History
Lua was created in 1993 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, members of the Computer Graphics Technology Group (Tecgraf) at the Pontifical Catholic University of Rio de Janeiro, inif
, while
, repeat
/until
), but also had taken influence from Features
Lua is commonly described as a " multi-paradigm" language, providing a small set of general features that can be extended to fit different problem types. Lua does not contain explicit support for inheritance, but allows it to be implemented with metatables. Similarly, Lua allows programmers to implement namespaces, classes, and other related features using its single table implementation; first-class functions allow the employment of many techniques fromSyntax
The classicControl flow
Lua has one type of if then end
with optional else
and elseif then
execution control constructs.
The generic if then end
statement requires all three keywords:
else
keyword may be added with an accompanying statement block to control execution when the if
condition evaluates to false
:
elseif then
keywords:
while
loop, the repeat
loop (similar to a do while
loop), the numeric for
loop, and the generic for
loop.
for
loop:
_G
using the standard iterator function pairs
, until it returns nil
.
Loops can also be nested (put inside of another loop).
Functions
Lua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified:print
will now be routed through the new function, and because of Lua's lexical scoping, the old print function will only be accessible by the new, modified print.
Lua also supports closures, as demonstrated below:
x
is created every time addto
is called, so that each new anonymous function returned will always access its own x
parameter. The closure is managed by Lua's garbage collector, just like any other object.
Tables
Tables are the most important data structures (and, by design, the only built-in composite data type) in Lua and are the foundation of all user-created types. They are associative arrays with addition of automatic numeric key and special syntax. A table is a collection of key and data pairs, where the data is referenced by key; in other words, it is a hashed heterogeneous associative array. Tables are created using the
constructor syntax.
nil
and NaN, including functions.
1
is distinct from a string key "1"
.
t
is defined to be any integer index n
such that t /code> is not nil
and t +1/code> is nil
; moreover, if t /code> is nil
, n
can be zero. For a regular array, with non-nil values from 1 to a given n
, its length is exactly that n
, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t
can be any of the indices that directly precedes a nil
value (that is, it may consider any such nil value as the end of the array).
ExampleTable =
print(ExampleTable 3]) -- Prints "3"
print(ExampleTable 4]) -- Prints "8"
A table can be an array of objects.
function Point(x, y) -- "Point" object constructor
return -- Creates and returns a new object (table)
end
array = -- Creates array of points
-- array = ;
print(array y) -- Prints 40
Using a hash map to emulate an array is normally slower than using an actual array; however, Lua tables are optimized for use as arrays to help avoid this issue.
Metatables
Extensible semantics is a key feature of Lua, and the metatable concept allows powerful customization of tables. The following example demonstrates an "infinite" table. For any n
, fibs /code> will give the n
-th Fibonacci number using dynamic programming and memoization
In computing, memoization or memoisation is an optimization technique used primarily to speed up computer programs by storing the results of expensive function calls and returning the cached result when the same inputs occur again. Memoization ...
.
fibs = -- Initial values for fibs and fibs
setmetatable(fibs, )
Object-oriented programming
Although Lua does not have a built-in concept of classes, object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
can be emulated using functions and tables. An object is formed by putting methods and fields in a table. Inheritance (both single and multiple) can be implemented with metatables, delegating nonexistent methods and fields to a parent object.
There is no such concept as "class" with these techniques; rather, prototypes are used, similar to Self or JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of Website, websites use JavaScript on the Client (computing), client side ...
. New objects are created either with a factory method (that constructs new objects from scratch) or by cloning an existing object.
Creating a basic vector object:
local Vector =
local VectorMeta =
function Vector.new(x, y, z) -- The constructor
return setmetatable(, VectorMeta)
end
function Vector.magnitude(self) -- Another method
return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end
local vec = Vector.new(0, 1, 0) -- Create a vector
print(vec.magnitude(vec)) -- Call a method (output: 1)
print(vec.x) -- Access a member variable (output: 0)
Here, tells Lua to look for an element in the table if it is not present in the table. , which is equivalent to , first looks in the table for the element. The table does not have a element, but its metatable delegates to the table for the element when it's not found in the table.
Lua provides some syntactic sugar to facilitate object orientation. To declare member functions inside a prototype table, one can use , which is equivalent to . Calling class methods also makes use of the colon: is equivalent to .
That in mind, here is a corresponding class with syntactic sugar:
local Vector =
Vector.__index = Vector
function Vector:new(x, y, z) -- The constructor
-- Since the function definition uses a colon,
-- its first argument is "self" which refers
-- to "Vector"
return setmetatable(, self)
end
function Vector:magnitude() -- Another method
-- Reference the implicit object using self
return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end
local vec = Vector:new(0, 1, 0) -- Create a vector
print(vec:magnitude()) -- Call a method (output: 1)
print(vec.x) -- Access a member variable (output: 0)
Inheritance
Lua supports using metatables to give Lua class inheritance. In this example, we allow vectors to have their values multiplied by a constant in a derived class.
