Type Annotation in Hack Language Facebook

Basic/Primitive Types:

 int, float, string, bool, array.

Other Types:

 mixed, void, resource, array, Vector, tuple (type1, type2, . . .)

Example :

<?hh
class ClassName {
    public int intVal = 10;  

                 const int constVal = 0;
                 private string $str = '';
                 public function add(int $x): int {
                 $ret = $x + $intVal;
                 return $ret;
    }
}