Kotlin Coroutines with returning value

To return exactly Int, you need to get out of the coroutine world and that's what runBlocking is for:

fun sum(): Int = runBlocking {
    funA().await() + funB().await()
}

See Bridging blocking and non-blocking worlds in the coroutine guide, and Composing suspending functions for how to do it if you want to use sum inside coroutines.

Adding another way of achieving it.

fun sum(): Int {
    var sum: Int = 0
    runBlocking {
        val jobA = async { funA() }
        val jobB = async { funB() }
        runBlocking{
           sum = jobA.await() + jobB.await()
        }
    }
    return sum
}

suspend fun funA(): Int {
    return 1
}

suspend fun funB(): Int {
    return 2
}

It may be late to answer this question but hopefully someone will find it useful. The code snippet below calculates sum of 3 values A + B + C. Each value is calculated independently in its own background thread in parallel and then all interim results consolidated into one final result and returned to the main thread to display it on a screen.

So it takes 5 seconds to calculate the final value (not 10 seconds = 2 + 3 + 5) and the result is 6 obviously and it's non-blocking, the main thread can handle other events while sum() execution is not complete.

suspend fun sum(scheduler: ThreadPoolExecutor): Int = coroutineScope {

    withContext(scheduler.asCoroutineDispatcher()) {
        val a = async { funA() }
        val b = async { funB() }
        val c = async { funC() }

        a.await() + b.await() + c.await()
    }
}

fun funA(): Int {
    Thread.sleep(2000L)
    return 1
}

fun funB(): Int {
    Thread.sleep(3000L)
    return 2
}

fun funC(): Int {
    Thread.sleep(5000L)
    return 3
}

class MainActivity : AppCompatActivity(), View.OnClickListener {
    private val tripletsPool = ThreadPoolExecutor(3, 3, 5L, TimeUnit.SECONDS, LinkedBlockingQueue())

   ...

    override fun onClick(view: View?) {
        if (view == null) {
            return
        }

        when (view.id) {
            R.id.calculate -> {
                GlobalScope.launch(Dispatchers.Main, CoroutineStart.DEFAULT) {
                    progressBar.visibility = View.VISIBLE
                    result.setText("${sum(tripletsPool)}")
                    progressBar.visibility = View.GONE
                }
            }
        }
    }
}