I'm playing with coroutines, trying to round out/b...

# getting-started

[Travis Griggs]

I'm playing with coroutines, trying to round out/better understand them instead of skirting around them. reading through the docs (https://kotlinlang.org/docs/coroutines-basics.html#scope-builder) and playing with variations in the online playground, I found that if I do this:

fun main() = runBlocking {
    launch { foo("a", 2) }
    launch { foo("b", 3) }
}

I get a "No main method found in project" error. But if I add a println statement then things work?

fun main() = runBlocking {
    launch { foo("a", 2) }
    launch { foo("b", 3) }
    println("tada")
}

I can put that anywhere (top, middle, last) for it to make it so I don't get the "No main method" error. If I try a different statement, such as just a 3 + 4, I get the same error. But if I store it in a variable (e.g. val total = 3 + 4 ) , then it works.

What's going on here? Is this something to do with the Playground? or a nuance of runBlocking? Or something else?

[Youssef Shoaib [MOD]]

launch returns a Job object, which allows you to manually cancel that launched coroutine. runBlocking returns whatever your block returns. So you end up with a fun main(): Job, which won't run since the expectation is that your main method returns Unit. println returns Unit. What you can do is simply annotate it fun main(): Unit = runBlocking {}
Nothing to do with coroutines and all to do with type inference and expression blocks for functions

[Javier]

you can do suspend fun main which I think it is sugar to the run blocking method

[Youssef Shoaib [MOD]]

IIRC suspend fun main is ever so slightly different than runBlocking (it doesn't even rely on coroutines lol) but I think it is considered "better" than runBlocking

[Travis Griggs]

I went with the Unit annotation. Subtle that, and I learned something. Just the reason I decided to sort of go back to the basics, read the docs thoroughly, and sort of play with it all from all sides. Thanks

[CLOVIS]

In IntelliJ, I highly recommend enabling inlay hints in the settings, they show the type inference information directly in the code, which makes it much easier to understand these cases.