An example about abandoning coroutine in Kotlin may not execute the finally clause immediately

In Kotlin, a coroutine can be abandoned by not resuming it. If a coroutine is abandoned while suspending in a try-block with finally, the finally clause will not be executed until the coroutine is collected.

For example, the following Kotlin code

val seqWithFinally = sequence {
    try {
        yield(Unit)
        yield(Unit)
    } finally {
        println("finally")
    }
}

for (i in seqWithFinally) {
    println("for")
    break
}

only outputs for.

As comparisons, both the following C# code

static IEnumerable<int> YieldWithFinally()
{
    try
    {
        yield return 0;
        yield return 0;
    }
    finally
    {
        Console.WriteLine("finally");
    }
}

static void Main(string[] args)
{
    foreach (int i in YieldWithFinally())
    {
        Console.WriteLine("for");
        break;
    }
}

and the following JS code

function* yieldWithFinally() {
    try {
        yield 0
        yield 0
    } finally {
        console.log("finally")
    }
}

for (let i of yieldWithFinally()) {
    console.log("for")
    break
}

output

for
finally

For the reasons why Kotlin is designed that way, check KEEP.

Jason5Lee/kotlin-abandoned-coroutines.md