`awaitFirst` function

Core library misses of an Promise.race equivalent function.

I propose a little POC of joinFirst/awaitFirst, or a more generic selectFirst.

What do you think?

import kotlinx.coroutines.experimental.*
import kotlinx.coroutines.experimental.selects.SelectClause0
import kotlinx.coroutines.experimental.selects.SelectClause1
import kotlinx.coroutines.experimental.selects.select
import kotlin.reflect.KProperty1

fun main(args: Array<String>) = runBlocking {
    val jobs = listOf(launch { delay(3) }, launch { delay(4) }, launch { delay(5) })
    jobs.selectFirst(Job::onJoin) {
        println("$it done")
    }

    val asyncs = listOf(async { 3 }, async { 4 }, async { 5 })
    asyncs.selectFirst(Deferred<Int>::onAwait) { deferred, value ->
        println("$deferred done with result $value")
    }

    println("First result is " + asyncs.awaitFirst())
}

// common functions

suspend fun <E : Job> Iterable<E>.joinFirst(): E = select {
    for (job in this@joinFirst) {
        job.onJoin { job }
    }
}

suspend fun <E : Deferred<R>, R> Iterable<E>.awaitFirst(): R = joinFirst().getCompleted()

// generic funtions

suspend fun <E, R> Iterable<E>.selectFirst(selectProperty: KProperty1<E, SelectClause0>, block: (E) -> R): R =
        select {
            for (task in this@selectFirst) {
                selectProperty.get(task).invoke { block(task) }
            }
        }

suspend fun <E, V, R> Iterable<E>.selectFirst(selectProperty: KProperty1<E, SelectClause1<V>>, block: (E, V) -> R): R =
        select {
            for (task in this@selectFirst) {
                selectProperty.get(task).invoke { block(task, it) }
            }
        }

[qwwdfsad]

Closing as obsolete

[elizarov]

@SolomonSun2010 We’ve designed Kotlin coroutines for more ergonomics. In Kotlin these examples look like this:

for (request in requestServer) {
    handleRequest(request)
}

It is cleaner and with less boiler-plate in Kotlin (no need to explicitly write await in Kotlin if you use suspending functions and channels). As an added bonus, it is not hardcoded into the language (unlike Dart), but is implemented in a library, so we can have even more variety of useful constructs without having to release an update to the language. I welcome you to study coroutines guide to learn about other features we offer.