in()

in() allows only values that match the supplied list using strict comparison.

use Fynix\Rule;

$status = Rule::string('status')
    ->in(['draft', 'published']);

An unlisted value produces the value.not_allowed error code.

The comparison is strict, so numeric and string representations are different:

$priority = Rule::integer('priority')->in([1, 2, 3]);

$priority->validate(2);   // null
$priority->validate('2'); // type or allow-list error

in() is useful for statuses, account types, enum-like request fields, and bounded numeric choices. It returns a new immutable rule.