is_null

<?php     public function equals(Collection $that, callable $func = null): bool

    {

        if (is_null($func)) {

            return get_class($this) === get_class($that) && $this->items === $that->items;

        }

<?php     public function filter(?callable $f = null): self

    {

        return is_null($f)

            ? new static(array_values(array_filter($this->items)))

            : new static(array_values(array_filter($this->items, $f)));

    }