ConcurrentMap.kt

fun <T, R> Iterable<T>.map(
    dispatcher: CoroutineDispatcher, 
    transform: (T) -> R
): List<R> = runBlocking {
    map { item -> async(dispatcher) { transform(item) } }.awaitAll()
}

fun doSomething(users: List<User>) {
    users.map(Dispatchers.Default) { user -> user.toSomething() /* `toSomething()` is heavy method */ }
}