Being explicit

Concurrency needs to be thought about and designed for, and because of that, it's important to make it explicit in terms of when a computation should run concurrently. Suspendable computations will run sequentially by default. Since they don't block the thread when suspended, there's no direct drawback:

fun main(args: Array<String>) = runBlocking {
    val time = measureTimeMillis {
        val name = getName()
        val lastName = getLastName()
        println("Hello, $name $lastName")
    }
    println("Execution took $time ms")
}

suspend fun getName(): String {
    delay(1000)
    return "Susan"
}

suspend fun getLastName(): String {
    delay(1000)
    return "Calvin"
}

In this code, main() executes the suspendable computations getName() and getLastName() in the current thread, sequentially.

Executing main() will print the following:

This is convenient because it's possible to write non-concurrent code that doesn't block the thread of execution. But after some time and analysis, it becomes clear that it doesn't make sense to have getLastName() wait until after getName() has been executed since the computation of the latter has no dependency on the former. It's better to make it concurrent:

fun main(args: Array<String>) = runBlocking {
    val time = measureTimeMillis {
        val name = async { getName() }
        val lastName = async { getLastName() }

        println("Hello, ${name.await()} ${lastName.await()}")
    }
println("Execution took $time ms")
}

Now, by calling async {...} it's clear that both of them should run concurrently, and by calling await() it's requested that main() is suspended until both computations have a result: