[hack/en] Add english Hack language page

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+---
+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.
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