finally.php

<?php

class Future
{
    public function finally(callable $onSettle): self
    {
        $state = new FutureState();

        $this->state->subscribe(static function (?\Throwable $error, mixed $value) use ($state, $onSettle): void {
            try {
                try {
                    if ($error) {
                        throw $error;
                    }
                } finally {
                    $onSettle();
                }

                $state->complete($value);
            } catch (\Throwable $exception) {
                $state->error($exception);
            }
        });

        return new self($state);
    }
}