mapping Either left to IO left. In BIO it would be very easy. I have written it for my own purpose and can push pull-request to 0.11.0 branch ;)

pakoito

01/29/2020, 8:54 PM

IO.monad<Error>().fx.monad {

don’t use this, use just

IO.fx<MyError> { ... }

pakoito

01/29/2020, 8:59 PM

should it be raiseException or raiseError? cc @aballano and @simon.vergauwen for opinion