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Hack Anti-Patterns Overview

Hack, despite being designed to improve upon PHP with static typing and other modern features, still has several anti-patterns that can lead to code quality issues, performance problems, and maintenance challenges. Here are the most important anti-patterns to avoid when writing Hack code.

Ignoring Static Typing

// Anti-pattern: Ignoring static typing
function process(mixed $data): void {
  // Using mixed and dynamic type checking
  if (is_array($data)) {
    foreach ($data as $item) {
      echo $item; // $item is still mixed
    }
  } else if (is_string($data)) {
    echo $data;
  }
}

// Better approach: Use proper static typing
function processArray(array<string> $data): void {
  foreach ($data as $item) {
    echo $item; // $item is string
  }
}

function processString(string $data): void {
  echo $data;
}

function process(mixed $data): void {
  if (is_array($data)) {
    $typed_data = TypeAssert\matches<array<string>>($data);
    processArray($typed_data);
  } else if (is_string($data)) {
    processString($data);
  }
}

One of Hack’s main advantages over PHP is its static typing system. Avoid using mixed types excessively or falling back to dynamic type checking with functions like is_array(). Instead, leverage Hack’s type system to catch errors at compile time rather than runtime.

Not Using Collections

// Anti-pattern: Using arrays instead of collections
function getUserNames(array<int, User> $users): array<int, string> {
  $names = [];
  foreach ($users as $id => $user) {
    $names[$id] = $user->getName();
  }
  return $names;
}

// Better approach: Use collections
function getUserNames(Map<int, User> $users): Map<int, string> {
  return $users->map($user ==> $user->getName());
}

// Or with Vector
function getUserNames(Vector<User> $users): Vector<string> {
  return $users->map($user ==> $user->getName());
}

Prefer Hack’s collection classes (Vector, Map, Set, etc.) over PHP arrays. Collections provide type safety, better performance in many cases, and useful higher-order functions for transformations.

Excessive Use of Nullable Types

// Anti-pattern: Excessive use of nullable types
function getUserName(?User $user): ?string {
  if ($user === null) {
    return null;
  }
  return $user->getName();
}

// Usage that leads to null checking chains
$userName = getUserName($user);
if ($userName !== null) {
  echo $userName;
} else {
  echo "Unknown user";
}

// Better approach: Use Option type pattern or provide defaults
function getUserName(User $user): string {
  return $user->getName();
}

function getDisplayName(?User $user): string {
  return $user !== null ? getUserName($user) : "Unknown user";
}

While nullable types (?T) are useful, excessive use can lead to null-checking chains and defensive programming. Consider using default values, the Option type pattern, or restructuring your code to minimize nullable types.

Not Using Shapes and Structural Typing

// Anti-pattern: Not using shapes for structured data
function processUserData(array<string, mixed> $userData): void {
  $name = $userData['name'] ?? '';
  $email = $userData['email'] ?? '';
  $age = $userData['age'] ?? 0;
  
  // Process data...
}

// Better approach: Use shapes
type UserShape = shape(
  'name' => string,
  'email' => string,
  ?'age' => int, // Optional field
);

function processUserData(UserShape $userData): void {
  $name = $userData['name'];
  $email = $userData['email'];
  $age = $userData['age'] ?? 0;
  
  // Process data...
}

Use Hack’s shape types to define the structure of your data instead of using untyped arrays. Shapes provide compile-time checking of field names and types, making your code more robust.

Ignoring Async

// Anti-pattern: Blocking I/O operations
function fetchUserData(int $userId): array<string, mixed> {
  $response = file_get_contents("https://api.example.com/users/{$userId}");
  return json_decode($response, true);
}

// Better approach: Use async/await
async function fetchUserDataAsync(int $userId): Awaitable<array<string, mixed>> {
  $response = await HttpClient::requestAsync(
    HttpMethod::GET,
    "https://api.example.com/users/{$userId}"
  );
  return json_decode($response->getBody(), true);
}

// Usage
async function processUsersAsync(): Awaitable<void> {
  $userIds = vec[1, 2, 3, 4, 5];
  $awaitables = $userIds->map(
    $id ==> fetchUserDataAsync($id)
  );
  
  // Process all requests concurrently
  $results = await AwaitAllWaitHandle::fromVec($awaitables);
  
  // Process results...
}

Take advantage of Hack’s async/await features for I/O-bound operations. This allows your code to handle multiple operations concurrently without blocking, improving performance and scalability.

Not Using XHP for HTML Generation

// Anti-pattern: String concatenation for HTML
function renderUserProfile(User $user): string {
  $html = '<div class="user-profile">';
  $html .= '<h1>' . htmlspecialchars($user->getName()) . '</h1>';
  $html .= '<p>Email: ' . htmlspecialchars($user->getEmail()) . '</p>';
  $html .= '</div>';
  return $html;
}

// Better approach: Use XHP
function renderUserProfile(User $user): XHPRoot {
  return 
    <div class="user-profile">
      <h1>{$user->getName()}</h1>
      <p>Email: {$user->getEmail()}</p>
    </div>;
}

Use XHP (XML in Hack) for generating HTML instead of string concatenation. XHP provides compile-time checking of HTML structure, automatic escaping of values, and a more readable syntax.

