features of Hack Language

• Type Annotations allow for PHP code to be explicitly typed on parameters, class member variables and return values:

  <?hh
  class MyClass {
    const int MyConst = 0;
    private string $x = '';
    public function increment(int $x): int {
      $y = $x + 1;
      return $y;
    }
  }

• Generics allow classes and methods to be parameterized (i.e., a type associated when a class is instantiated or a method is called) in the same vein as statically type languages like C# and Java):

  <?hh
  class Box<T> {
    protected T $data;
  
    public function __construct(T $data) {
      $this->data = $data;
    }
  
    public function getData(): T {
      return $this->data;
    }
  }

• Nullable Types are supported by Hack through use of the ? operator. This introduces a safer way to deal with nulls and is very useful for primitive types that don’t generally allow null as one of their values, such as bool and int (using ?bool and ?int respectively). The operator can be used on any type or class.
• Collections enhance the experience of working with PHP arrays, by providing first class, built-in parameterized types such as Vector (an ordered, index-based list), Map (an ordered dictionary), Set (a list of unique values), and Pair (an index-based collection of exactly two elements).
• Lambdas offer similar functionality to PHP closures, but they capture variables from the enclosing function body implicitly and are less verbose:

  <?hh
  function foo(): (function(string): string) {
    $x = 'bar';
    return $y ==> $x . $y;
  }
  function test(): void {
    $fn = foo();
    echo $fn('baz'); // barbaz
  }

Other significant features of Hack include Shapes, Type Aliasing, Async support, and much more.

The JIT Compiler

Rather than directly interpret or compile PHP code directly to C++, HHVM compiles Hack and PHP into an intermediate bytecode. This bytecode is then translated into x64 machine code dynamically at runtime by a just-in-time (JIT) compiler. This compilation process allows for all sorts of optimizations that cannot be made in a statically compiled binary, thus enabling higher performance of your Hack and PHP programs.

HHVM

HHVM is an open-source virtual machine designed for executing programs written in Hack and PHP. HHVM uses a just-in-time (JIT) compilation approach to achieve superior performance while maintaining the development flexibility that PHP provides.

HHVM runs much of the world’s existing PHP. Developers and hosts are adopting HHVM. We are aware of minor incompatibilities (please open issues when you find them), but we can run the top 20 Github PHP frameworks out of the box. The HHVM team, along with many wonderful community members, has made it a stated, high priority goal to run all existing PHP code existing out in the wild.

HHVM Features

• The Hack Language
• JIT Compilation
• HNI
• FastCGI support
• Increasing PHP5 Parity
• hphpd debugger
• …. and many more