We know how to execute parallel suspend functions in a CoroutineScope, but how to do it within a suspend function that has a return type.

The Problem Link to heading

For what we know it is not hard to run multiple suspend functions parallelly in a CoroutineScope using async {} block. async’s return type is Deferred<T> which then can be awaited so it is possible to chain multiple Deferred<T> jobs using awaitAll which solves the problem of parallelism.

suspend fun taskList1() = "Some text data"
suspend fun taskList2() = "Another random data"

suspend fun parallelNoReturnType() {
    CoroutineScope(Dispatchers.IO).launch {
        val task1 = async { taskList1() }
        val task2 = async { taskList2() }

        val total: List<String> = awaitAll(task1, task2)

        // process data & return unit
    }
}

Certainly, this is not a problem but what if the return type of the function is other than Unit meaning there is need for a non Unit return type. Certainly returning the total variable from above will not work as it is in the @launch scope, also returning the CoroutineScope will not work since it’s return type is Job not List<String> which we are expecting.

// Eg: 1 - Returning total variable (will not compile)
suspend fun parallelReturnType() : List<String>  {
    CoroutineScope(...).launch {
        ...
        val total: List<String> = ...

        return total // Error
    }
}

// Eg: 2 - Returning CoroutineScope (will not compile)
suspend fun parallelReturnType() : List<String> {
    // Valid return type but it's not List<String>; Error
    return CoroutineScope(...).launch {
        ...
        val total: List<String> = ...
        total // hoping this would be the result just like `run` function
    }
}

Solution Link to heading

The easy fix is to use coroutineScope { } suspend function which has a signature public suspend fun <R> coroutineScope(block: suspend CoroutineScope.() -> R): R.

For us the R is List<String>, using it in the above code requires minimal edit.

suspend fun parallelReturnType() : List<String> = coroutineScope scope@{
    val task1 = async { taskList1() }
    val task2 = async { taskList2() }

    val total: List<String> = awaitAll(task1, task2)

    return@scope total
}

The coroutineScope uses the default coroutineContext associated with the suspend function i.e the parent’s coroutine context which was used to invoke this suspend function.

If you want to dispatch the jobs on some other CoroutineDispatcher you can use .async {} on CoroutineScope.

suspend fun parallelReturnType() : List<String> = CoroutineScope(Dispatchers.Default).async {
    val task1 = async { taskList1() }
    val task2 = async { taskList2() }

    val total: List<String> = awaitAll(task1, task2)

    return@async total
}.await() // important otherwise the return type will be Deferred<List<String>>