I'm sure there have been great advances. But you are showcasing this as if it was a completely new idea, even though PHP has been using it for a long time. Correct me if I'm wrong.
http://research.microsoft.com/apps/pubs/default.aspx?id=1961...
I think we can go much farther in this area than we have with Hindley Milner. Of course, this is still research, but as long as you are claiming "biggest advance in the last few decades" I might as well just throw this out there.
$f = function helloAction(Http\Request $request) {
$response = new Http\Response("Hello " . $request->query->get('name'), 200);
$response->setMa // at this point the IDE will show a list of methods like 'setMaxAge($time)', because it knows its type
};
$someObject->someMethod($f);
protected method someMethod($f) {} // no checks!
protected method someMethod(\Closure $f) {} // only an argument of type Closure will be accepted.
Response $r = new Response(...);
Of course, in this example, it's silly and type inference would be used to ensure that you don't need to write the left side.
The advantage is that before running your code you can suss out much greater degrees of what your code "could possibly mean". The \Closure bit is a start, but it needs to fail prior to running to be statically typed. It also could potentially include much more information like (\Closure[Http\Request -> Http\Response]) and reject even more bad arguments.