The other day I came across `suspend fun main()` f...

# coroutines

[Erik]

The other day I came across suspend fun main() for the first time. Where is it documented? How does it work?

[wasyl]

It’s been added some time ago already https://youtrack.jetbrains.com/issue/KT-17679. There aren’t many details in the issue but it seems like a shortcut for fun main(args: Array<String>) = runBlocking { ... }

[Zach Klippenstein (he/him) [MOD]]

I think the only difference is that suspend fun main uses the Default dispatcher

[Erik]

I think it's not obvious that it uses runBlocking under the hood. As this is a suspend fun I'm blind to the coroutine scope and context, right? So it's not within my power to know and control whether or not my coroutines run blocking or are backed by many threads?

[Zach Klippenstein (he/him) [MOD]]

It’s definitely not obvious. You can always use withContext to be explicit about that anyway though.

[Erik]

So I can escape the synchrony of runBlocking that way? 😎 I'll have to try that some time soon

[Erik]

Btw, if I'd had to guess, I would have thought that it was a shorthand for GlobalSope.launch

[Zach Klippenstein (he/him) [MOD]]

So I can escape the synchrony of runBlocking that way?

Not really. Regardless of which dispatchers your coroutines are running on, main has to block at some point or your program will just exit immediately without running any coroutines.

[wasyl]

Yep, I kind of read the issue which said it should be equivalent, but clearly the implementation is different. The bytecode for such suspend main is:

// signature ([Ljava/lang/String;Lkotlin/coroutines/Continuation<-Lkotlin/Unit;>;)Ljava/lang/Object;
// declaration:  main(java.lang.String[], kotlin.coroutines.Continuation<? super kotlin.Unit>)
public final static main([Ljava/lang/String;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;

Then there’s synthetic method:

// $FF: synthetic method
public static void main(String[] var0) {
  RunSuspendKt.runSuspend(new AppKt$$$main(var0));
}

And the RunSuspend class uses empty coroutine context:

private class RunSuspend : Continuation<Unit> {
    override val context: CoroutineContext
        get() = EmptyCoroutineContext

[gildor]

Suspend main doesn't use runBlocking under the hood and not shortcut for GlobalScope.launch, runBlocking is not a part of stdlib, this is a library function
The main difference it doesn't provide coroutine scope, but in terms of threading it should be the same as runBlocking on the main thread, it uses main thread