A question about doing parallel async work using map More in kotlinlang #coroutines

Channels

lincheck

kobweb

koalaql

kotlin-multiplatform-contest

osgi

brikk

revolver

tricity-kotlin-user-group

cincinnati-user-group

prozis-android-backup

refactoring-to-kotlin

russian-kotlinasfirst

androidgithubprojects

navigation-architecture-component

stayhungrystayfoolish

practical-functional-programming

tensorflow

swagger-gradle-codegen

kotlinultimatechallenge

competitive-programming

skia-wasm-interop-temp

java-to-kotlin-refactoring

event21-community-content

myndocs-oauth2-server

jvm-ir-backend-feedback

Title

Stylianos Gakis

09/26/2021, 6:04 PM

A question about doing parallel async work using map. (More in Thread 🧵)

👀 1

I’ve seen a function signature that looks like this

suspend fun <A, B> Iterable<A>.pmap(f: suspend (A) -> B): List<B> = coroutineScope {
    map { async { f(it) } }.awaitAll()
}

However I am curious to understand how this would be different from me doing something like this manually in the call site.

something.map {
    async { f(it) } // f() is suspending
}.awaitAll()

The one difference I see here is that the

pmap

function wraps everything with

coroutineScope

but I don’t think I understand why that is. How does it make it any different and will mine break in some case where one of them throws an exception, is cancelled or something like that?

ephemient

09/26/2021, 6:22 PM

1. the function won't work without some coroutine scope either created there or passed in 2. behavior-wise, the first ensures that all these mapped parallel asyncs are cancelled as soon as any one fails, whereas in the second it won't happen until whatever scope your function uses is cancelled (which may be later, in case of a supervisor scope, or never, in case of global scope)

Stylianos Gakis

09/26/2021, 6:38 PM

1. For the second case, I do imply that it’s inside a for example

viewModelScope.launch { //here }

so it is part of some scope, if that’s what you meant. 2. I wonder why it is that if one fails they all cancel immediately in the case where they are wrapped with

coroutineScope

. What makes that happen, I am completely lost on this one. How does it being wrapped inside a

coroutineScope

make it behave differently from it being wrapped in a

.launch

scope. Is this some special behaviour of

async

? What and where can I read about this to understand it better?

ephemient

09/27/2021, 3:51 AM

it's a behavior of coroutineScope, https://elizarov.medium.com/structured-concurrency-722d765aa952

👍 1

2 Views

#coroutines Join Slack