Kotlin strange behaviour with chunked, async and await

user_2096052 0
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I'm doing some code that involves a number of parallel requests in batchs from elements in a list and I'm facing a strange behaviour:

import kotlinx.coroutines.*

suspend fun main() {
    coroutineScope {
        val hello = listOf("hello", "world")
        val chunks = hello.chunked(1) {
            async { translate(it.first()) }
        }
        chunks.forEach {
            print(it.await())
        }
    }
}

private fun translate(word: String): String {
    return if(word == "hello") {
        "hola"
    } else {
        "mundo"
    }
}

It should display "holamundo" but sometimes this example prints "mundomundo" instead.

I made it available on Kotlin Playground as well.

What I'm missing here?

kotlin-coroutines kotlin asynchronous
1 Answers

The reason for this behavior is that the code inside async is not run immediately. It is only scheduled. The async block returns before the lambda is invoked. This is why the it inside async block always points to the ["world"] window and hence the final output is "mundomundo".

Take this example:

fun main() {
    runBlocking {
        var a = 1
        val deferred = async {
            println(a)
        }
        a++
        deferred.await()
    }
}

Playground
In this example the output will be 2 and not 1 because a++ is processed before the async lambda.

Edit: As @IR42 pointed out in the comment, the documentation clearly mentions the the list passed to the transform function changes very quickly, so if you want to use it asynchronously you should first make a copy of it.

val chunks = hello.chunked(1) {
    val copy = it.toList()
    async { translate(copy.first()) }
}

This code will give you the expected output.

Another solution (thanks to @broot) is to first calculate the entire chunked list and then map it to the list of deferred.

val chunks = hello.chunked(1).map {
    async { translate(it.first()) }
}
user_133089910
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