---
language: Hack
contributors:
    - ["Stephen Holdaway", "https://github.com/stecman"]
filename: learnhack.hh
---

Hack is a superset of PHP that runs under a virtual machine called HHVM. Hack
is almost completely interoperable with existing PHP code and adds a bunch of
useful features from statically typed languages.


Only Hack-specific features are covered here. Details about PHP's syntax are
available in the [PHP article](php.html.markdown).

```php
<?hh

// Hack syntax is only enabled for files starting with an <?hh marker
// <?hh markers cannot be interspersed with HTML the way <?php can be.
// Using the marker "<?hh //strict" puts the type checker in strict mode.


// Scalar parameter type hints
function repeat(string $word, int $count)
{
    $word = trim($word);
    return str_repeat($word . ' ', $count);
}

// Type hints for return values
function add(...$numbers) : int
{
    return array_sum($numbers);
}

// Functions that return nothing are hinted as "void"
function truncate(resource $handle) : void
{
    // ...
}

// Type hints must explicitly allow being nullable
function identity(?string $stringOrNull) : ?string
{
    return $stringOrNull;
}

// Type hints can be specified on class properties
class TypeHintedProperties
{
    public ?string $name;
    
    protected int $id;

    private float $score = 100.0;

    // Hack's type checker enforces that typed properties either have a
    // default value or are set in the constructor.
    public function __construct(int $id)
    {
        $this->id = $id;
    }
}


// Concise anonymous functions (lambdas)
$multiplier = 5;
array_map($y ==> $y * $multiplier, [1, 2, 3]);


// Generics
class Box<T>
{
    protected T $data;

    public function __construct(T $data) {
        $this->data = $data;
    }

    public function getData(): T {
        return $this->data;
    }
}

function openBox(Box<int> $box) : int
{
    return $box->getData();
}


// Shapes
// 
// Hack adds the concept of shapes for defining struct-like arrays with a
// guaranteed, type-checked set of keys
type Point2D = shape('x' => int, 'y' => int);

function distance(Point2D $a, Point2D $b) : float
{
    return sqrt(pow($b['x'] - $a['x'], 2) + pow($b['y'] - $a['y'], 2));
}

distance(
    shape('x' => -1, 'y' => 5),
    shape('x' => 2, 'y' => 50)
);


// Type aliasing
// 
// Hack adds a bunch of type aliasing features for making complex types readable
newtype VectorArray = array<int, Vector<int>>;

// A tuple containing two integers
newtype Point = (int, int);

function addPoints(Point $p1, Point $p2) : Point
{
    return tuple($p1[0] + $p2[0], $p1[1] + $p2[1]);
}

addPoints(
    tuple(1, 2),
    tuple(5, 6)
);


// First-class enums
enum RoadType : int
{
    Road = 0;
    Street = 1;
    Avenue = 2;
    Boulevard = 3;
}

function getRoadType() : RoadType
{
    return RoadType::Avenue;
}


// Constructor argument promotion
// 
// To avoid boilerplate property and constructor definitions that only set
// properties, Hack adds a concise syntax for defining properties and a
// constructor at the same time.
class ArgumentPromotion
{
    public function __construct(public string $name,
                                protected int $age,
                                private bool $isAwesome) {}
}

class WithoutArugmentPromotion
{
    public string $name;

    protected int $age;

    private bool $isAwesome;

    public function __construct(string $name, int $age, bool $isAwesome)
    {
        $this->name = $name;
        $this->age = $age;
        $this->isAwesome = $isAwesome;
    }
}


// Co-oprerative multi-tasking
// 
// Two new keywords "async" and "await" can be used to perform mutli-tasking
// Note that this does not involve threads - it just allows transfer of control
async function cooperativePrint(int $start, int $end) : Awaitable<void>
{
    for ($i = $start; $i <= $end; $i++) { 
        echo "$i ";

        // Give other tasks a chance to do something
        await RescheduleWaitHandle::create(RescheduleWaitHandle::QUEUE_DEFAULT, 0);
    }
}

// This prints "1 4 7 2 5 8 3 6 9"
AwaitAllWaitHandle::fromArray([
    cooperativePrint(1, 3),
    cooperativePrint(4, 6),
    cooperativePrint(7, 9)
])->getWaitHandle()->join();


// Attributes
// 
// Attributes are a form of metadata for functions. Hack provides some
// special built-in attributes that introduce useful behaviour. 

// The __Memoize special attribute causes the result of a function to be cached
<<__Memoize>>
function doExpensiveTask() : ?string
{
    return file_get_contents('http://example.com');
}

// The function's body is only executed once here:
doExpensiveTask();
doExpensiveTask();


// The __ConsistentConstruct special attribute signals the Hack type checker to
// ensure that the signature of __construct is the same for all subclasses.
<<__ConsistentConstruct>>
class ConsistentFoo
{
    public function __construct(int $x, float $y)
    {
        // ...
    }

    public function someMethod()
    {
        // ...
    }
}

class ConsistentBar extends ConsistentFoo
{
    public function __construct(int $x, float $y)
    {
        // Hack's type checker enforces that parent constructors are called
        parent::__construct($x, $y);

        // ...
    }

    // The __Override annotation is an optional signal for the Hack type
    // checker to enforce that this method is overriding a method in a parent
    // or trait. If not, this will error.
    <<__Override>>
    public function someMethod()
    {
        // ...
    }
}

class InvalidFooSubclass extends ConsistentFoo
{
    // Not matching the parent constructor will cause a type checker error:
    //  
    //  "This object is of type ConsistentBaz. It is incompatible with this object
    //   of type ConsistentFoo because some of their methods are incompatible"
    //
    public function __construct(float $x)
    {
        // ...
    }

    // Using the __Override annotation on a non-overriden method will cause a
    // type checker error:
    //  
    //  "InvalidFooSubclass::otherMethod() is marked as override; no non-private
    //   parent definition found or overridden parent is defined in non-<?hh code"
    //
    <<__Override>>
    public function otherMethod()
    {
        // ...
    }
}


// Traits can implement interfaces (standard PHP does not support this)
interface KittenInterface
{
    public function play() : void;
}

trait CatTrait implements KittenInterface
{
    public function play() : void
    {
        // ...
    }
}

class Samuel
{
    use CatTrait;
}


$cat = new Samuel();
$cat instanceof KittenInterface === true; // True

```

## More Information

Visit the [Hack language reference](http://docs.hhvm.com/manual/en/hacklangref.php)
for detailed explainations of the features Hack adds to PHP, or the [official Hack website](http://hacklang.org/)
for more general information.

Visit the [official HHVM website](http://hhvm.com/) for HHVM installation instructions.

Visit [Hack's unsupported PHP features article](http://docs.hhvm.com/manual/en/hack.unsupported.php).]
for details on the backwards incompatibility between Hack and PHP.