IO is being deprecated in Arrow in favor of Arrow Fx Coroutines which transparently integrates with Kotlin suspend functions and the KotlinX Coroutines library.

Arrow has adopted suspend as system to model monadic computations and offers the same api and additional features as top level extensions functions over suspend () -> A whereas before it was IO<A>. Some functions like flatMap are now replaced by simple function invocation.

Why suspend () -> A instead of IO<A>

This section explains the rationale about why Arrow dropped IO in favor of suspend.

The reason for using IO is because you care about writing side-effecting code in a safe and referential transparent manner. Additionally, IO offers powerful concurrent operators and cancellation in addition of offering a referential transparent runtime. These properties are what makes using IO powerful, and suspend offers the exact same properties but natively in the language with support from the compiler.

Below we discuss a couple topics why we favor suspend over IO.


IO requires a wrapper in the return type: fun number(): IO<Int>, and thus we always have to work with the IO type to access the value we care about within. A typical pattern for this using flatMap, so let’s say we want to calculate 3 numbers and return them as a Triple.

fun number(): IO<Int> = IO.just(1)

fun triple(): IO<Triple<Int, Int, Int>> =
  number().flatMap { a ->
    number().flatMap { b ->
      number().map { c ->
        Triple(a, b, c)

So simply to call a function 3 times, and combine the result into a Triple we had to use flatMap twice and map. What that means under the hood we’ll discuss in the performance section but in terms of ergonomics this is not ideal. Especially not if we can compare it to the following suspend code:

suspend fun number(): Int = 1

suspend fun triple(): Triple<Int, Int, Int> = 
  Triple(number(), number(), number())

Here we can see that we can simply forget about flatMap & map and we can simply construct the Triple and call number() three times directly in the constructor.

The ergonomics of suspend are clearly better here, and this is a very important point in Kotlin since the language aims for high ergonomics and developer friendly constructs.

Safety / Purity / Referential transparency

So what guarantees does IO bring, and does suspend offer the same guarantees?

IO<A> always results in Either<Throwable, A> whether we run it with unsafeRunSync (throws Throwable), unsafeRunAsync or unsafeRunAsyncCancellable. This is done so that any exceptions that occur within the IO API can be safely returned to the user, and it can be recovered from at any point in the code. Important here is that a Throwable that occurs in an async thread is safely captured in the IO as well and can be recovered from at any point.

suspend always results in Result<A> which is isomorphic to Either<Throwable, A>, and it can be used to offer the same safety guarantees as IO. So the suspend API can also always return any exception safely to the user, and it can be recovered from at any point in the code. In contrast to IO, we can only find startCoroutine in the standard library and it’s isomorphic to unsafeRunAsync. Instead of f: (Either<Throwable, A>) -> Unit you provide f: (Result<A>) -> Unit to run the suspend () -> A program.

So suspend () -> A offers us the exact same guarantees as IO<A>.

Both also forces us to go through certain set of methods to execute/run the effectful program.

Monad transformers vs effect mixing

IO<Either<E, A» vs suspend () -> Either<E, A>

When writing functional code style we often want to express our domain errors as clearly as possible, a popular pattern is to return Either<DomainError, SuccessValue>. Let’s assume following domain, and compare two snippets one using IO<Either<E, A>> and another suspend () -> Either<E, A>.

import arrow.core.Either
import arrow.core.Either.Left
import arrow.core.Either.Right

/* inline */ class Id(val id: Long)
object PersistenceError

data class User(val email: String, val name: String)
data class ProcessedUser(val id: Id, val email: String, val name: String)

suspend fun fetchUser(): Either<PersistenceError, User> =
  Right(User("", "Simon"))

suspend fun User.process(): Either<PersistenceError, ProcessedUser> =
  if (email.contains(Regex("^(.+)@(.+)$"))) Right(ProcessedUser(Id(1), email, name))
  else Left(PersistenceError)
IO<Either<E, A»
import arrow.fx.*

fun ioProgram(): IO<Either<PersistenceError, ProcessedUser>> =
  IO.fx {
    val res = !IO.effect { fetchUser() }

