Lua

For use of the Lua programming language in Wikipedia, see Wikipedia:Lua.

Lua

Paradigm	Multi-paradigm: scripting, imperative (procedural, prototype-based object-oriented), functional
Designed by	Roberto Ierusalimschy Waldemar Celes Luiz Henrique de Figueiredo
First appeared	1993
Stable release	5.2.2 / March 27, 2013 (2013-03-27)
Typing discipline	dynamic, strong, duck
OS	Cross-platform
License	MIT License
Website	www.lua.org
Major implementations
Lua, LuaJIT, LLVM-Lua, Lua Alchemy
Dialects
Metalua, Idle, GSL Shell
Influenced by
C++, CLU, Modula, Scheme, SNOBOL
Influenced
Io, GameMonkey, Squirrel, Falcon, MiniD

Lua (/ˈluːə/ LOO-ə, from Template:Lang-pt Template:IPA-pt meaning moon; explicitly not "LUA" ) is a lightweight multi-paradigm programming language designed as a scripting language with "extensible semantics" as a primary goal. Lua is cross-platform since it is written in ISO C. Lua has a relatively simple C API, thus "Lua is especially useful for providing end users with an easy way to program the behavior of a software product without getting too far into its innards."

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, in Brazil.

From 1977 until 1992, Brazil had a policy of strong trade barriers (called a market reserve) for computer hardware and software. In that atmosphere, Tecgraf's clients could not afford, either politically or financially, to buy customized software from abroad. Those reasons led Tecgraf to implement the basic tools it needed from scratch.

Lua's historical "father and mother" were data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language). They had been 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 Petrobras felt a growing need to add 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.

Lua 1.0 was designed in such a way that its object constructors, being then slightly different from the current light and flexible style, incorporated the data-description syntax of SOL (hence the name Lua – sol is Portuguese for sun; lua is moon). 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"), 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.

Current Lua semantics were borrowed 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.

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.

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 – the full reference interpreter is only about 180 kB 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 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.

Example code

The classic hello world program can be written as follows:

print 'Hello World!'

Comments use the following syntax, similar to that of Ada, Eiffel, Haskell, SQL and VHDL:

-- A comment in Lua starts with a double-hyphen and runs to the end of the line.
--[[ Multi-line strings & comments
     are adorned with double square brackets. ]]
--[=[ Comments like this can have other --[[comments]] nested. ]=]

The factorial function is implemented as a function in this example:

function factorial(n)
	local x = 1
	for i = 2,n do
		x = x * i
	end
	return x
end

Loops

Lua has four types of loops: the while loop, the repeat loop (similar to a do while loop), the for loop, and the generic for loop. (The local variables defined are to simply make the program complete. User variables are expected to be the normal input parameters for these functions.)

The while loop has the syntax:

while condition do
    --Statements
end

The repeat loop:

local cond = false
repeat
    --Statements
until cond

executes the loop body at least once, and would keep looping until cond becomes true.

The for loop:

for index = 1,5 do
    print(index)
end

would repeat the loop body 5 times, outputting the numbers 1 through 5 inclusive.

Another form of the for loop is:

local start,finish,delta = 10,1,-1 --delta may be negative, allowing the for loop to count down or up.
for index = start,finish,delta do
    print(index)
end

The generic for loop:

for key,value in pairs(_G) do
    print(key,value)
end

would iterate over the table _G using the standard iterator function pairs, until it returns nil.

Functions

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

do
  local oldprint = print   -- Store current print function as oldprint
  function print(s)        -- Redefine print function, the usual print function can still be used
    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 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:

function addto(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 would be shorter than that of this anonymous
         function, Lua creates a closure.]]
    return x + y
  end
end
fourplus = addto(4)
print(fourplus(3))  -- Prints 7

A new closure for the variable 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 structure (and, by design, the only built-in composite data type) in Lua, and are the foundation of all user-created types. They are conceptually similar to associative arrays in PHP and dictionaries in Python.

A table is a collection of key and data pairs, where the data is referenced by key; in other words, it's a hashed heterogeneous associative array. A 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:

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

We can insert/remove values/indexes from tables, as well.

local myTable={"a","b"}
table.insert(myTable,"c") --print(unpack(myTable)) --> a   b   c
table.remove(myTable,2) --print(unpack(myTable)) --> a   c

As record

A table is often used as structure (or record) 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

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

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 other 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.

The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] 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).

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

Using a hash map to emulate an array normally is 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 Lua’s tables to be customized in powerful ways. The following example demonstrates an "infinite" table. For any $n$, fibs[n] will give the $n$^th Fibonacci number using dynamic programming and memoization.

fibs = { 1, 1 }                          -- Initial values for fibs[1] and fibs[2].
setmetatable(fibs, {
  __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
})

Another example, with the __call metamethod to create an Object Oriented Programming feel:

  newPerson = {}                         -- Creates a new table called 'newPerson'.

  setmetatable(newPerson, {
  __call = function(table,name,age)      -- Turns the newPerson table into a functable.
    local person = {Name = name, Age = age} -- Creates a local variable which has all the properties of the person you create later on.
    return person                        -- Returns the table person so when you create it, it will set the variables in the table person.
  end
})

Bill = newPerson("Bill Raizer", 21)    -- Creates a new person.
print(Bill.Name, Bill.Age)             -- Prints the name and age.

