*/ abstract class ImmutableCollection implements IteratorAggregate, Countable { /** @var T[] */ protected $values; /** * @param T[] $values * * Keep the constructor protected to keep immutability, the subclasses should not change this constructor * and rely on a static factory method for their construction: * * public static function from(T ...$values): static */ protected function __construct(array $values) { $this->values = $values; } /** * @return ArrayIterator|T[] */ public function getIterator(): Traversable { return new ArrayIterator($this->values); } public function count(): int { return count($this->values); } /** * @return T */ public function first() { return reset($this->values); } /** * @return static */ public function filter(callable $callback): self { return new static(array_filter($this->values, $callback)); } public function isEmpty(): bool { return empty($this->values); } }