Readonly: Syntax

The readonly keyword can be applied to various positions in Hack.

Parameters and return values

Parameters and return values of any callable (e.g. a function or method) can be marked readonly.

class Bar {
  public function __construct(
    public Foo $foo,
class Foo {
  public function __construct(
    public int $prop,
  ) {}

function getFoo(readonly Bar $x): readonly Foo {
  return $x->foo;

A readonly parameter signals that the function/method will not modify that parameter when called. A readonly return type signals that the function returns a readonly reference to an object that cannot be modified.

Static and regular properties

Static and regular properties marked as readonly cannot be modified.

class Bar {}
class Foo {
  private static readonly ?Bar $static_bar = null;
  public function __construct(
    private readonly Bar $bar,

A readonly property represents a property that holds a readonly reference (specifically, that the nested object within the property cannot be modified).

Lambdas and function type signatures

readonly is allowed on inner parameters and return types on function typehints.

class Bar {}
function call(
    (function(readonly Bar) : readonly Bar) $f,
    readonly Bar $arg,
   ) : readonly Bar {
   return readonly $f($arg);


readonly can appear on expressions to convert mutable values to readonly.

class Foo {}
function foo(): void {
  $x = new Foo();
  $y = readonly $x;

Functions / Methods

readonly can appear as a modifier on instance methods, signaling that $this is readonly (i.e, that the method promises not to modify the instance).

class C {
  public function __construct(public int $prop) {}
  public readonly function foo() : void {
    $this->prop = 4; // error, $this is readonly.

Note that readonly objects can only call readonly methods, since they promise not to modify the object.

class Data {}
class Box {
  public function __construct(public Data $data) {}
  public readonly function getData(): readonly Data {
    return $this->data;
  public function setData(Data $d) : void {
    $this->data = $d;
function readonly_method_example(readonly Box $b): void {
  $y = readonly $b->getData(); // ok, $y is readonly
  $b->setData(new Data()); // error, $b is readonly, it can only call readonly methods

Closures and function types

A function type can be marked readonly: (readonly function(T1): T). Denoting a function/closure as readonly adds the restriction that the function/closure captures all values as readonly:

function readonly_closure_example(): void {
  $x = new Foo();
  $f = readonly () ==> {
    $x->prop = 4; // error, $x is readonly here!

One way to make sense of this behavior is to think of closures as an object whose properties are the values it captures, which implement a special invocation function that executes the closure. A readonly closure is then defined as a closure whose invocation function is annotated with readonly.

Readonly closures affect Hack’s type system; readonly closures are subtypes of their mutable counterparts. That is, a (readonly function(T1):T2) is a strict subtype of a (function(T1): T2).

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