Called using HH\Asio\join(new Forever()->loop()); in the top level

Forever.php

<?hh // strict
class Forever {
	private ?Awaitable<?(mixed, int)> $hook = null;
	public function __construct() {
	}
	public async function loop(): Awaitable<void> {
		$generator = $this->sub();
		await ($this->hook = $generator->next());
	}
	private async function sub(): \AsyncIterator<int> {
		await \HH\Asio\usleep(1);
		invariant(!is_null($this->hook), '');
		await $this->hook;
		yield 1;
	}
}