local Vector =
Vector.__index = Vector
function Vector:new(x, y, z) -- The constructor
-- Here, self refers to whatever class's "new"
-- method we call. In a derived class, self will
-- be the derived class; in the Vector class, self
-- will be Vector
return setmetatable(, self)
end
function Vector:magnitude() -- Another method
-- Reference the implicit object using self
return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end
-- Example of class inheritance
local VectorMult =
VectorMult.__index = VectorMult
setmetatable(VectorMult, Vector) -- Make VectorMult a child of Vector
function VectorMult:multiply(value)
self.x = self.x * value
self.y = self.y * value
self.z = self.z * value
return self
end
local vec = VectorMult:new(0, 1, 0) -- Create a vector
print(vec:magnitude()) -- Call a method (output: 1)
print(vec.y) -- Access a member variable (output: 1)
vec:multiply(2) -- Multiply all components of vector by 2
print(vec.y) -- Access member again (output: 2)
Lua also supports multiple inheritance; can either be a function or a table. Operator overloading can also be done; Lua metatables can have elements such as , , and so on.
Implementation
Lua programs are not interpreted directly from the textual Lua file, but are compiled into bytecode, which is then run on the Lua virtual machine
In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...
. The compilation process is typically invisible to the user and is performed during run-time, especially when a JIT compiler is used, but it can be done offline in order to increase loading performance or reduce the memory footprint of the host environment by leaving out the compiler. Lua bytecode can also be produced and executed from within Lua, using the dump
function from the string library and the load/loadstring/loadfile
functions. Lua version 5.3.4 is implemented in approximately 24,000 lines of C code.
Like most CPUs, and unlike most virtual machines (which are stack-based
Stack-oriented programming, is a programming paradigm which relies on a stack machine model for passing parameters. Stack-oriented languages operate on one or more stacks, each of which may serve a different purpose. Programming constructs i ...
), the Lua VM is register-based, and therefore more closely resembles an actual hardware design. The register architecture both avoids excessive copying of values and reduces the total number of instructions per function. The virtual machine of Lua 5 is one of the first register-based pure VMs to have a wide use. Parrot and Android
Android may refer to:
Science and technology
* Android (robot), a humanoid robot or synthetic organism designed to imitate a human
* Android (operating system), Google's mobile operating system
** Bugdroid, a Google mascot sometimes referred to ...
's Dalvik are two other well-known register-based VMs. PCScheme's VM was also register-based.
This example is the bytecode listing of the factorial function defined above (as shown by the luac
5.1 compiler):
function (9 instructions, 36 bytes at 0x8063c60)
1 param, 6 slots, 0 upvalues, 6 locals, 2 constants, 0 functions
1 LOADK 1 -1 ; 1
2 LOADK 2 -2 ; 2
3 MOVE 3 0
4 LOADK 4 -1 ; 1
5 FORPREP 2 1 ; to 7
6 MUL 1 1 5
7 FORLOOP 2 -2 ; to 6
8 RETURN 1 2
9 RETURN 0 1
C API
Lua is intended to be embedded into other applications, and provides a C API for this purpose. The API is divided into two parts: the Lua core and the Lua auxiliary library. The Lua API's design eliminates the need for manual reference management
Reference management software, citation management software, or bibliographic management software is software for scholars and authors to use for recording and utilising bibliographic citations (references) as well as managing project references ...
in C code, unlike Python's API. The API, like the language, is minimalistic. Advanced functionality is provided by the auxiliary library, which consists largely of preprocessor macros which assist with complex table operations.
The Lua C API is stack
Stack may refer to:
Places
* Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group
* Blue Stack Mountains, in Co. Donegal, Ireland
People
* Stack (surname) (including a list of people ...
based. Lua provides functions to push and pop most simple C data types (integers, floats, etc.) to and from the stack, as well as functions for manipulating tables through the stack. The Lua stack is somewhat different from a traditional stack; the stack can be indexed directly, for example. Negative indices indicate offsets from the top of the stack. For example, −1 is the top (most recently pushed value), while positive indices indicate offsets from the bottom (oldest value). Marshalling data between C and Lua functions is also done using the stack. To call a Lua function, arguments are pushed onto the stack, and then the lua_call
is used to call the actual function. When writing a C function to be directly called from Lua, the arguments are read from the stack.
Here is an example of calling a Lua function from C:
#include
#include // Lua main library (lua_*)
#include // Lua auxiliary library (luaL_*)
int main(void)
Running this example gives:
$ cc -o example example.c -llua
$ ./example
Result: 8
The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At LUA_GLOBALSINDEX
prior to Lua 5.2 is the globals table, _G
from within Lua, which is the main namespace. There is also a registry located at LUA_REGISTRYINDEX
where C programs can store Lua values for later retrieval.
Modules
Besides standard library (core) modules it is possible to write extensions using the Lua API. Extension modules are shared objects which can be used to extend the functionality of the interpreter by providing native facilities to Lua scripts. Lua scripts may load extension modules using require
, just like modules written in Lua itself, or with package.loadlib
. When a C library is loaded via require("foo")
Lua will look for the function luaopen_foo
and call it, which acts as any C function callable from Lua and generally returns a table filled with methods . A growing collection of modules known as ''rocks'' are available through a package management system called LuaRocks, in the spirit of CPAN, RubyGems and Python eggs
setuptools is a package development process library designed to facilitate packaging Python projects by enhancing the Python standard library (distribution utilities). It includes:
*Python package and module definitions
*Distribution package met ...
