Built In Types: Nothing

The type nothing is the bottom type in the Hack typesystem. This means that there is no way to create a value of the type nothing. nothing only exists in the typesystem, not in the runtime.

The concept of a bottom type is quite difficult to grasp, so I'll first compare it to the supertype of everything mixed. mixed is the most general thing you can imagine within the hack typesystem. Everything "extends" mixed if you will. nothing is the exact opposite of that.

Let's work out the hierarchy of scalar types. Forget about nullable types and dynamic for the moment, they would make this example far more complex without adding much value.

  • mixed is at the top. Everything is a subtype of mixed, either directly (types that have no other supertypes) or indirectly (via their supertypes).
  • num is a subtype of mixed.
  • arraykey is a subtype of mixed.
  • bool is a subtype of mixed.
  • int is a subtype of num and arraykey.
  • float is a subtype of num.
  • string is a subtype of arraykey.
  • nothing is a subtype of int, float, string, and bool.

The important thing to note here is that nothing is never between two types. nothing only shows up right below a type with no other subtypes.


When defining a function that will never return (it either throws, loops forever, or terminates the request) you can use nothing for the return type. This gives more information to the caller than void and is more flexible than noreturn. nothing can be used in expressions (like nullable T ?? nothing) and it will typecheck "as if it wasn't there", since (T | nothing) is just T.

nothing can be used to create a throw expression in this way.

function throw_as_an_expression(\Throwable $t): nothing {
  throw $t;

function returns_an_int(?int $nullable_int): int {
  // You can not use a `throw` statement in an expression bodied lambda.
  // You need to add curly braces to allow a `throw` statement.
  $throwing_lambda = () ==> {
    throw new \Exception();

  $throwing_expr_lambda = () ==> throw_as_an_expression(new \Exception());

  // You can't write a statement on the RHS of an operator, because it operates on expressions.
  // The type of the `??` operator is `(nothing | int)`, which simplifies to `int`,
  // so this return statement is valid.
  return $nullable_int ?? throw_as_an_expression(new \Exception());

async function main_async(): Awaitable<void> {
  echo returns_an_int(1);

When writing a new bit of functionality, you may need to pass a value to a function you can't produce without a lot of work. nothing can be used as a placeholder value in place of any type without causing type errors. The typechecker will continue checking the rest of your program and the runtime will throw if this code path gets executed. I have called this function undefined, as an homage to Haskell undefined.

type undefined = nothing;

function undefined(): undefined {
  throw new Exception('NOT IMPLEMENTED: `undefined` cannot be produced.');

And here is how to use it

interface MyInterface {
  public function isAmazed(): bool;

function do_something(MyInterface $my_interface): bool {
  return $my_interface->isAmazed();

async function main_async(): Awaitable<void> {
  $my_interface = undefined();
  // We won't ever reach this line, since `undefined()` will halt the program by throwing.
  // We can't produce a MyInterface just yet, since there are no classes which implement it.
  // `undefined` is a placeholder for now.
  // We can continue writing our business logic and come back to this later.
  if (do_something($my_interface)) {
    // Write the body first, worry about the condition later.

You could make your staging environment remove the file which declared the undefined() function. That way you'll get a typechecker error when you accidentally push code that has these placeholders in it. This prevents you from accidentally deploying unfinished code to production.

When making a new / empty Container<T>, Hack will infer its type to be Container<nothing>. It is not that there are actual value of type nothing in the Container<T>, it is just that this is a very nice way of modeling empty Container<T>s.

Should you be able to pass an empty vec where a vec<string> is expected? Yes, there is no element inside that is not a string, so that should be fine. You can even pass the same vec into a function that takes a vec<bool> since there are no elements that are not of type bool. What are you allowed to do with the $nothing of this foreach? Well, you can do anything to it. Since nothing is a subtype of everything, you can pass it to any method and do all the things you want to.

function takes_vec_of_strings(vec<string> $_): void {}
function takes_vec_of_bools(vec<bool> $_): void {}

async function main_async(): Awaitable<void> {
  $empty_vec = vec[];

  foreach ($empty_vec as $nothing) {

To make an interface that requires that you implement a method, without saying anything about its types. This does still make a requirement about the amount of parameters that are required parameters.

interface DontForgetToImplementShipIt {
  public function shipIt(nothing $_): mixed;

abstract class Software implements DontForgetToImplementShipIt {

class HHVM extends Software {
  public function shipIt(string $version): string {
    return 'Shipping HHVM version '.$version.'!';

class HSL extends Software {
  private function __construct(public bool $has_new_functions) {

  public function shipIt(bool $has_new_functions): HSL {
    return new HSL($has_new_functions);

class HHAST extends Software {
  public function shipIt(Container<string> $linters): void {
    foreach ($linters as $linter) {
        Str\ends_with($linter, 'Linter'),
        'Linter %s does not have a name that ends in "Linter"!',

It is important to note that Software::shipIt() is not directly callable without knowing what subtype of Software you have.

Contravariant generic types can use nothing to allow all values to be passed. This acts in a similar way that mixed acts of covariant generics, such as vec<mixed>.

final class MyClass<-T> {
  public function consume(T $value): void {}
  public function someOtherMethod(): void {}

// We don't use the `T` from `->consume(T): void` in the function,
// so we can use `nothing` for the generic and accept any and all MyClass instances.
function some_function(MyClass<nothing> $a, MyClass<nothing> $b): void {
  if ($a !== $b) {
    echo "different\n";

async function main_async(): Awaitable<void> {

  $my_class_int = new MyClass<int>();
  $my_class_string = new MyClass<string>();

  some_function($my_class_int, $my_class_string);
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