Facebook engineers Bryan O’Sullivan, Julien Verlaguet, and Alok Menghrajani spent the last few years building a programming language unlike any other.
Working alongside a handful of others inside the social networking giant, they fashioned a language that lets programmers build complex websites and other software at great speed while still ensuring that their software code is precisely organized and relatively free of flaws — a combination that few of today’s languages even approach. In typical Facebook fashion, the new language is called Hack , and it already drives almost all of the company’s website — a site that serves more than 1.2 billion people across the globe.
What is Hack?
Hack is a programming language for HHVM that interoperates seamlessly with PHP. Hack reconciles the fast development cycle of PHP with the discipline provided by static typing, while adding many features commonly found in other modern programming languages.
Hack provides instantaneous type checking via a local server that watches the filesystem. It typically runs in less than 200 milliseconds, making it easy to integrate into your development workflow without introducing a noticeable delay.
The New PHP
You can think of Hack as a new version of PHP. It too runs on the Hip Hop Virtual Machine, but it lets coders use both dynamic typing and static typing. This is what’s called gradual typing , and until now, it has mostly been an academic exercise. Facebook, O’Sullivan says, is the first to bring gradual typing to a “real, industrial strength” language.
What this means is that Facebook was able to gradually replace its existing PHP code with Hack — move from the old dynamically typed system to a statically typed arrangement. “It allows you to slide the dial yourself on the continuum between dynamic types and statics — so you can start out with dynamically typed code and then gradually add more statically typed code, benefiting from each little bit of work you do as you go along,” O’Sullivan says.
In doing so, he explains, Facebook built much more precise code — code with fewer flaws. Hack provides a kind of safety net for developers. What’s more, engineers can more easily understand code when they revisit it. Static typing acts a lot like documentation.
But the big trick is that Hack provides these benefits without slowing down the developer: Unlike other statically type languages, Hack can run without compiling. “You edit a file and you reload a webpage and you immediately get the feedback of: Here’s what the page looks like after I made that change. There is no delay,” O’Sullivan says. “You get both safety and speed.”
Nils Adermann, a software engineer and the co-founder of a company called Forumatic, has used the language, and he says he knows of nothing else quite like it. James Miller and Simon Welsh, engineers at a company called PocketRent, who have also used Hack, agree. The closest thing, they say, is Haskell, a statically typed language that provides a way of executing code relatively quickly. But Hack, they indicate, takes the idea much further.
Hack will be particularly attractive, Adermann says, to existing PHP shops. “Ironically,” he says, “its chief advantage is how little it differs from PHP.” Like Facebook, these shops can gradually move their operations from one language to the another. But Adermann also believes that some developers will adopt the language even if they’re not already using PHP. “While PHP is the most widely used language on the web, it’s unpopular in many places because of its inconsistencies,” he says. “Hack addresses these … and thereby makes the language more attractive to users of other languages.”
But the biggest endorsement for the new language is that Facebook already uses it to run its own site, the world’s most popular social network. It’s not every day that a new language debuts with such an impressive track record. Some, however, question whether Hack should really be called a new programming language. There’s a fine line here between an update to PHP and a replacement for PHP. Where does Facebook draw that line? “That,” Sullivan says, “is a good question to discuss late at night over whiskeys.”
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