Managing mutable state

The main concern (and nightmare fuel) when we deal with asynchronous code is how to handle mutable state. We covered how to reduce mutable state with a functional style in Chapter 3, Immutability - It's Important. But sometimes it is impossible to use a functional immutable style. Coroutines offer some alternatives to this problem.

In the following example, we'll use several coroutines to update a counter:

import kotlin.system.measureTimeMillis

suspend fun repeatInParallel(times: Int, block: suspend () -> Unit) {
   val job = launch {
      repeat(times) {
         launch(coroutineContext) {
            block()
         }
      }
   }
   job.join()
}

fun main(args: Array<String>) = runBlocking {
   var counter = 0

   val time = measureTimeMillis {
      repeatInParallel(1_000_000) {
         counter++
      }
   }
   println("counter = $counter")
   println("time = $time")
}

On smaller numbers, counter is right, but once we start increasing the size, we'll...