Hack is a
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming l ...
for the
HipHop Virtual Machine (HHVM), created by
Meta as a dialect of
PHP
PHP is a General-purpose programming language, general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementati ...
. The language implementation is
open-source, licensed under the
MIT License
The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts only very limited restriction on reuse and has, therefore, high license co ...
.
Hack allows programmers to use both
dynamic typing
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
and
static typing
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
. This kind of a
type system
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
is called
gradual typing, which is also implemented in other programming languages such as
ActionScript
ActionScript is an object-oriented programming language originally developed by Macromedia Inc. (later acquired by Adobe). It is influenced by HyperTalk, the scripting language for HyperCard. It is now an implementation of ECMAScript (meani ...
. Hack's type system allows types to be specified for
function arguments, function
return value
In computer programming, a return statement causes execution to leave the current subroutine and resume at the point in the code immediately after the instruction which called the subroutine, known as its return address. The return address is s ...
s, and
class properties; however, types of
local variables are always inferred and cannot be specified.
History
Hack was introduced on March 20, 2014. Before the announcement of the new programming language, Facebook had already implemented the code and tested it on a large portion of its web site.
Features
Hack is designed to interoperate seamlessly with PHP, which is a widely used open-source
scripting language
A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled.
A scripti ...
that has a focus on
web development
Web development is the work involved in developing a website for the Internet (World Wide Web) or an intranet (a private network). Web development can range from developing a simple single static page of plain text to complex web application ...
and can be embedded into
HTML
The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
. A majority of valid PHP scripts are also valid in Hack; however, numerous less frequently used PHP features and language constructs are not supported in Hack.
Hack extends the
type hinting
PHP is a General-purpose programming language, general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementati ...
available in PHP 5 through the introduction of static typing, by adding new type hints (for example, for scalar types such as integer or string), as well as by extending the use of type hints (for example, for class properties or function return values). However, types of local variables cannot be specified.
Since Hack uses a gradual typing system, in the default mode, type annotations are not mandatory even in places they cannot be inferred; the type system will assume the author is correct and admit the code. However, a "strict" mode is available which requires such annotations, and thus enforces fully sound code.
Syntax and semantics
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
opening markup tag and forbids using top-level declarations.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 call both main :
<<__EntryPoint>>
function main(): void
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.
Functions
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
See also
* Parrot virtual machine
* Phalanger
References
External links
* and
Facebook Q&A: Hack brings static typing to PHP world
A tutorial on converting an existing PHP project to Hack
KPHP: An Executable Formal Semantics for PHP
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2014 software
Dynamically typed programming languages
Facebook
PHP software
Programming languages created in 2014
Scripting languages
Statically typed programming languages
Programming languages compiled to bytecode