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 look up 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, one 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, { __index = Vector })  -- Inheritance
  return object
end
function Vector:magnitude()   -- Another member function
  -- Reference the implicit object using self
  return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end

vec = Vector:new(0, 1, 0)     -- Create a vector
print(vec:magnitude())        -- Call a member function using ":" (output: 1)
print(vec.x)                  -- Access a member variable using "." (output: 0)

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), 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. Perl's Parrot and Android's Dalvik are two other well-known register-based VMs.

This example is the bytecode listing of the factorial function defined above (as shown by the luac 5.1 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

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 and the Lua auxiliary library.

The Lua API is fairly straightforward because its 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.

Stack

The Lua C API is stack 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 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.

Example

Here is an example of calling a Lua function from C:

#include <stdio.h>
#include <stdlib.h>
#include <lua.h>
#include <lauxlib.h>

int main()
{
    lua_State *L = luaL_newstate();
    if (luaL_dostring(L, "function foo (x,y) return x+y end")) exit(1);
    lua_getglobal(L, "foo");
    lua_pushinteger(L, 5);
    lua_pushinteger(L, 3);
    lua_call(L, 2, 1);
    printf("Result: %d\n", lua_tointeger(L, -1));
    lua_close(L);
    return 0;
}

Running this example gives:

$ gcc -o example example.c -llua
$ ./example
Result: 8

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.

Extension and binding

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. From the Lua side, such a module appears as a namespace table holding its functions and variables. Lua scripts may load extension modules using require, just like modules written in Lua itself.

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. Other sources include LuaForge and the Lua Addons directory of lua-users.org wiki.

There are several packages for creating graphical user interfaces, Perl/POSIX regular expressions, encryption, file compression, and many others. 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

Video games

Main category: Lua-scripted video games

In video game development, Lua is widely used as a scripting language by game programmers, perhaps owing to how easy it is to embed, its fast execution, and its short learning curve.

In 2003, a poll conducted by GameDev.net showed Lua as a most popular scripting language for game programming. On January 12, 2012, Lua was announced as a winner of the Front Line Award 2011 from the magazine Game Developer in the category Programming Tools.

Other

Other applications using Lua include:

