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Arrow v0.10.3 is now available

Arrow v0.10.3 is now available

The latest Arrow release includes some new and exciting features that bring us another step closer to 1.0 with Arrow Meta on the horizon. If you’re new to Arrow, it’s an open-source library that’s packed with data types and type classes that empower pure functional programming in Kotlin.

Deprecation of Try

Arrow’s goal is to offer a public API consisting of only pure functions. Therefore, we mark side-effects with suspend or we wrap them into an effectful type such as IO.

Try can never yield pure functions since it’s eager and executes the effects on construction; it can only be made pure by marking it as suspend. Since Try<A> is isomorphic to Either<Throwable, A>, we can deprecate Try and replace its constructor for a smart constructor for Either.

data class User(val name: String)

suspend fun <A> Either.Companion.catch(f: suspend () -> A): Either<Throwable, A> =
try { f().right() } catch (t: Throwable) { t.left() }

suspend fun getUser(): Either<Throwable, User> Either.catch {

What’s new in Arrow Fx

Simplifying Fx

The Fx typeclass has been removed from Arrow and its functionality is now merged into the typeclass hierarchy. If you were using Fx for concrete data types like IO, Option, Either, etc. you can now do so by calling the fx function on their companion objects.

import arrow.core.fx

val option = Option.fx {
  val one = !Some(1)
  val two = !Some(2)
  one + two

To use fx within the typeclass hierarchy ,you can call the property fx and then the named DSL for the syntax you want enabled:

import arrow.fx.Concurrent

fun io(CF: Concurrent<ForIO>): IO<Int> = CF.run {
  fx.concurrent {
    val one = !IO(1)
    val two = !IO(2)
    one + two

The following DSLs are available:

Typeclass New API Old API
Monad fx.monad binding
MonadThrow fx.monadThrow bindingCatch
MonadFilter fx.monadFilter bindingFilter
Async fx.async /
Comonad/Bimonad fx.comonad /
MonadDefer / bindingCancelable
Concurrent fx.concurrent bindingConcurrent

Note that bindingCancelable was removed because it belongs to the typeclass ConcurrentEffect, which is still a work in progress.

With the upcoming release of Arrow Meta, this could be further cleaned up!


Effect types can now interoperate with suspend and you can go back and forth between IO, Single, Mono, etc. and suspend using suspended() and effect. effect already exists in the typeclass hierarchy, and a typeclass for suspended will soon be included in Arrow Fx.

suspend fun main(): Unit {
  val request: Either<Throwable, User> =
    IO.effect(Dispatchers.IO) { getUser() }
  val text = request.fold({ "No user" }, { it.name })
  IO.effect(Dispatchers.Main) { displayUi(text) }.suspended()

New concurrency tools

Arrow v.0.10.3 includes a bunch of powerful new concurrency operators such as parTraverse, parSequence, and sleep.

val io = IO.fx {
  !listOf(1, 2, 3).parTraverse { i ->
    IO.effect { println("$i ==> ${Thread.currentThread().name} ") }

  !IO.effect { println("Going to take a nap now!") }
  !IO.effect { println("Woke up after an extreme powernap!") }

For a full list of changes and features in 0.10.3 please visit the official changelog.

We would like to give special thanks to the following contributors for this version (in no particular order):

For a full list of the contributors who have helped Arrow get to where it is today, please check out: Arrow Contributors. We’re always looking for additional help, please view the guidelines for contributing and take a look at the help-wanted issues. All levels are welcome and we offer 1:1 mentoring through Arrow’s Slack and Gitter channels.

If you would like to support Arrow, but don’t have the time to invest in contributing, consider picking up some Arrow swag! All proceeds are invested back into advancing the library.


The active development of Arrow is proudly sponsored by 47 Degrees, a Functional Programming consultancy with a focus on the Scala, Kotlin, Haskell, and Swift programming languages.

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