ATK4.3 default form submission handler syntax

gistfile1.php

$form->onSubmit(function($f){
  $errors=[];
  if($f['name'] != 'Joe') $errors[] = $f->error('You must be Joe');
  if($f['surname'] != 'Blogs') $errors[] = $f->error('You must be Blogs');
  
  if($errors) return $errors;
  
  $f->update();
  
  return 'Success!';
});