Lua (programming language)

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Lua

Paradigm	Multi-paradigm: scripting, imperative, functional, object-oriented, prototype-based
Appeared in	1993
Designed by	Roberto Ierusalimschy Waldemar Celes Luiz Henrique de Figueiredo
Latest release	5.1.4/ August 22, 2008
Typing discipline	dynamic, weak ("duck")
Major implementations	Lua, LuaJIT, LLVM-Lua, Kahlua, Mochalua, Lua.NET/LuaCLR, Nua, Yueliang
Dialects	LuaPlus, Metalua
Influenced by	Scheme, Icon
Influenced	Io, Squirrel, Dao, MiniD, GameMonkey
OS	Cross-platform
License	MIT License
Website	http://lua.org

In computing, Lua (pronounced /ˈluː.ə/ LOO-uh) is a lightweight, reflective, imperative and functional programming language, designed as a scripting language with extensible semantics as a primary goal. The name means ‘moon’ in Portuguese. It has enjoyed great popularity in the videogames industry and is known for having a simple yet powerful C API.

Some of its closest relatives include Icon for its design and Python for its ease of use by non-programmers. Perhaps because both languages were influenced by Scheme, Lua and JavaScript feature many common semantics, despite the great differences in syntax.

Apart from games, Lua has been used in many applications, both commercial and non-commercial. See the Applications section for a detailed list.

[edit] 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 PUC-Rio, the Pontifical University of Rio de Janeiro, in Brazil.

From 1977 until 1992, Brazil had a policy of strong trade barriers (called a 'market reserve') for computer hardware and software motivated by a nationalistic feeling that Brazil could and should produce its own hardware and software. In that atmosphere, Tecgraf's clients could not afford, either politically or financially, to buy customized software from abroad. Added to the natural geographical isolation of Brazil from other research and development centers, those reasons led Tecgraf to implement from scratch the basic tools it needed.

Lua's historical 'father and mother' were data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).^[1] They had being independently developed at Tecgraf in 1992-1993 to add some flexibility into two different projects (both were interactive graphical programs for engineering applications at Petrobras company). There was a lack of any flow control structures in SOL and DEL, and a continuously growing need for a full programming power to them. "In 1993, the only real contender was Tcl, which had been explicitly designed to be embedded into applications. However, Tcl had unfamiliar syntax, did not offer good support for data description, and ran only on Unix platforms. We did not consider LISP or Scheme because of their unfriendly syntax. Python was still in its infancy. In the free, do-it-yourself atmosphere that then reigned in Tecgraf, it was quite natural that we should try to develop our own scripting language... Because many potential users of the language were not professional programmers, the language should avoid cryptic syntax and semantics. The implementation of the new language should be highly portable, because Tecgraf's clients had a very diverse collection of computer platforms. Finally, since we expected that other Tecgraf products would also need to embed a scripting language, the new language should follow the example of SOL and be provided as a library with a C API."^[2]

So as a consequence, the Lua was born. Lua 1.0 object constructors, being then slightly different from the current light and flexible style, had incorporated data-description syntax of SOL. (Lua name is thence — sol stands for sun in portuguese.) Lua syntax for control structures was mostly borrowed from Modula (if, while, repeat/until), but also had taken influence from CLU (multiple assignments and multiple returns from function calls, as a simpler alternative to reference parameters or explicit pointers), C++ ("neat idea of allowing a local variable to be declared only where we need it"^[2]), SNOBOL and AWK (associative arrays). In an article published in Dr. Dobb's Journal, Lua's creators also state that Lisp and Scheme with their single, ubiquitous data structure mechanism (the list) were a major influence on their decision to develop the table as the primary data structure of Lua.^[3]

Current Lua semantics was gained mainly from Scheme: "Semantically, Lua has many similarities with Scheme, even though these similarities are not immediately clear because the two languages are syntactically very different. The influence of Scheme on Lua has gradually increased during Lua's evolution: initially, Scheme was just a language in the background, but later it became increasingly important as a source of inspiration, especially with the introduction of anonymous functions and full lexical scoping".^[2]

Versions of Lua prior to version 5.0 were released under a license similar to the BSD license. From version 5.0 onwards, Lua has been licensed under the MIT License.

[edit] 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, rather than providing a more complex and rigid specification to match a single paradigm. Lua, for instance, does not contain explicit support for inheritance, but allows it to be implemented relatively easily 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 powerful techniques from functional programming; and full lexical scoping allows fine-grained information hiding to enforce the principle of least privilege.

