Introduction

Attributes attach metadata to Hack definitions.

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

<<__Memoize>>
function add_one(int $x): int {
  return $x + 1;
}

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

<<__ConsistentConstruct>>
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 Contributors implements HH\ClassAttribute {
  public function __construct(private string $author, private ?keyset<string> $maintainers = null) {}
  public function getAuthor(): string {
    return $this->author;
  }
  public function getMaintainers(): keyset<string> {
    return $this->maintainers ?? keyset[$this->author];
  }
}

<<Contributors("John Doe", keyset["ORM Team", "Core Library Team"])>>
class MyClass {}

<<Contributors("You")>>
class YourClass {}

The full set of attribute interfaces is shown below. Each interface restricts where the attribute may be attached.

Interface	Applies to
HH\ClassAttribute	a class
HH\ClassConstantAttribute	a constant of a class
HH\EnumAttribute	an enum
HH\EnumClassAttribute	an enum class
HH\TypeAliasAttribute	a type, newtype or case type
HH\FunctionAttribute	a function
HH\MethodAttribute	a method
HH\InstancePropertyAttribute	an instance property
HH\StaticPropertyAttribute	a static property
HH\ParameterAttribute	a parameter
HH\TypeParameterAttribute	a type parameter
HH\TypeConstantAttribute	a type constant
HH\FileAttribute	a file
HH\LambdaAttribute	a lambda expression
HH\ModuleAttribute	a module

See the Hack interface reference for the full signature of each interface.

Accessing attribute arguments

You need to use reflection to access attribute arguments.

Given the MyClass example defined above:

$rc = new ReflectionClass('MyClass');
$my_class_contributors = $rc->getAttributeClass(Contributors::class);
$my_class_contributors?->getAuthor(); // "John Doe"
$my_class_contributors?->getMaintainers(); // keyset["ORM Team", "Core Library Team"]