To write unit tests for your Hack code, you can utilize testing frameworks and libraries that provide the necessary tools and structures to define and run tests. Here's a step-by-step guide to writing unit tests in Hack:

1. Choose a Testing Framework : Start by selecting a testing framework that aligns with your requirements. Popular choices for Hack include PHPUnit, HackTest, and HHVM's built-in test framework. These frameworks provide the necessary infrastructure for defining and executing tests.

2. Create a Test File : Create a new file specifically for your unit tests. Conventionally, test files are named with the suffix `Test.hack` or `_Test.hack` to distinguish them from regular code files.

3. Define Test Functions : Within your test file, define functions that represent individual tests. Each test function should focus on a specific aspect or behavior of your code that you want to verify.

// MyCode.hack

function addNumbers(int $a, int $b): int {
  return $a + $b;
}

// MyCodeTest.hack

function testAddNumbers(): void {
  $result = addNumbers(2, 3);
  assert($result === 5, "Expected result is 5, but got $result");
}​

In the above example, the `testAddNumbers` function tests the `addNumbers` function and checks if the result matches the expected value.

4. Use Assertions : Inside your test functions, employ assertions to validate the expected behavior of your code. Assertions verify conditions and raise an exception if the condition fails, indicating that a test has failed.

assert($result === 5, "Expected result is 5, but got $result");​

The above assertion verifies if the `$result` variable is equal to 5. If the condition fails, the assertion throws an exception with the provided error message.

5. Run the Tests : Execute your tests using the testing framework's command-line tool or IDE integration. The framework will discover and run all the test functions defined in your test file.

6. Interpret Test Results : Review the test results to determine which tests passed or failed. The testing framework typically provides a detailed report, indicating the number of tests run, passed, failed, and any error messages or stack traces associated with failed tests.

7. Refine and Iterate : Analyze the test results and investigate any failures or unexpected behavior. If a test fails, debug the issue, make necessary code modifications, and rerun the tests. Ensure that your code passes all the tests and meets the desired behavior.

8. Maintain and Expand Test Suite : As your codebase evolves, continue adding new tests to cover additional functionality, edge cases, and potential issues. Maintain a comprehensive suite of unit tests to validate the behavior of your code and catch regressions.