In general, Lua strives to provide flexible meta-features that can be extended as needed, rather than supply a feature-set specific to one programming paradigm. As a result, the base language is light — in fact, the full reference interpreter is only about 150kB compiled — and easily adaptable to a broad range of applications.

Lua is a dynamically typed language intended for use as an extension or scripting language, and is compact enough to fit on a variety of host platforms. It supports only a small number of atomic data structures such as boolean values, numbers (double-precision floating point by default), and strings. Typical data structures such as arrays, sets, lists, and records can be represented using Lua’s single native data structure, the table, which is essentially a heterogeneous associative array.

Lua has built-in support for namespaces and object-oriented programming. The table structure provides a 'class'-like and 'struct'-like object system. You can make members of your table, as well as associated methods, all part of the normal table structure. Namespaces can be similarly created by simply wrapping the appropriate code sections in a table (much like a namespace does).

Lua implements a small set of advanced features such as first-class functions, garbage collection, closures, proper tail calls, coercion (automatic conversion between string and number values at run time), coroutines (cooperative multitasking) and dynamic module loading.

By including only a minimum set of data types, Lua attempts to strike a balance between power and size.

[edit] Example code

The classic hello world program can be written as follows:

print("Hello World!")   -- A comment in Lua starts with a double-hyphen
                        -- and runs to the end of the line

Frequently used multi-line syntax is also supported:

--[[  Multi-line strings & comments are
      adorned with double square brackets ]]
print([[Hello,
  world!]])

The factorial is an example of a recursive function:

function factorial(n)
  if n == 0 then
    return 1
  else
    return n * factorial(n - 1)
  end
end                                     
 
function factorial2(n)             -- Shorter equivalent of the above
  return n==0 and 1 or n*factorial2(n - 1)
end

The second form of factorial function originates from Lua's short-circuit evaluation of boolean operators.

Lua’s treatment of functions as first-class variables is shown in the following example, where the print function’s behavior is modified:

do
  local oldprint = print           -- Store current print function as oldprint
  print = function(s)              -- Redefine print function
    if s == "foo" then
      oldprint("bar")
    else 
      oldprint(s) 
    end
  end
end

Any future calls to ‘print’ will now be routed through the new function, and thanks to Lua’s lexical scoping, the old print function will only be accessible by the new, modified print.

Lua also supports closures, as demonstrated below:

function makeaddfunc(x)
  -- Return a new function that adds x to the argument
  return function(y)
    -- When we refer to the variable x, which is outside of the current
    -- scope and whose lifetime is shorter than that of this anonymous
    -- function, Lua creates a closure.
    return x + y
  end
end
plustwo = makeaddfunc(2)
print(plustwo(5)) -- Prints 7

A new closure for the variable x is created every time makeaddfunc is called, so that the anonymous function returned will always access its own x parameter. The closure is managed by Lua’s garbage collector, just like any other object.

Extensible semantics is a key feature of Lua, and the “metatable” concept allows Lua’s tables to be customized in powerful and unique ways. The following example demonstrates an “infinite” table. For any n, fibs[n] will give the nth Fibonacci number using dynamic programming and memoization.

fibs = { 1, 1 }                                -- Initial values for fibs[1] and fibs[2].
setmetatable(fibs, {                           -- Give fibs some magic behavior.
  __index = function(name, n)                  -- Call this function if fibs[n] does not exist.
    name[n] = name[n - 1] + name[n - 2]        -- Calculate and memoize fibs[n].
    return name[n]
  end
})

[edit] Tables

Tables are the most important data structure (and, by design, the only complex data structure) in Lua, and are the foundation of all user-created types.

The table is a collection of key and data pairs (known also as hashed heterogeneous associative array), where the data is referenced by key. The key (index) can be of any data type except nil. An integer key of 1 is considered distinct from a string key of "1".

Tables are created using the {} constructor syntax:

a_table = {}     -- Creates a new, empty table

Tables are always passed by reference:

-- Creates a new table, with one associated entry. The string x mapping to
-- the number 10.
a_table = {x = 10}
-- Prints the value associated with the string key,
-- in this case 10.
print(a_table["x"])
b_table = a_table
a_table["x"] = 20    -- The value in the table has been changed to 20.
print(a_table["x"])  -- Prints 20.
-- Prints 20, because a_table and b_table both refer to the same table.
print(b_table["x"])

[edit] Table as structure

Tables are often used as structures (or objects) by using strings as keys. Because such use is very common, Lua features a special syntax for accessing such fields. Example:

point = { x = 10, y = 20 }   -- Create new table
print(point["x"])            -- Prints 10
print(point.x)               -- Has exactly the same meaning as line above

[edit] Table as namespace

By using a table to store related functions, it can act as a namespace.