    !res.fold({ error ->
    }, { user ->
      IO.effect { user.process() }

// Or unwrapped in `suspend`
suspend suspendedIOProgram(): Either<PersistenceError, ProcessedUser> =
suspend () -> Either<E, A>
import arrow.core.computations.either

suspend fun suspendProgram(): Either<PersistenceError, ProcessedUser> =
  either {
    val user = fetchUser().bind()
    val processed = user.process().bind()

The above two examples demonstrate how much simpler suspend is over its IO counterpart and how the either computation block allows us to bind values of Either to extract their right side all while inside suspend.

Arrow Fx allows to intermix effects in suspension. What otherwise would have required the EitherT transformer over IO now it can just be expressed by wrapping in either instead

suspend R.() -> A

We can use extension functions to do functional dependency injection with similar semantics as Reader or Kleisli. They allow us to elegantly define syntax for a certain type. Let’s see a simple example.

Let’s reuse our previous domain ofUser, ProcessedUser, but let’s introduce Repo and Persistence layers to mimick what could be a small app with a couple layers.

interface Repo {
    suspend fun fetchUsers(): List<User>

interface Persistence {
    suspend fun User.process(): Either<PersistenceError, ProcessedUser>

    suspend fun List<User>.process(): Either<PersistenceError, List<ProcessedUser>> =
        either { map { it.process().bind() } }

Given the above defined layers we can easily compose them by creating a product which implements the dependencies by delegation.

class DataModule(
    persistence: Persistence,
    repo: Repo
) : Persistence by persistence, Repo by repo

We can also define top-level functions based on constraints on the receiver. Here we define getProcessedUsers which can only be called where R is both Repo and Persistence.

 * Generic top-level function based on syntax enabled by [Persistence] & [Repo] constraint
suspend fun <R> R.getProcessedUsers(): Either<PersistenceError, List<ProcessedUser>>
        where R : Repo,
              R : Persistence = fetchUsers().process()


suspend is extremely fast in comparison to IO<A>, since IO<A> is build at runtime and suspend is build by the compiler.

Working with an ADT such as IO<A> implies that each composition of our program has some allocation cost. This happens because the IO ADT uses different data classes to move computation from the stack to the heap in order to compose them and preserve properties such as lazy evaluation semantics.

In contrast, when using suspend, the Kotlin compiler is aware of function composition on each suspension point and can desugar and specialise the program into more efficient target code. The generated code by the Kotlin compiler is better in terms of allocations and throughput when compared to other IO ADT based impls in the JVM.

Let’s take our previous example from ergonomics:

import arrow.fx.IO

fun number(): IO<Int> = IO.just(1)

fun triple(): IO<Triple<Int, Int, Int>> =
  number().flatMap { a ->
    number().flatMap { b ->
      number().map { c ->
        Triple(a, b, c)

If we translate this piece of code to the data class it uses it results in the following:

fun number(): IO<Int> = IO.Just(1)

fun triple(): IO<Triple<Int, Int, Int>> =
  IO.Bind(IO.Just(1)) { a ->
    IO.Bind(IO.Just(1)) { b ->
      IO.Map(IO.Just(1)) { c -> Triple(a, b, c) }

We can see that for this simple program we allocated 6 IO cases of the IO sealed class. This is necessary so when unsafeRun is invoked the IO program can find the branch representing the kind of operation of IO that needs to be interpreted. In the example above IO.Bind, IO.Map or IO.Just

In contrast, suspend can simply be wired by the Kotlin compiler eliminating the need for additional sealed class declarations and allocations keeping computations in the stack instead of maintaining value level in memory representations of our program. The Kotlin compiler rewrites the suspend program to a super fast runtime which uses a switch table and mutable state machine to run the suspend program. Furthermore, the Kotlin compiler applies other optimisations focused on the speed of suspend and memory usage of suspension making it suitable for hotspots and places where otherwise allocations or ADT based data types are not an option.

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