• 3DMLW plugin uses Lua scripting for animating 3D and handling different events.
• Adobe Photoshop Lightroom uses Lua for its user interface.
• Apache HTTP Server can use Lua anywhere in the request process (since version 2.3, via the core mod_lua module).
• Artweaver graphics editor uses Lua for scripting filters.
• Awesome, a window manager, is written partly in Lua, also using it as its configuration file format
• Barracuda Application Server includes Lua Server Pages, a technology that enables easy device management of industrial electronics
• The Canon Hack Development Kit (CHDK), an open source firmware for Canon cameras, uses Lua as one of two scripting languages.
• Celestia, the astronomy educational program, uses Lua as its scripting language.
• Cheat Engine, a memory editor/debugger, enables Lua scripts to be embedded in its "cheat table" files, and even includes a GUI designer.
• Cisco uses Lua to implement Dynamic Access Policies within the Adaptive Security Appliance.
• Corona lets developers use integrated Lua, layered on top of C++/OpenGL, to build graphic mobile applications for the iPhone, iPad, and Android devices.
• Custom applications for the Creative Technology Zen X-Fi2 portable media player can be created in Lua.
• Damn Small Linux uses Lua to provide desktop-friendly interfaces for command-line utilities without sacrificing lots of disk space.
• Dolphin Computer Access uses Lua scripting to make inaccessible applications accessible for visually impaired computer users with their screen reader – SuperNova.
• Eyeon's Fusion compositor uses embedded Lua for internal and external scripts and also plugin prototyping.
• A fork of the NES emulator FCE Ultra called FCEUX allows for extensions or modifications to games via Lua scripts.
• Flame, a large and highly sophisticated piece of malware being used for cyber espionage.
• Foldit, a science-oriented game in protein folding, uses Lua for user scripts. Some of those scripts have been the aim of an article in PNAS.
• FreePOPs, an extensible mail proxy, uses Lua to power its web front-end.
• Freeswitch, An open-source telephony platform designed to facilitate the creation of voice and chat driven products in which Lua can be used as a scripting language for call control and call flow among other things.
• Ginga, the middleware for Brazilian Digital Television System (SBTVD or ISDB-T), uses Lua as a script language to its declarative environment, Ginga-NCL. In Ginga-NCL, Lua is integrated as media objects (called NCLua) inside NCL (Nested Context Language) documents.
• GrafX2, a pixel-art editor, can run Lua scripts for simple picture processing or generative illustration.
• iClone, a 3D real-time animation studio to create animation movies uses Lua in the controls of its new physic simulation.
• The drawing editor Ipe (mainly used for producing figures with LaTeX labeling) uses Lua for its functionality and script extensions.
• Lego Mindstorms NXT and NXT 2.0 can be scripted with Lua using third-party software.
• lighttpd web server uses Lua for hook scripts as well as a modern replacement for the Cache Meta Language.
• Version 2.01 of the profile management software for Logitech’s G15 gaming keyboard uses Lua as its scripting language.
• LuaTeX, the designated successor of pdfTeX, allows extensions to be written in Lua.
• Multimedia Fusion Developer 2 has a Lua extension that allows games and applications created with it to run Lua scripts.
• MySQL Workbench uses Lua for its extensions & add-ons.
• Nginx has a powerful embedded Lua module that provides an API for accessing Nginx facilities like socket handling, for example.
• nmap network security scanner uses Lua as the basis for its scripting language, called nse.
• Wavecom's GSM firmware Open AT OS allows user applications to be written in Lua.
• Project Dogwaffle Professional offers Lua scripting to make filters through the DogLua filter. Lua filters can be shared between Project Dogwaffle, GIMP, Pixarra Twistedbrush and ArtWeaver.
• Prosody is a cross-platform Jabber/XMPP server written in Lua.
• Reason Digital Audio Workstation, Lua is used to describe Remote codecs.
• Redis is an open source key-value database, in which Lua can be used (starting with version 2.6) to write complex functions that run in the server itself, thus extending its functionality.
• Renoise audio tracker, in which Lua scripting is used to extend functionality.
• Roblox.com, uses it for scripting in games.
• Rockbox, the open-source digital audio player firmware, supports plugins written in Lua.
• New versions of SciTE editor can be extended using Lua.
• Snort intrusion detection system includes a Lua interpreter since 3.0 beta release.
• The Squeezebox music players from Logitech support plugins written in Lua on recent models (Controller, Radio and Touch).
• Tarantool/Box NoSQL database uses Lua as its stored procedures language
• TI-Nspire calculators contain applications written in Lua, since TI added Lua scripting support with a calculator-specific API in OS 3+.
• Vim has Lua scripting support starting with version 7.3.
• VLC media player uses Lua to provide scripting support.
• WeeChat IRC client allows scripts to be written in Lua.
• Since February–March 2013, Lua is used as a new template scripting language on Wikipedia and other Wikimedia Foundation wikis.
• WinGate proxy server allows event processing and policy to execute lua scripts with access to internal WinGate objects.
• Wireshark network packet analyzer allows protocol dissectors and post-dissector taps to be written in Lua.

Further reading

Books

• Figueiredo, L. H.; Celes, W.; Ierusalimschy, R., eds. (2008). Lua Programming Gems. Lua.org. ISBN 978-85-903798-4-3.
• Gutschmidt, T. (2003). Game Programming with Python, Lua, and Ruby. Course Technology PTR. ISBN 1-59200-077-0.
• Ierusalimschy, R. (2006). Programming in Lua (2nd ed.). Lua.org. ISBN 85-903798-2-5. (The 1st ed. is available online.)
• Jung, K.; Brown, A. (2007). Beginning Lua Programming. Wrox Press. ISBN 0-470-06917-1.
• Schuytema, P.; Manyen, M. (2005). Game Development with Lua. Charles River Media. ISBN 1-58450-404-8.
• Takhteyev, Yuri (2012). Coding Places: Software Practice in a South American City. The MIT Press. ISBN 0262018071. Chapters 6 and 7 are dedicated to Lua, while others look at software in Brazil more broadly.
• Varma, Jayant (2012). Learn Lua for iOS Game Development. Apress. ISBN 1430246626.

Articles

• Matheson, Ash (29 April 2003). "An Introduction to Lua". GameDev.net. Retrieved 3 January 2013.
• Fieldhouse, Keith (16 February 2006). "Introducing Lua". ONLamp.com. O'Reilly Media.
• Streicher, Martin (28 April 2006). "Embeddable scripting with Lua". developerWorks. IBM.
• Quigley, Joseph (1 June 2007). "A Look at Lua". Linux Journal.
• Hamilton, Naomi (11 September 2008). "The A-Z of Programming Languages: Lua". Computerworld. IDG. Interview with Roberto Ierusalimschy.
• Ierusalimschy, Roberto; de Figueiredo, Luiz Henrique; Celes, Waldemar (12 May 2011). "Passing a Language through the Eye of a Needle". ACM Queue. ACM. How the embeddability of Lua impacted its design.
• Lua papers and theses

External links

• Official website
• lua-users.org – Community website for and by users (and authors) of Lua
• eLua – Embedded Lua
• Projects in Lua
• Where Lua Is Used
• Lua operating system project
• SquiLu Squirrel modified with Lua libraries