In Hack, asynchronous operations allow you to perform tasks concurrently and efficiently without blocking the execution of the main program. Asynchronous programming is particularly useful for handling I/O operations, network requests, and other tasks that may take time to complete. Here are the main features and techniques for asynchronous operations in Hack:

1. Async Functions : Async functions are functions that can be executed asynchronously. They are defined using the `async` keyword before the function declaration. An async function returns an `Awaitable<T>` object, where `T` represents the type of the eventual result.

async function fetchData(): Awaitable<string> {
  // Perform asynchronous operations
  // ...
  return $result;
}​

In the example above, the `fetchData` function is defined as an async function, indicating that it can be executed asynchronously. It returns an `Awaitable<string>` object representing a future result of type `string`.

2. Await Expressions : Within an async function, you can use the `await` keyword to pause the execution and wait for the completion of an asynchronous operation. The `await` expression is used to retrieve the eventual result of an `Awaitable` object.

async function processAsync(): Awaitable<void> {
  $result = await fetchData();
  // Process the result
  // ...
}​

In the example above, the `await` expression is used to pause the execution of the `processAsync` function until the `fetchData` async function completes. The value of `$result` will be the eventual result of the asynchronous operation. 3. Async Control Structures : Hack provides async versions of control structures like `if`, `foreach`, and `while` that allow you to work with async operations.

async function processListAsync(array<int> $numbers): Awaitable<void> {
  foreach async ($numbers as $number) {
    $result = await processNumberAsync($number);
    // Process the result
    // ...
  }
}​

In the example above, the `foreach async` loop allows you to iterate over the `$numbers` array asynchronously, processing each number using the `processNumberAsync` function.

4. Parallel Execution : Hack provides the `HH\Asio\v()` function, also known as the "vector await" function, which allows you to execute multiple async operations in parallel. It takes an array of `Awaitable` objects and returns an `Awaitable<array<mixed>>` object representing the results.

async function fetchMultiple(): Awaitable<void> {
  $results = await HH\Asio\v(fetchData(), fetchOtherData());
  // Access individual results
  // ...
}​

In the example above, the `fetchData()` and `fetchOtherData()` functions are executed concurrently using `HH\Asio\v()`, allowing the async function to await both operations in parallel.

These are the main features and techniques for performing asynchronous operations in Hack. Asynchronous programming enables you to improve the responsiveness and efficiency of your code, especially when dealing with I/O-bound tasks and concurrency.