So when someone announces a "Major New Language", I tend to have a look-in. When Facebook announces a new language, well...
Here's the thing. I don't want to be mean, but it's going to come out, because some of the criticisms I have border on cruel and unusual. In fact the more I read the spec, the more I wonder what they were thinking...
Actually, I do know what they were thinking. They wanted to improve PHP. That is a laudable goal, but frankly, and this is going to sound mean, the fastest way to do that is to stop using it. Hack is designed to solve problems which are central to Facebook's business, but irrelevant to the majority of platforms and users that run PHP. For those platforms, PHP's shortcoming are not a bug, they are a feature.
Case in point: Async. Hack has some lovely Awaitable metas (sort of similar to generics) which you stick on a function and when you call it, you instantly get back a kind of 'handle' to the result which should be available eventually.
That's nice. It makes parallelizing parts of the code really simple.
But it's not that you couldn't already do something similar in PHP (granted, clunky as hell) it's just that most Hosting providers had the PCE (Process Control Extensions) turned off, or would suspend your account if you called too many long-running shell scripts, besides killing your script after 30 seconds of runtime, no matter what you set the timeout to.
Hack Async solves the problem internally for Facebook where they trust all their code, but doesn't do a thing for Dreamhost or Gator clients. In fact it guarantees they can't run it, because even the simplest script can now balloon out into infinite parallel threads, and there's no resource management to limit it.
Also, easy creation of parallel algorithms means easy creation of deadlock problems.. where are the tools to solve those?
Other issues that are important to a company with a huge monolithic codebase are static typing and annotations (check) immutable database tuples (check) generics (check) collections (check) and UML modelling. (um, no!)
Then there are missed opportunities. Traits are broken for the one thing they should be useful for - macros which prevent copypasta and tortuous subclassing in cases where even generics can't cope. It could have been a "literate programming" primitive. Now I don't know what it's for. "Implementing Static Interfaces" apparently.
What really amazes me is that "Lambda Expressions" are more than halfway down the list. "Nullables" (the ability to make static types include 'null') is given more prominence. That's like casually mentioning "oh, and we've got warp drive." after explaining the many benefits of the cup-holder.
But none of that addresses my original point; which is that Hosting providers chose PHP because it was limited and constrained, and could be used to sandbox user accounts on a shared machine. Everything in Hack was already available in C++ and UNIX, but we were not allowed that full power, because abuses (or bad code) brought down the system for everyone, partly because hosting providers are quite terrible sysadmins.
But also because our languages are really bad at allocating finite resources like memory, bandwidth, and CPU power among sub-processes. Or informing us how much is available, or being consumed by our own processes. But that's another discussion...
Because PHP was largely cut-off from the OS, (deliberately!) Hack has incorporated large chunks of the OS inside itself to make that interface frictionless and performant. Switching on all that power will go wonderfully inside the walled garden of the Facebook server farm and other such corporate installations, but it doesn't address anyone else's needs.
This is an heroic effort to get Facebook out of the language dead-end they had painted themselves into, a way to transition from PHP to C++ incrementally, without re-writing the whole damn codebase. Because they're right: PHP starts breaking down when you get too large and complex. I'm amazed they've pushed it this far. It's designed to spam out a web page in 100ms, not multithread a distributed computation. Fortunately, Turing Complete means what it says on the box.
But if you're looking for a cool server-side language for your next project, I'd still go with node.js, unless you too have a million lines of legacy PHP hung around your neck like a dead albatross.
That's not the most resounding accolade, granted, but at least Hack has a definite use-case. (More than can be said for many languages) For PHP programmers with a formal background, it offers a partial escape from our sad and sorry lives. Well, if the sysadmin would just install it...