Facebook's Hack: a new programming language for HHVM

Facebook has released a programming language called Hack, which marries the ease of PHP with the rigorous safety controls of older languages such as C++.

PHP programmers should easily understand Hack, which replicates many of the same features and functions of PHP, and adds a few of its own for greater productivity, said Bryan O’Sullivan, a Facebook engineer on the project.

Over the past year, Facebook has converted nearly all of its PHP code base to Hack, which makes up the core of its website.

In creating Hack, Facebook took an approach that was similar to Microsoft’s with TypeScript, which is basically a superset of JavaScript that, like Hack, adds static typing.

Both projects set out to strengthen a popular dynamic programming language so it can be more easily used by large software teams to design mission-critical applications.

Individuals would also benefit by using Hack, O’Sullivan said, both in terms of increasing performance of their websites and improving the overall quality of their code.

Extending PHP

Hack requires Facebook’s HHVM (Hip Hop Virtual Machine) to run. HHVM is a virtual machine that compiles PHP, normally an interpreted language, into byte code, so it can run more quickly.

Hack is basically an extension of the PHP language with built-in static typing, a feature found in more traditional programming languages such as C/C++ and Java, O’Sullivan said.

Many of the newer Web-oriented programming languages, such as PHP and JavaScript, do not have static typing, hence they are referred to as dynamically typed languages.

With dynamic typing, “there is no explicit information in the source code that describes what kind of information the program is dealing with,” O’Sullivan said.

In contrast, static typing requires the programmer to define the data type for each variable before that program is compiled or run.

Though it takes extra work to implement, static typing prevents run-time errors occurring when the wrong data type is entered into the program, either by human input or some other computer function.

“There are certain kinds of errors and crashes that can occur,” if the programmer is not careful about what data is assigned to variables, O’Sullivan said. “These latent errors can hide for a long time in a dynamically typed languages.”

The HHVM virtual machine has a built-in type checker to ensure that all of the typed information is correct. Hack even allows the programmer to define unique data types.

“Syntactically, Hack is very close to PHP. We allowed it to be possible to run PHP and Hack code side-by-side so you can gradually convert your language codebase from PHP to Hack,” O’Sullivan said.

Certain deprecated PHP features, however, are not supported in Hack, and neither are a handful of features that don’t work well with static typing.

Hack also comes with a number of additions not found in PHP. One is Collections, a way to create arrays with more nuance than the array function offered by PHP itself, O’Sullivan said.

Hack also eases the use of closures through the use of Lambda expressions. Closures,which were added to Java 8, “make it easy to succinctly write fairly complicated data transformations,” O’Sullivan said.

Hack’s Lambda expressions provide a way to create closures “with a fewer number of keystrokes, which is a big win for productivity,” he said.

Facebook has supplied a number of text editor plug-ins on the Hack website to help coders write in the language, though the company is hoping volunteers will build a few more elaborate ones.

O’Sullivan didn’t reveal any specific plans to offer the Hack augmentations back to the keepers of PHP, though he did note that the company plans to “work closely with the open-source community,” to further develop the language.

M.S. Saggoo is a Software Engineer and founder of Programming Inspiration, a popular  programming news and development blog since 2013.

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