How to access optional shape field in hack?

Question

To give an example, let's assume I have a type foo of the following shape,

type foo = shape(
  ?'bar' => float,
  ...
);

Now if I try to access value of field bar in the following way,

do_something_with($rcvd['bar']);

where $rcvd is of foo type, it doesn't work as bar is an optional member and might not exist for the instance $rcvd. So for this given example, the question would be - how to access the member bar of $rcvd?

Answer 1

Ok, found it: https://docs.hhvm.com/hack/reference/class/HH.Shapes/idx/

So the correct way is,

Shapes::idx($rcvd, 'bar');

Answer 2

You can use the Shapes::idx method as you mentioned in your answer.

but you can also use null coalesce operator, which you might be more familiar with from other programming languages such as PHP and C#.

see : https://docs.hhvm.com/hack/expressions-and-operators/coalesce

example :

type Foo = shape(
  'bar' => ?int,
  ?'baz' => int,
);

function qux(Foo $foo): int {
  $default = 1;
  // foo[bar] is null ? set default.
  $bar = $foo['bar'] ?? $default;
  // foo[baz] doesn't exist ? set default.
  $baz = $foo['baz'] ?? $default;
  return $baz + $bar;
}

<<__EntryPoint>>
async function main(): Awaitable<noreturn> {
  echo qux(shape(
    'bar' => null,
  )); // 2
  echo qux(shape(
    'bar' => 0,
  )); // 1
  echo qux(shape(
    'bar' => 1,
  )); // 2
  echo qux(shape(
    'bar' => 4,
    'baz' => 2,
  )); // 6
  echo qux(shape(
    'bar' => null,
    'baz' => 0
  )); // 1
}

Hack also support null coalesce equal operator.

example :

// foo[bar] is null ? set foo[bar] to default
$foo['bar'] ??= $default;

// foo[baz] is missing ? set foo[baz] to default
$foo['baz'] ??= $default;