Suspending functions

Throughout the book, we have written many suspending algorithms, but most of the time it has been done using coroutine builders such as launch(), async(), and runBlocking(). When calling a coroutine builder, the code that we pass to it is a suspending lambda. Now it's time to take a detailed look at how to write suspending code that exists as a function instead:

suspend fun greetDelayed(delayMillis: Int) {
delay(delayMillis)
println("Hello, World!")
}

To create a suspending function, you only need to add the modifier suspend to its signature. Notice that suspending functions can call other suspending functions such as delay() directly – no need to wrap inside a coroutine builder–  making our code clean and easy to read.

Nevertheless, if we try calling this function outside of a coroutine it will not work. Consider this example:

fun main(args: Array<String>) {
greetDelayed(1000)
}

This code will not compile because, as we already know, suspending computations can be...