CI4 Validation - how use/work with calleble functions

Hi everyone, I've read CI4-manual for validation library and not found example code how use/work with calleble or anonymous functions? When I use example code from CI3 - this code don't works     . May be anybody know how use calleble/ anonymous functions validation in CI4-version. Sorry for my English.    

My exaple rule for email code:

PHP Code:

'email' => [

                    'label'  => 'Email',
                    'rules'  => [

                        'required',
                        'valid_email',
                        [
                            'email_exist', function( $str )
                            {
                                return ! $this->checkExistsUserEmail( $str );
                            }
                        ]
                    ],
                    'errors' => [

                        'email_exist' =>'Email not found'
                    ]
                ]