Point = {}
Point.new = function (x, y)
  return {x = x, y = y}
end
 
Point.set_x = function (point, x)
  point.x = x
end

[edit] Table as array

By using a numerical key, the table resembles an array data type. Lua arrays are 1-based: the first index is 1 rather than 0 as it is for many programming languages (though an explicit index of 0 is allowed).

A simple array of strings:

array = { "a", "b", "c", "d" }   -- Indices are assigned automatically.
print(array[2])                  -- Prints "b". Automatic indexing in Lua starts at 1.
print(#array)                    -- Prints 4.  # is the length operator for tables and strings.
array[0] = "z"                   -- Zero is a legal index.
print(#array)                    -- Still prints 4, as Lua arrays are 1-based.

An array of objects:

function Point(x, y)        -- "Point" object constructor
  return { x = x, y = y }   -- Creates and returns a new object (table)
end
array = { Point(10, 20), Point(30, 40), Point(50, 60) }   -- Creates array of points
print(array[2].y)                                         -- Prints 40

[edit] Object-oriented programming

Although Lua does not have a built-in concept of classes, they can be implemented using two language features: first-class functions and tables. By placing functions and related data into a table, an object is formed. Inheritance (both single and multiple) can be implemented via the “metatable” mechanism, telling the object to lookup nonexistent methods and fields in parent object(s).

There is no such concept as “class” with these techniques, rather “prototypes” are used as in the programming languages Self or JavaScript. New objects are created either with a factory method (that constructs new objects from scratch) or by cloning an existing object.

Lua provides some syntactic sugar to facilitate object orientation. To declare member functions inside a prototype table, you can use function table:func(args), which is equivalent to function table.func(self, args). Calling class methods also makes use of the colon: object:func(args) is equivalent to object.func(object, args).

Creating a basic vector object:

Vector = { } -- Create a table to hold the class methods
function Vector:new(x, y, z) -- The constructor
  local object = { x = x, y = y, z = z }
  setmetatable(object, {
    -- Overload the index event so that fields not present within the object are
    -- looked up in the prototype Vector table
    __index = Vector
  })
  return object
end
-- Declare another member function, to determine the
-- magnitude of the vector
function Vector:mag()
  -- Reference the implicit object using self
  return math.sqrt(self.x * self.x + self.y * self.y + self.z * self.z)
end
vec = Vector:new(0, 1, 0)   -- Create a vector
print(vec:mag())            -- Call a member function using ":"
print(vec.x)                -- Access a member variable using "."

[edit] Internals

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. The compilation process is typically transparent to the user and is performed during run-time, 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.

Like most CPUs, and unlike most virtual machines (which are stack-based), 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 the first register-based VM to have a wide use.^[4] Parrot (currently in development) is an another well-known register-based VM.

This example is the bytecode listing of the factorial function described above (as shown by luac 5.1.1 compiler):

function <factorial.lua:1,6> (10 instructions, 40 bytes at 003D5818)
1 param, 3 slots, 0 upvalues, 1 local, 3 constants, 0 functions
        1       [2]     EQ              0 0 -1  ; - 0
        2       [2]     JMP             2       ; to 5
        3       [3]     LOADK           1 -2    ; 1
        4       [3]     RETURN          1 2
        5       [5]     GETGLOBAL       1 -3    ; factorial
        6       [5]     SUB             2 0 -2  ; - 1
        7       [5]     CALL            1 2 2
        8       [5]     MUL             1 0 1
        9       [5]     RETURN          1 2
        10      [6]     RETURN          0 1

There is also a free, third-party just-in-time compiler for the latest version (5.1) of Lua called LuaJIT[1]. It is very small (under 32kB of additional code) and can often improve the performance of a Lua program significantly.^[5]

[edit] C API

Lua is intended to be embedded into other applications, and accordingly it provides a robust, easy to use C API. The API is divided into two parts: the Lua core [2], and the Lua auxiliary library [3].

The Lua API is fairly straightforward because its unique design eliminates the need for manual reference management 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 make complex table operations more palatable.

[edit] Stack

The Lua API makes extensive use of a global stack which is used to pass parameters to and from Lua and C functions. 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 last element), while positive indices indicate offsets from the bottom.

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 popped from the stack.

[edit] Special tables

The C API also provides several special tables, located at various “pseudo-indices” in the Lua stack. At LUA_GLOBALSINDEX 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.

[edit] Extension modules

It is possible to write extension modules 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.^[6] A growing collection of modules known as rocks are available through a package management system called LuaRocks [4], in the spirit of RubyGems.

