Generics: Type Constraints

A Type Constraint is a restriction on the type of a type parameter. Type constraints are specified using the as keyword. For example, the following type is generic over a type parameter T, which must be a subtype of iterable<int, string>:

class Box<T as iterable<int, string>> {
    public function __construct(private readonly T $value) {}
}

The Box class can be used with any type that is a subtype of iterable<int, string>:

$box = new Box::<dict<int, string>>(dict[
    1 => "Hello",
    2 => "World",
]);

$box = new Box::<vec<string>>(vec[
    "Hello",
    "World",
]);

$box = new Box::<(string, string, string)>((
    "Hello",
    "World",
    "!",
));

`where` Method Type Constraints

The where keyword can be used to specify a type constraint on a method. For example, the following type is generic over a type parameter T, which must be a subtype of iterable<int, string>:

class Box<T> {
    public function __construct(private readonly T $value) {}

    public function get(): T where T as iterable<int, string> {
        return $this->value;
    }
}

The get() method can be used with any type that is a subtype of iterable<int, string>:

$box = new Box::<dict<int, string>>(dict[
    1 => "Hello",
    2 => "World",
]);

$box->get(); // dict[1 => "Hello", 2 => "World"]

$box = new Box::<vec<string>>(vec[
    "Hello",
    "World",
]);

$box->get(); // vec["Hello", "World"]

$box = new Box::<(string, string, string)>((
    "Hello",
    "World",
    "!",
));

Using get() with a type that is not a subtype of iterable<int, string> will result in a typechecker error:

$box = new Box::<int>(42);

$box->get(); // error: method type constraint violation, expected `iterable<int, string>`, got `int`

Generics: Type Constraints #

where Method Type Constraints #

Generics: Type Constraints

`where` Method Type Constraints