Can't reify a suspendable block passed to forEach - kotlin

Question

Given some
suspend fun a(): Int
This works:
launch(Unconfined) {
(1..10).forEach {
val a = a()
println("Result is $a")
}
}
But this fails at compile time:
val action: (Int) -> Unit = {
// Suspend function should be called only from a coroutine
// or another suspend function:
val a = a()
println("Result is $a")
}
launch(Unconfined) {
(1..10).forEach(action)
}
Furthermore, it isn't fixable because:
val action: suspend (Int) -> Unit = {
val a = a()
println("Result is $a")
}
launch(Unconfined) {
// suspend (Int) -> Unit cannot be applied to (T) -> Unit
(1..10).forEach(action)
}
What is the story here in terms of the static type system? The current situation looks like a quick hack where an inline block containing a suspend fun call is still inferred to a non-suspend type signature.
Is this an area where the design will be improved before being finalized?

Answer 1

The suspend and normal functional types are not subtypes of each other and thus cannot be assigned or passed to a function in place of each other:
val f: () -> Unit = { }
val g: suspend () -> Unit = f // Type mismatch
val f: suspend () -> Unit = { }
val g: () -> Unit = f // Type mismatch
This is why a suspend (Int) -> Unit cannot be passed to forEach.
Basically, the restriction for suspend functions to be called only in other suspend functions works irrespective to the type system. Such calls should simply be placed inside a suspend function or a suspend lambda or inlined into one. So, this should also work:
val action: suspend (Int) -> Unit = {
val a = a()
println("Result is $a")
}
launch(Unconfined) {
(1..10).forEach { action() } // The call is inlined into a suspend lambda
}
I've filed an issue about supporting (1..10).forEach(action) as well: KT-22186