ConcurrentMap.kt

fun <T, R> Iterable<T>.map(
    concurrency: Int,
    transform: (T) -> R
): List<R> = runBlocking {
    // Create semaphore with permit specified as `concurrency`
    val semaphore = Semaphore(concurrency)
   
    map { item ->
        // Before processing each item, acquire the semaphore permit
        // This will be suspended until permit is available.
        semaphore.acquire()
        
        async(Dispatchers.Default) {
            try {
                transform(item)
            } finally {
                // After processing (or failure), release a semaphore permit
                semaphore.release()
            }
        }
    }.awaitAll()
}

fun doSomething(users: List<User>) {
    // Concurrently 5 users will be processed
    users.map(concurrency = 5) { user -> user.toSomething() /* `toSomething()` is heavy method */ }
}