Using 'mixed' type is Hack Language Facebook

In certain situations, it is difficult to determine the return type of a function prior to its call. In that situation we use the ‘mixed’ type.
Example:

<?hh
function add(mixed $val): mixed {
  if (is_array($val) || $val instanceof Vector) {
    $sum = 0;
    foreach ($val as $val) {
      $sum += $val;
    }
    return $sum;
  }
  return $val;
}