Introduction to coroutines

Let's start with a simple example without coroutines:

import kotlin.concurrent.thread

fun main(args: Array<String>) {
   thread {
      Thread.sleep(1000)
      println("World!")
   }
   print("Hello ")
   Thread.sleep(2000)
}

The thread function executes a block of code in a different thread. Inside the block, we are simulating an expensive I/O computation (such as accessing data from a microservice over HTTP) with Thread.sleep. Thread.sleep will block the current thread for the number of milliseconds passed as a parameter. In this example, we don't wait until the computation finishes to keep working on other things; we print another message, "Hello", while the other computation is being executed. At the end, we wait for two seconds until the computation is finished.

That's not a pretty code, and...