Lua (programming language)
Lua is a lightweight, high-level, multi-paradigm programming language designed primarily for embedded use in applications. Lua is cross-platform, since the interpreter of compiled bytecode is written in ANSI C, and Lua has a relatively simple C API to embed it into applications.
Lua was originally designed in 1993 as a language for extending software applications to meet the increasing demand for customization at the time. It provided the basic facilities of most procedural programming languages, but more complicated or domain-specific features were not included; rather, it included mechanisms for extending the language, allowing programmers to implement such features. As Lua was intended to be a general embeddable extension language, the designers of Lua focused on improving its speed, portability, extensibility, and ease-of-use in development.
HistoryLua was created in 1993 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, members of the Computer Graphics Technology Group at the Pontifical Catholic University of Rio de Janeiro, in Brazil.
From 1977 until 1992, Brazil had a policy of strong trade barriers 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 predecessors were the data-description/configuration languages SOL and DEL. They had been independently developed at Tecgraf in 1992–1993 to add some flexibility into two different projects. 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 The Evolution of Lua, the language's authors wrote:
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. Lua syntax for control structures was mostly borrowed from Modula, but also had taken influence from CLU, C++, SNOBOL and AWK. 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 were a major influence on their decision to develop the table as the primary data structure of Lua.
Lua semantics have been increasingly influenced by Scheme over time, especially with the introduction of anonymous functions and full lexical scoping. Several features were added in new Lua versions.
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. Both are permissive free software licences and are almost identical.
FeaturesLua 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 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 simple, 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 247 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, 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, coroutines and dynamic module loading.
SyntaxThe 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, similar to Ada, Eiffel, Haskell, SQL and VHDL. Multi-line strings and comments are adorned with double square brackets.
The factorial function is implemented as a function in this example:
local x = 1
for i = 2, n do
x = x * i
Control flowLua has four types of loops: the
repeatloop, the numeric
forloop, and the generic
--condition = true
while condition do
for i = first, last, delta do --delta may be negative, allowing the for loop to count down or up
for key, value in pairs do
would iterate over the table
_Gusing the standard iterator function
pairs, until it returns
You can also do a nested loop, which is a loop inside of another loop.
local grid =
for y, row in ipairs do
for x, value in ipairs do
FunctionsLua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified:
local oldprint = print
-- Store current print function as oldprint
-- Redefine print function. The usual print function can still be used
through oldprint. The new one has only one argument.
Any future calls to
Lua also supports closures, as demonstrated below:
-- Return a new function that adds x to the argument
return x + y
fourplus = addto
print -- Prints 7
--This can also be achieved by calling the function in the following way:
-- This is because we are calling the returned function from 'addto' with the argument '3' directly.
This also helps to reduce data cost and up performance if being called iteratively.
A new closure for the variable
xis created every time
addtois called, so that each new anonymous function returned will always access its own
xparameter. The closure is managed by Lua's garbage collector, just like any other object.
TablesTables are the most important data structures 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
a_table = -- Creates a new, empty table
Tables are always passed by reference.
A key can be any value except
niland NaN, including functions.
a_table = -- Creates a new table, with one entry mapping "x" to the number 10.
print -- Prints the value associated with the string key, in this case 10.
b_table = a_table
b_table = 20 -- The value in the table has been changed to 20.
print -- Prints 20.
print -- Also prints 20, because a_table and b_table both refer to the same table.
A table is often used as structure by using strings as keys. Because such use is very common, Lua features a special syntax for accessing such fields.
point = -- Create new table
print -- Prints 10
print -- Has exactly the same meaning as line above. The easier-to-read dot notation is just syntactic sugar.
By using a table to store related functions, it can act as a namespace.
Point.new = function
return -- return
Point.set_x = function
point.x = x -- point = x;
Tables are automatically assigned a numerical key, enabling them to be used as an array data type. The first automatic index is 1 rather than 0 as it is for many other programming languages.
A numeric key
1is distinct from a string key
array = -- Indices are assigned automatically.
print -- Prints "b". Automatic indexing in Lua starts at 1.
print -- Prints 4. # is the length operator for tables and strings.
array = "z" -- Zero is a legal index.
print -- Still prints 4, as Lua arrays are 1-based.
The length of a table
tis defined to be any integer index
nil; moreover, if
ncan 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", then
#tcan be any of the indices that directly precedes a
print -- Prints "3"
print -- Prints "8"
A table can be an array of objects.
function Point -- "Point" object constructor
return -- Creates and returns a new object
array = -- Creates array of points
-- array = ;
print -- 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.
MetatablesExtensible 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
fibswill give the
n-th Fibonacci number using dynamic programming and memoization.
fibs = -- Initial values for fibs and fibs.
Object-oriented programmingAlthough Lua does not have a built-in concept of classes, object-oriented programming can be achieved using two language features: first-class functions and tables. By placing functions and related data into a table, an object is formed. Inheritance can be implemented using the metatable mechanism, telling the object to look up nonexistent methods and fields in parent object.
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.
Creating a basic vector object:
local Vector =
Vector.__index = Vector
function Vector:new -- The constructor
function Vector:magnitude -- Another method
-- Reference the implicit object using self
local vec = Vector:new -- Create a vector
print) -- Call a method
print -- Access a member variable
ImplementationLua 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 invisible 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. Lua bytecode can also be produced and executed from within Lua, using the
dumpfunction from the string library and the
load/loadstring/loadfilefunctions. Lua version 5.3.4 is implemented in approximately 24,000 lines of C code.
Like most CPUs, and unlike most virtual machines, 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'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 :
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 APILua 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 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 based. Lua provides functions to push and pop most simple C data types 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, 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_callis 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:
// Lua main library
// Lua auxiliary library
Running this example gives:
$ cc -o example example.c -llua
The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At
LUA_GLOBALSINDEXprior to Lua 5.2 is the globals table,
_Gfrom within Lua, which is the main namespace. There is also a registry located at
LUA_REGISTRYINDEXwhere C programs can store Lua values for later retrieval.
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. 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.
ApplicationsIn video game development, Lua is widely used as a scripting language by programmers, mainly due to its perceived easiness to embed, fast execution, and short learning curve.
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, a text editor, and Nginx, a web server.
Through the Scribunto extension, Lua is available as a server side scripting language in the MediaWiki software that powers Wikipedia and other wikis. Among its uses are allowing the integration of data from Wikidata into articles, and powering the.