Why is this an issue?

There are two ways to define asynchronous functions in Kotlin:

• using the modifier suspend in the function declaration
• creating an extension function on CoroutineScope

The suspend modifier is generally used for functions that might take some time to complete. The caller coroutine might potentially be suspended.

Functions that start a coroutine in the background and return before said coroutine has completed running should be extension functions on CoroutineScope. This helps to clarify the intention of such a function. Further, such functions should not be suspending, as suspending functions should only return once all the work they are designed to perform is complete.

Functions returning Flow or Channel should return the result immediately and may start a new coroutine in the background. As a consequence, such functions should not be suspending and if they launch a coroutine in the background, they should be declared as extension functions on CoroutineScope.

Noncompliant code example

suspend fun f(): Flow<Int> {
    val flow = flow {
        emit(1)
    }
    delay(500L)
    return flow
}

suspend fun f(): Channel<Int> {
    val ch = Channel<Int>()
    ch.send(1)
    return ch
}

Compliant solution

fun f(): Flow<Int> = flow {
    emit(1)
}

fun CoroutineScope.f(): Channel<Int> {
    val ch = Channel<Int>()
    launch {
        ch.send(1)
    }
    return ch
}

Resources

• Coroutine Context and Scope