[edit] Development

Please help improve this section by expanding it with: some intro phrases on Lua development; more distros with proper descriptions. Further information might be found on the talk page. (December 2008)

[edit] Distributions

Similar to Linux kernel, the official Lua distribution provides C sources only. Similar to GNU/Linux distributions, there are many third-party distributions of Lua.

Only major or for some reason noticeable distributions are listed here. For a complete list see LuaDistributions and LuaImplementations pages of lua-users.org wiki.

[edit] Generic

Desktop distros for general scripting.

• LuaBinaries [5] tries to maintain precompiled versions of Lua reference interpreter (both shared library and standalone) for all major platforms.
• Lua for Windows [6] is a 'batteries included environment' for the Lua scripting language on Windows. Includes installer, interpreter/compiler, many libraries, documentation, examples, and a SciTE-based editor.
• murgaLua [7] is a complete Lua runtime in a single 500kb file for Windows, Linux, and MacOSX, bundled with LuaFltk, LuaSocket, LuaSQLite, LuaFileSystem and Copas. Supplied with the FLUID GUI builder and documentation for all the APIs (even FLTK).
• wxLua [8] is distributed as a 6Mb standalone scripting runtime as well as a Lua extension module package. Renders cross-platform native-looking GUI widgets along with sockets, streams, printing, clipboard access, and much more. Brings the wxWidgets advantages to Lua.
• ...

[edit] Domain-specific

These are specialized 'Lua scripting environments' — domain-specific, platform-generic development tools which contain or can substitute a reference Lua interpreter. (If such environment promotes Lua scripting only as a 'plugin' feature, it should rather go to the Applications section.)

• Kepler is an extremely lightweight and portable web application framework (see also Orbit, Sputnik, Nanoki [9]).
• LÖVE [10] is a free open-source (zlib-licensed) cross-platform 2D game engine which relies on OpenGL graphics and the SDL multimedia library. Encourages rapid game development and prototyping in Lua.
• Doris [11] is a small OpenGL scripting runtime bundled with GLUT utility toolkit, GLUI widget library, and LuaSocket. Can also serve as a tool for learning 3D graphics and experimenting with OpenGL.
• ...

[edit] Platform-specific

Lua compilers and interpreters — ports or reimplementations.

• Kahlua [12] and Mochalua [13] are two independent implementations of Lua VM for J2ME (CLDC 1.1).
• Plua is a port of Lua (along with a small IDE) for the Palm Computing platform.
• Lua Player is a port designed to run on Sony PSP to allow entry-level programming.
• eLua [14] is a small Lua runtime for microcontrollers with special consideration for limited performance and low level hardware access.
• Lua89 [15] is an experimental port of Lua to the TI89 graphing calculator.

[edit] Bindings to other languages

• LuaInterface [16] for CLR based languages.^[7][8][9]
• Tao.Lua [17] for .Net and Mono.
• LuaJava [18] for Java.
• RubyLuaBridge [19] for Ruby.
• Lunatic Python [20] for Python.

There are many other bindings, wrappers, libraries, tools and IDEs listed on the Lua Addons page of lua-users.org wiki. Large part of them is hosted on LuaForge.

[edit] Applications

Lua, as a compiled binary, is small. Coupled with it being relatively fast and having the liberal MIT license, it has gained a following among game developers for providing a viable scripting interface.^{[citation needed]}

[edit] Games

In video game development, Lua is widely used as scripting language, see also game programmer. Here is a list of notable games using Lua:

