Hack (programming language)

Hack

Developer		Meta Platforms
First appeared	2014; 11 years ago (2014)
Stable release	4.172  / 2 November 2022; 2 years ago (2 November 2022)
dynamic, weak, gradual
OS	Cross-platform
License	MIT[2]
Website	hacklang.org
Influenced by
PHP, OCaml, Java, C#, Scala, Haskell

Hack is a

Hack was introduced on March 20, 2014.[7] Before the announcement of the new language, Facebook had already implemented the code and tested it on a large part of its web site.

Hack is designed to interoperate seamlessly with PHP, which is a widely used open-source scripting language that has a focus on web development and can be embedded into HTML. A majority of valid PHP scripts are also valid in Hack; however, many less-often used PHP features and language constructs are unsupported in Hack.^[8]

Hack extends the

The basic file structure of a Hack script is similar to a PHP script with a few changes. A Hack file does not include the <?php opening markup tag and forbids using top-level declarations.[11] Code must be placed in an entrypoint function. These are automatically executed if they are in the top-level file, but not if the file is included via include, require, or the autoloader. Like other functions in Hack, the function names must be unique within a project – i.e., projects with multiple entrypoints can not both be called main:

<<__EntryPoint>>
function main(): void {
  echo 'Hello, World!';
}

The above script, similar to PHP, will be executed and the following output is sent to the browser:

Hello, World!

Unlike PHP, Hack and HTML code do not mix; either XHP or another template engine needs to be used.^[8]

Like PHP 7, Hack allows types to be specified for function arguments and function return values. Functions in Hack are thus annotated with types like the following:

// Hack functions are annotated with types.
function negate(bool $x): bool {
    return !$x;
}

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