Change the value of a lexically scoped variable in a HHVM/Hack lambda expression?

Question

Is it possible to change the value of a lexically scoped variable in a Hack lambda expression?

function allTrue(Map<string, bool> $map): bool {
    $valid = 1;
    $map->map($a ==> $valid &= $a);
    return $valid === 1;
}

$map = Map{'foo' => true, 'bar' => false };
var_dump(allTrue($map)); // true

I would expect the return value to be false

Answer 1

Unfortunately Hack lambda expressions do not support this. However you can still fall back on PHP 5.3 closure syntax.

function allTrue(Map<string, bool> $map): bool {
    $valid = 1;
    $map->map(function ($a) use (&$valid) {
        $valid &= $a;
    });
    return $valid === 1;
}

$map = Map{'foo' => true, 'bar' => false };
var_dump(allTrue($map)); // false 

This works because: Objects of type Closure created by lambda expression syntax vs. PHP 5.3's closure syntax are interchangeable;

From the docs

Note:
There are currently some limitations to lambda expressions that are being worked on or considered:

Lambda expressions don't support capturing variables by reference. If the programmer wants to capture variables by reference, they must use PHP 5.3 closure syntax and put "&" in front of the variable in the "use(..)" list.

Lambda expressions don't support returning by reference at present, but support could be added in the future.

Answer 2

As of 2019 use can also use Ref to work around this.

function allTrue(Map<string, bool> $map): bool {
  $valid = Ref(1);
  $map->map($a ==> $valid->set($a));
  return $valid->get() === 1;
}

$map = Map{'foo' => true, 'bar' => false };
var_dump(allTrue($map));

See: https://hhvm.com/blog/2019/10/01/deprecating-references.html