Not Using Type Aliases and Newtype

// Anti-pattern: Using primitive types directly
function createUser(string $email, string $password): User {
  // Validate email and password
  // Create user...
}

// Usage - easy to mix up parameters
$user = createUser($password, $email); // Oops, parameters swapped!

// Better approach: Use type aliases and newtype
newtype Email = string;
newtype Password = string;

function createUser(Email $email, Password $password): User {
  // Validate email and password
  // Create user...
}

// Usage - now type-safe
$email = Email($emailString);
$password = Password($passwordString);
$user = createUser($email, $password); // Compile error if swapped

Use type aliases and newtype to create more specific types for your domain concepts. This improves type safety, documentation, and prevents accidental misuse of values.

Not Using Enums

// Anti-pattern: Using string constants
const string STATUS_PENDING = 'pending';
const string STATUS_ACTIVE = 'active';
const string STATUS_SUSPENDED = 'suspended';

function updateUserStatus(User $user, string $status): void {
  // No compile-time checking of status values
  $user->setStatus($status);
}

// Better approach: Use enums
enum UserStatus: string {
  PENDING = 'pending';
  ACTIVE = 'active';
  SUSPENDED = 'suspended';
}

function updateUserStatus(User $user, UserStatus $status): void {
  $user->setStatus($status);
}

// Usage
updateUserStatus($user, UserStatus::ACTIVE);

Use enums to represent a fixed set of related values instead of string or integer constants. Enums provide type safety, better documentation, and prevent invalid values.

Not Using Generics

// Anti-pattern: Not using generics
class Repository {
  public function find(int $id): mixed {
    // Find entity by ID
    return $entity;
  }
  
  public function findAll(): array<mixed> {
    // Find all entities
    return $entities;
  }
}

// Usage requires casting
$user = $userRepository->find(1) as User;

// Better approach: Use generics
class Repository<T> {
  public function find(int $id): ?T {
    // Find entity by ID
    return $entity;
  }
  
  public function findAll(): vec<T> {
    // Find all entities
    return $entities;
  }
}

// Usage with type safety
$userRepository = new Repository<User>();
$user = $userRepository->find(1); // Returns ?User

Use generics to create reusable components that work with different types while maintaining type safety. This reduces the need for type casting and makes your code more robust.

Not Using Immutable Data Structures

// Anti-pattern: Mutable data structures
function processUserData(Map<string, mixed> $userData): void {
  // Modify the map in-place
  $userData['processed'] = true;
  $userData['timestamp'] = time();
  
  // More processing...
}

// Better approach: Use immutable data structures
function processUserData(ImmMap<string, mixed> $userData): ImmMap<string, mixed> {
  // Create a new map with modifications
  return $userData
    ->toMap() // Convert to mutable
    ->add('processed', true)
    ->add('timestamp', time())
    ->toImmMap(); // Convert back to immutable
}

Prefer immutable data structures (ImmVector, ImmMap, ImmSet) over mutable ones when appropriate. Immutable data structures prevent unexpected modifications and make your code easier to reason about, especially in concurrent contexts.

Not Using Traits for Code Reuse

// Anti-pattern: Duplicated code across classes
class UserController {
  public function validateInput(array<string, mixed> $input): bool {
    // Validation logic duplicated in multiple controllers
    return $isValid;
  }
}

class ProductController {
  public function validateInput(array<string, mixed> $input): bool {
    // Same validation logic duplicated
    return $isValid;
  }
}

// Better approach: Use traits for shared behavior
trait InputValidation {
  public function validateInput(array<string, mixed> $input): bool {
    // Common validation logic
    return $isValid;
  }
}

class UserController {
  use InputValidation;
  
  // User-specific methods...
}

class ProductController {
  use InputValidation;
  
  // Product-specific methods...
}

Use traits to share behavior across classes without using inheritance. Traits provide a way to reuse code in a more flexible way than class inheritance.

Not Using Proper Error Handling

// Anti-pattern: Poor error handling
function processFile(string $filename): array<string, mixed> {
  $content = file_get_contents($filename); // Might fail
  $data = json_decode($content, true); // Might fail
  return $data;
}

// Better approach: Use exceptions with try/catch
function processFile(string $filename): array<string, mixed> {
  try {
    $content = file_get_contents($filename);
    if ($content === false) {
      throw new Exception("Failed to read file: {$filename}");
    }
    
    $data = json_decode($content, true);
    if ($data === null) {
      throw new Exception("Failed to parse JSON from file: {$filename}");
    }
    
    return $data;
  } catch (Exception $e) {
    // Log the error
    error_log($e->getMessage());
    // Rethrow or return a default value
    throw $e;
  }
}

Implement proper error handling in your code. Use exceptions for exceptional conditions and handle them appropriately. Avoid silently ignoring errors or returning null/false without proper documentation.

