The Hack Programming Language

Published on 07 May 2019 (Updated: 23 August 2023)

Welcome to the Hack page! Here, you'll find a description of the language as well as a list of sample programs in that language.

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According to Wikipedia, Hack is a dialect of PHP used by Facebook. Beyond that, Wikipedia was really only able to tell me that the open source language was released in 2014. So for once, I had to dig into the documentation.

Apparently, the most important features of Hack are:

Now, I think the most interesting feature has to be the type annotations. That's because Hack is actually dynamically typed like Python.

However, if you've used Python, then you know that it has a relatively new type hinting feature. Typing hinting allows you to arbitrarily enforce static type checking in your code. I say arbitrarily because no one is forcing you to use type hints.

At any rate, Hack has essentially the same feature, but it's restricted to parameters, class variables, and return values. For example:

function foo(int $x): int {
  return $x * 2;

Here we've defined some function foo which takes an integer parameter and returns an integer. Of course, nothing is stopping us from removing those annotations:

function bar($x) {
  return $x * 2;

Now, this function would run exactly the same except we wouldn't discover any type issues until runtime.

So, what kind of typing system does Hack have? Is it static? How about dynamic? Well, there's actually a new term for the kind of type system that languages like Hack and Python have. It's called gradual typing, and it allows users to specify exactly when they want static or dynamic typing.

Once again, I think I've explored a topic a bit too deeply, so I'll stop there.