Hack Type casting - No error emited

Question

I'm learning about the Hack langage and I wanted to create a specific type like this one :

    type Points = array<array<int, int>>;

And then I defined that function :

    function printPoint(Points $point){
        var_dump($point[0], $point[1]);
    }

When I pass this array as parameter : [0, 0], there no error emited and the output is :

    int(0)
    int(0)

And when I pass this one : [[0, 0], [0, 0]], and the output is like it should :

    array(2) {
        [0]=>
            int(0)
        [1]=>
            int(0)
    }

    array(2) {
        [0]=>
            int(0)
        [1]=>
            int(0)
    }

My problem here is : why does this work ? (I know it's not the usual question x) ). Why is there not a notice or even a fatal type error in the first test ?

Thanks ;)

Answer 1

At run time, generics are erased. This means that the runtime only knows that Points is an array, not what type of array.

Instead, you want to use the type checker -- hh_client -- for this. With a file like:

<?hh

type Points = array<array<int, int>>;

function printPoint(Points $point) {
  var_dump($point[0], $point[1]);
}

function test(): void {
  printPoint([0, 0]);
}

It gives the error you expect:

test.php:10:14,14: Invalid argument (Typing[4110])
  test.php:3:21,25: This is an array (used like a hashtable)
  test.php:10:15,15: It is incompatible with an int