Not Using Namespaces

// Anti-pattern: Not using namespaces
class User {}
class UserController {}
class UserRepository {}
class UserService {}

// Better approach: Use namespaces
namespace App\Domain\User;

class User {}

namespace App\Controller;

use App\Domain\User\User;

class UserController {}

namespace App\Repository;

use App\Domain\User\User;

class UserRepository {}

Organize your code into namespaces based on functionality or domain concepts. This prevents naming conflicts and makes your codebase more maintainable as it grows.

Not Using Dependency Injection

// Anti-pattern: Hard-coded dependencies
class UserService {
  private UserRepository $repository;
  
  public function __construct() {
    $this->repository = new UserRepository();
  }
  
  public function getUser(int $id): ?User {
    return $this->repository->find($id);
  }
}

// Better approach: Use dependency injection
class UserService {
  private UserRepository $repository;
  
  public function __construct(UserRepository $repository) {
    $this->repository = $repository;
  }
  
  public function getUser(int $id): ?User {
    return $this->repository->find($id);
  }
}

Use dependency injection to provide a class with its dependencies rather than having the class create them. This makes your code more testable, flexible, and loosely coupled.

Not Using Interfaces

// Anti-pattern: Direct dependency on concrete classes
class UserService {
  private MySQLUserRepository $repository;
  
  public function __construct(MySQLUserRepository $repository) {
    $this->repository = $repository;
  }
}

// Better approach: Depend on interfaces
interface UserRepositoryInterface {
  public function find(int $id): ?User;
  public function findAll(): vec<User>;
  public function save(User $user): void;
}

class MySQLUserRepository implements UserRepositoryInterface {
  // Implementation...
}

class MongoUserRepository implements UserRepositoryInterface {
  // Alternative implementation...
}

class UserService {
  private UserRepositoryInterface $repository;
  
  public function __construct(UserRepositoryInterface $repository) {
    $this->repository = $repository;
  }
}

Define interfaces for your components and depend on those interfaces rather than concrete implementations. This allows for easier testing and more flexible architecture.

Not Using Type Refinement

// Anti-pattern: Repeated type checking
function processValue(mixed $value): void {
  if (is_int($value)) {
    echo "Processing integer: {$value}";
    // More code using $value as int
    $result = $value * 2;
    // $value is still mixed here
  } else if (is_string($value)) {
    echo "Processing string: {$value}";
    // More code using $value as string
    $length = strlen($value);
    // $value is still mixed here
  }
}

// Better approach: Use type refinement
function processValue(mixed $value): void {
  if (is_int($value)) {
    // $value is refined to int in this block
    echo "Processing integer: {$value}";
    $result = $value * 2; // $value is known to be int
  } else if (is_string($value)) {
    // $value is refined to string in this block
    echo "Processing string: {$value}";
    $length = strlen($value); // $value is known to be string
  }
}

Take advantage of Hack’s type refinement feature, which narrows the type of a variable within conditional blocks based on type checks. This reduces the need for type assertions and makes your code safer.

Not Using Attributes

// Anti-pattern: Not using attributes for metadata
class UserController {
  public function getUser(int $id): User {
    // Method implementation
  }
  
  public function createUser(string $name, string $email): User {
    // Method implementation
  }
}

// Better approach: Use attributes
class UserController {
  <<Route("/users/:id", "GET")>>
  public function getUser(int $id): User {
    // Method implementation
  }
  
  <<Route("/users", "POST")>>
  <<RequiresAuth>>
  public function createUser(string $name, string $email): User {
    // Method implementation
  }
}

Use attributes (annotations) to add metadata to your classes and methods. Attributes can be used for routing, validation, authorization, and other cross-cutting concerns.

Not Writing Tests

// Anti-pattern: No tests

// Better approach: Write unit tests
<<TestCase>>
class UserServiceTest extends HackTest {
  <<Test>>
  public function testGetUserReturnsNullForInvalidId(): void {
    // Arrange
    $repository = new MockUserRepository();
    $repository->method('find')->willReturn(null);
    
    $service = new UserService($repository);
    
    // Act
    $result = $service->getUser(999);
    
    // Assert
    $this->assertNull($result);
  }
  
  <<Test>>
  public function testGetUserReturnsUserForValidId(): void {
    // Arrange
    $user = new User(1, 'Test User');
    $repository = new MockUserRepository();
    $repository->method('find')->willReturn($user);
    
    $service = new UserService($repository);
    
    // Act
    $result = $service->getUser(1);
    
    // Assert
    $this->assertNotNull($result);
    $this->assertEquals(1, $result?->getId());
    $this->assertEquals('Test User', $result?->getName());
  }
}

Write tests for your Hack code. Even with Hack’s strong type system, tests are still important for verifying business logic, edge cases, and integration between components.

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