. Prewritten Lua bindings exist for most popular programming languages, including other scripting languages. For C++, there are a number of template-based approaches and some automatic binding generators.
Applications
In video game development, Lua is widely used as a 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 ...
by programmers, mainly due to its perceived easiness to embed, fast execution, and short learning curve. Notable games which use Lua include '' Roblox'', '' Garry's Mod'', '' World of Warcraft'', '' Payday 2'', '' Phantasy Star Online 2'', '' Dota 2'', ''Angry Birds Space
''Angry Birds Space'' is a physics-based puzzle game and the fifth game in the ''Angry Birds'' video game series. It is developed and published by Rovio Entertainment. Angry Birds Space was released on March 22, 2012. The game was featured in NASA ...
'', '' Crysis'', and many others. Some games that do not natively support Lua programming or scripting, have this functionality added by mods, such as ComputerCraft does for '' Minecraft''. In addition, Lua is also used in non-video game software, such as Adobe Lightroom, Moho, iClone, Aerospike and certain system software in FreeBSD and NetBSD, and is used as a template scripting language on MediaWiki
MediaWiki is a Free and open-source software, free and open-source wiki software. It is used on Wikipedia and almost all other Wikimedia movement, Wikimedia Website, websites, including Wiktionary, Wikimedia Commons and Wikidata; these sit ...
using the Scribunto extension.
In 2003, a poll conducted by GameDev.net showed Lua was the most popular scripting language for game programming. On 12 January 2012, Lua was announced as a winner of the Front Line Award 2011 from the magazine '' Game Developer'' in the category Programming Tools.
A large number of non-game applications also use Lua for extensibility, such as LuaTeX, an implementation of the TeX type-setting language, Redis, a key-value database, Neovim
Vim (;
"Vim is pronounced as one word, like Jim, not vi-ai-em. It's written with a capital, since it's a name, again like Jim.", a text editor, Nginx, a web server, and Wireshark, a network packet analyzer.
Through the Scribunto extension, Lua is available as a server-side scripting language in the MediaWiki
MediaWiki is a Free and open-source software, free and open-source wiki software. It is used on Wikipedia and almost all other Wikimedia movement, Wikimedia Website, websites, including Wiktionary, Wikimedia Commons and Wikidata; these sit ...
software that powers Wikipedia
Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system. Wikipedia is the largest and most-read ref ...
and other wikis. Among its uses are allowing the integration of data from Wikidata into articles, and powering the .
Derived languages
Languages that compile to Lua
* MoonScript is a dynamic, whitespace
White space or whitespace may refer to:
Technology
* Whitespace characters, characters in computing that represent horizontal or vertical space
* White spaces (radio), allocated but locally unused radio frequencies
* TV White Space Database, a mec ...
-sensitive 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 ...
inspired by CoffeeScript, which is compiled into Lua. This means that instead of using do
and end
(or
) to delimit sections of code it uses line breaks and indentation style. A notable usage of MoonScript is the video game distribution website Itch.io
Itch.io (stylized as itch.io) is a website for users to host, sell and download indie games. Launched in March 2013 by Leaf Corcoran, the service hosts over 500,000 games and items (assets, ebooks, music) .
Itch.io also allows users to host ...
.
* Haxe supports compilation to a Lua target, supporting Lua 5.1-5.3 as well as LuaJIT 2.0 and 2.1.
* Fennel, a Lisp dialect that targets Lua.
* Urn, a Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispi ...
dialect that is built on Lua.
* Amulet, an ML-like functional language, whose compiler outputs Lua files.
Dialects
* LuaJIT
* Luau from Roblox, Lua 5.1 language with gradual typing and ergonomic additions.
* Ravi, JIT-enabled Lua 5.3 language with optional static typing. JIT is guided by type information.
* Shine, a fork of LuaJIT with many extensions, including a module system and a macro system.
In addition, the Lua users community provides some ''power patches'' on top of the reference C implementation.
See also
* Comparison of programming languages
References
Further reading
* (The 1st ed. is availabl
online
)
*
*
*
*
* Chapters 6 and 7 are dedicated to Lua, while others look at software in Brazil more broadly.
*
*
*
*
*
* Interview with Roberto Ierusalimschy.
* How the embeddability of Lua impacted its design.
*
External links
*
Lua Users
Community
Lua Forum
LuaDist
Lua Rocks - Package manager
Projects in Lua
{{Portal bar, Free and open-source software, Computer programming
Articles with example C code
Brazilian inventions
Cross-platform free software
Cross-platform software
Dynamic programming languages
Dynamically typed programming languages
Embedded systems
Free compilers and interpreters
Free computer libraries
Free software programmed in C
Object-oriented programming languages
Pontifical Catholic University of Rio de Janeiro
Programming languages
Programming languages created in 1993
Prototype-based programming languages
Register-based virtual machines
Scripting languages
Software using the MIT license