Reified Generics

Reified Generics

A Reified Generic is a Generic with type information accessible at runtime.


Generics are currently implemented in HHVM through erasure, in which the runtime drops all information about generics. This means that generics are not available at runtime. Although the typechecker is able to use the generic types for static typechecking, we are unable to enforce generic types at runtime.

In Hack, generics are opt-in. The goal of opt-in reified generics is to bridge the gap between generics and runtime availability while keeping erasure available to maintain performance when reification is not needed. To mark a generic as reified, simply add the reify keyword at the declaration site.

Parameter and return type verification

class C<reify T> {}

function f(C<int> $c): void {}

function main(): void {
  $c1 = new C<int>();
  f($c1); // success
  $c2 = new C<string>();
  f($c2); // parameter type hint violation

The reified type parameter is checked as well:

class C<reify T> {}

function f<reify T>(T $x): C<T> {
  return new C<int>();

function main(): void {
  f<int>(1); // success
  f<int>("yep"); // parameter type hint violation
  f<string>("yep"); // return type hint violation

Type testing and assertion with is and as expressions

Suppose you have a vec<mixed> and you want to extract all types T from it. Prior to reified generics, you'd need to implement a new function for each type T but with reified generics you can do this in a generic way. Start by adding the keyword reify to the type parameter list.

function filter<<<__Enforceable>> reify T>(vec<mixed> $list): vec<T> {
  $ret = vec[];
  foreach ($list as $elem) {
    if ($elem is T) {
      $ret[] = $elem;
  return $ret;

function main(): void {
  filter<int>(vec[1, "hi", true]);
  // => vec[1]
  filter<string>(vec[1, "hi", true]);
  // => vec["hi"]

Notice that the reified type parameter has the attribute <<__Enforceable>>. In order to use type testing and type assertion, the reified type parameter must be marked as <<__Enforceable>>, which means that we can fully enforce this type parameter, i.e. it does not contain any erased generics, not a function type, etc.

class A {}
class B<reify T> {}
class C<reify Ta, Tb> {}

int // enforceable
A // enforceable
B<int> // enforceable
B<A> // enforceable
C<int, string> // NOT enforceable as C's second generic is erased

Creating an instance of a class with new

Prior to reified generics, in order to create a new instance of a class without a constant class name, you'd need to pass it as classname<T> which is not type safe. In the runtime, classnames are strings.

abstract class A {}

class B extends A {}
class C extends A {}

function f<<<__Newable>> reify T as A>(): T {
  return new T();

function main(): void {
  f<A>(); // not newable since it is abstract class
  f<B>(); // success
  f<C>(); // success

Notice that the reified type parameter has the attribute <<__Newable>>. In order for a type to be <<__Newable>>, the type must represent a class that's not abstract and has a consistent constructor or be a final class. Creating a new instance using the reified generics also carries across the generics given. For example,

final class A<reify T> {}

function f<<<__Newable>> reify T as A<string>>(): A<string> {
  return new T();

function demo(): void {
  // creates a new A<int> and since f's return type is A<string>,
  // this raises a type hint violation

Accessing a class constant / static class method

class C {
  const string class_const = "hi";
  public static function h<reify T>(): void {}

// Without reified generics
function f<T as C>(classname<T> $x): void {

// With reified generics
function g<reify T as C>(): void {

Accessing static class properties (T::$class_property) is currently not supported.

Hack Arrays

Hack Arrays can be used as inner type for classes since we do not need to check whether each element of the vec is string. Look at the limitations section for more information on when Hack Arrays cannot be used.

class Box<reify T> {}

function foo(): Box<vec<string>> {
  return new Box<vec<int>>(); // Type hint violation


  • No support for subtyping (reified type parameter on classes are invariant, they cannot be co/contra-variant
class C<reify +Ta> {} // Cannot make the generic covariant
  • Static class methods cannot use the reified type parameter of its class
class C<reify T> {
  public static function f(): void {
    return new T(); // Cannot use T
  • Hack arrays cannot be reified when used as containers with __Enforceable or __Newable
function f<<<__Enforceable>> reify T>(T $x): void {
  $x is vec<int>; // Cannot use vec<int>
  $x is T;

function demo(): void {
  f<vec<int>>(); // not enforceable

but can be used as inner type for classes

  • If a type parameter is reified, it must be provided at all call sites; it cannot be inferred.
function f<reify T>(): void {}

function foo(): void {
  f(); // need to provide the generics here

Migration Features

In order to make migrating to reified generics easier, we have added some Migration Features.

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