• Aleph One (an open-source enhancement of Marathon 2: Durandal) supports Lua, and it has been used in a number of scenarios (including Excalibur and Eternal).
• Aquaria, an underwater metroidvania, is programmed with Lua and accepts mods made with Lua.
• The Buzz! series of games make extensive use of Lua.
• Company of Heroes, a WW2 RTS. Lua is used for the console, AI, single player scripting, win condition scripting and for storing unit attributes and configuration information.
• Crysis, a first-person shooter from Crytek GmbH
• Far Cry, another first-person shooter of Crytek GmbH that used Lua extensively.
• Garry's Mod and Fortress Forever, mods for Half-Life 2, use Lua scripting for tools and other sorts of things for full customization.
• Grim Fandango and Escape from Monkey Island, both based on the GrimE engine. The historic “SCUMM Bar” is renovated and renamed to the "Lua Bar" as a reference.
• Heroes of Might and Magic V, a turn-based strategy computer game.
• Homeworld 2, a space based RTS.
• Monopoly Tycoon, a game that is derived from the Monopoly board game.
• PlayStation Home is programmed using Lua.
• Psychonauts has more Lua aboard than C++ (400KLOC vs. 260KLOC).^[10]
• Ragnarok Online uses Lua to allow players to fully customize the artificial intelligence of their homunculus to their liking, provided that they have an Alchemist to summon one.
• Richard Burns Rally, a realistic Rally Simulator/Game, uses Lua to describe spectator behavior.
• Roblox, an online game intended to be played by 7-14 year olds and lets the scripting language Lua be used by online players to create their own virtual worlds
• ROSE Online uses Lua scripting for AI dialogs.
• S.T.A.L.K.E.R.: Shadow of Chernobyl, has all game scripts written in Lua
• Star Wars: Empire at War and Universe at War: Earth Assault
• Star Wolves and Star Wolves 2, two commercial video games by X-Bow Software.
• Supreme Commander and its expansion Forged Alliance make extensive use of Lua for in-game scripting, camera control, user interfaces and game modifications
• The Guild 2: most of the game is programmed in Lua.
• Open Tibia: Open Tibia (OT) are mainly using lua as their scripting language.
• Toribash: The award winning fighting game uses Lua to script functions like movies and mod-making.
• Warhammer Online: Age of Reckoning: a fantasy MMORPG based upon the popular Warhammer table-top game.
• World of Warcraft, a fantasy MMORPG. Lua is used to allow users to customize its user interface.
• The Witcher uses Lua for game scripts
• Vendetta Online uses Lua for game scripts and missions.

[edit] Other applications

• Celestia uses Lua to expand its capabilities without recompiling its source code.
• Multimedia Fusion Developer 2 has a Lua extension that allows games and applications created with it to run Lua scripts.
• Adobe Photoshop Lightroom uses Lua for its user interface.
• The window manager Ion uses Lua for customization and extensibility.
• The packet sniffer Wireshark uses Lua for scripting and prototyping.
• Intellipool Network Monitor uses Lua for customization and extensibility.
• CMUcam uses Lua for customization and extensibility as part of the CMUcam 3 scripter.
• lighttpd uses Lua for hook scripts. Also, Cache Meta Language, a sophisticated way to describe caching behavior.
• The popular network mapping program nmap uses Lua as the basis for its scripting language, called nse.
• The version control system Monotone uses Lua for scripting hooks.
• The Freeswitch open source PBX project embeds Lua in its API.
• Eyeon's Fusion compositor uses embedded Lua for internal and external scripts and also plugin prototyping.
• The Snort Intrusion Detection/Prevention System version 3.0 uses Lua for its command line interpreter.
• New versions of SciTE allow Lua to be used to provide additional features.
• Version 2.01 of the profile management software for Logitech’s G15 gaming keyboard uses Lua as its scripting language.
• Cisco uses Lua to implement Dynamic Access Policies within the Adaptive Security Appliance.
• Project Dogwaffle uses Lua to let the end-user create new imaging filters. DogLua is based on a 'gluas' plugin spec developed initially for the GIMP. Lua scripting is also available in other digital painting programs, such as ArtWeaver and Twistedbrush. Some implementations have their respective extensions. When the core GIMP-original gluas syntax is used without proprietary extensions, these imaging filters can be shared and used across these applications for the benefit of other users. Some extensions from Project Dogwaffle have found their way also into others such as ArtWeaver.
• 3DMLW plugin uses Lua scripting for animating 3D and handling different events.
• FreePOPs is an extensible mail proxy. It enables checking and downloading of e-mail from webmails from any conventional POP3 client program, avoiding the need to use a Web browser.
• Damn Small Linux uses Lua to provide desktop-friendly interfaces for command-line utilities without sacrificing lots of disk space.
• LuaTeX, the designated successor of pdfTeX, allows extensions to be written in Lua.
• awesome (a window manager) uses Lua for customization and config from version 3.0.
• Dolphin Computer Access uses Lua scripting to make inaccessible applications accessible for visually impaired computer users with their screen reader - SuperNova.
• Match-IT Ltd embed Lua into their manufacturing software as a scripting language.
• Sailwave scoring software uses Lua as its scripting and publishing language.
• CHDK, a software extension for several Canon PowerShot digital cameras, can run scripts written in Lua as well as a BASIC variant.

[edit] References

[edit] External links

• Lua.org — official site.
• User projects — list of applications using Lua, compiled by the authors.
• lua-users.org — community website for and by users (and authors) of Lua.
• lua-users wiki — supplementary information and resources.
• lua-l archive — the official mailing list of Lua.
• Luai — an alternative interface to the reference manual.
• Lua on SWiK.
• An Introduction to Lua — GameDev.net.
• Introducing Lua — ONLamp.com.
• Interview with Roberto Ierusalimschy on Lua — Computerworld.

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