Attributes: Introduction

Attributes attach metadata to Hack definitions.

Hack provides built-in attributes that can change runtime or typechecker behavior.

function add_one(int $x): int {
  return $x + 1;

You can attach multiple attributes to a definition, and attributes can have arguments.

class OtherClass {
  <<__Memoize, __Deprecated("Use FooClass methods instead")>>
  public function addOne(int $x): int {
    return $x + 1;

Defining an attribute

You can define your own attribute by implementing an attribute interface in the HH namespace.

class Maintainers implements HH\ClassAttribute {
  public function __construct(string ...$args) {}

<<Maintainers("Team X", "Team Y")>>
class MyClass {}

Other common attribute interfaces are HH\FunctionAttribute, HH\MethodAttribute and HH\PropertyAttribute, but see the Hack interface reference for the full list.

Accessing attribute arguments

You need to use reflection to access attribute arguments.

Given the MyClass example defined above:

$rc = new ReflectionClass('MyClass');
$rc->getAttribute('Maintainers') // ["Team X", "Team Y"]