Expressions And Operators: Type Assertions

Hack provides the is and as operators for inspecting types at runtime. To convert primitive types, see casting.

The type checker also understands is and as, so it will infer precise types.

Checking Types with is

The is operator checks whether a value has the type specified, and returns a boolean result.

1 is int;        // true
'foo' is int;    // false

1 is num;        // true
1.5 is num;      // true
'foo' is num;    // false

The type checker understands is and refines values inside conditionals or after invariant calls.

function transport(Vehicle $v): void {
  if ($v is Car) {
  } else if ($v is Plane) {
  } else {
    invariant($v is Boat, "Expected a boat");

A common pattern with is refinement is to use nonnull rather than an explicit type.

function transport(?Car $c): void {
  if ($c is nonnull) {
    // Infers that $c is Car, but saves us
    // repeating the name of the type.


For enums, is also checks that the value is valid.

enum MyEnum: int {
  FOO = 1;

function demo(): void {
  1 is MyEnum;       // true
  42 is MyEnum;      // false
  'foo' is MyEnum;   // false


Since is provides a runtime check, it cannot be used with erased generics. For generic types, you must use _ placeholders for type parameters.

$v = vec[1, 2, 3];

// We can't use `is vec<int>` here.
$v is vec<_>; // true

If you need to check inner types at runtime, consider using reified generics instead.


For tuples and shapes, is validates the size and recursively validates every field in the value.

$x = tuple(1, 2.0, null);
$x is (int, float, ?bool); // true

$y = shape('foo' => 1);
$y is shape('foo' => int); // true


is also works with type aliases and type constants, by testing against the underlying runtime type.

type myint = int;

function demo(): void {
  1 is myint; // true

Enforcing Types with as and ?as

as performs the same checks as is.

However, it throws TypeAssertionException if the value has a different type. The type checker understands that the value must have the type specified afterwards, so it refines the value.

1 as int;        // 1
'foo' as int;    // TypeAssertionException

as enables you to narrow a type.

// Normally you'd want to make transport take a Vehicle
// directly, so you can check when you call the function.
function transport(mixed $m): void {
  // Exception if not a Vehicle.
  $v = $m as Vehicle;

  if ($v is Car) {
  } else {
    // Exception if $v is not a Boat.
    $v as Boat;

Hack also provides ?as, which returns null if the type does not match.

1 ?as int;        // 1
'foo' ?as int;    // null

Note that as can also be used in type signatures when using generics.

Legacy Type Predicates

Hack also provides type predicate functions is_int, is_bool and so on. You should use is instead.

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