Readonly: 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

• Syntax: Basic syntax for readonly keyword
• Subtyping: Rules and semantics for interacting with readonly and mutable values
• Explicit Readonly Keywords: Positions where the readonly keyword is explicitly required
• Containers and Collections: Interactions between collections of readonly values
• Advanced Features and Semantics: More complex features and interactions