The Identity monad.

identity.hh

<?hh

type F1<Ta, Tb> = (function(Ta): Tb);

class Identity<Ta> {
	public function __construct(private Ta $value) {}

	public function bind<Tb>(F1<Ta, Identity<Tb>> $f): Identity<Tb> {
		return $f($this->value);
	}

	public function run(): Ta {
		return $this->value;
	}
}

function identity<T>(T $value) {
	return new Identity /* <T> */($value);
}

$three = identity(1)->bind(
	$x ==> identity(2)->bind(
		$y ==> identity($x + $y)
	)
);

echo $three->run() . "\n";

Now with added No errors!.