Nullable Types

A type ?Foo is either a value of type Foo, or null.

function takes_nullable_str(?string $s): string {
  if ($s is null){
    return "default";
  } else {
    return $s;
  }
}

nonnull is any value except null. You can use it to check if a value is not null:

function takes_nullable_str2(?string $s): string {
  if ($s is nonnull){
    return $s;
  } else {
    return "default";
  }
}

This is slightly better than writing $s is string, as you don't need to repeat the type name.

nonnull is also useful when using generics.

function my_filter_nulls<T as nonnull>(vec<?T> $items): vec<T> {
  $result = vec[];
  foreach ($items as $item) {
    if ($item is null) {
      // Discard it.
    } else {
      $result[] = $item;
    }
  }
  return $result;
}