Built In Types: Null

The null type has only one possible value, the value null.

You can use the null type when refining with is.

function number_or_default(?int $x): int {
  if ($x is null) {
    return 42;
  } else {
    return $x;

See nullable types for a discussion of ?T types.

The null type is also useful when writing generics. Suppose you want to define a generic interface with a 1-argument function, but some instances don't need an argument.

// A toy interface that allows you to iterate over something,
// setting a start point.
interface TraverseFrom<Tv, Ti> {
  public function startAt(Ti $_): Traversable<Tv>;

You can use null to define a class implementing this interface, making it clear that you don't care about the argument to startAt.

class TraverseIntsFromStart implements TraverseFrom<int, null> {
  public function __construct(private vec<int> $items) {}

  public function startAt(null $_): Traversable<int> {
    return $this->items;
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