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Introduction

readonly is a keyword used to create immutable references to Objects and their properties.

How does it work?​

Expressions in Hack can be annotated with the readonly keyword. When an object or reference is readonly, there are two main constraints on it:

  • Readonlyness: Object properties cannot be modified (i.e. mutated).
  • Deepness: All nested properties of a readonly value are readonly.

Readonlyness​

Object properties of readonly values cannot be modified (i.e. mutated).

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

function test(readonly Foo $x) : void {
  $x->prop = 4; // error, $x is readonly, its properties cannot be modified
}

Deepness​

All nested properties of readonly objects are readonly.

function test(readonly Bar $x) : void {
  $foo = $x->foo;
  $foo->prop = 3; // error, $foo is readonly
}

How is readonly different from contexts and capabilities that control property mutation (such as write_props)?​

Contexts such as write_props affect an entire function (and all of its callees), whereas readonly affects specific values / expressions.

Topics covered in this section​