Lua programming language

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The Lua (pronouced LOO-ah, or /'lua/ in IPA) programming language is a lightweight, reflective, imperative and procedural language, designed as a scripting language with extensible semantics as a primary goal. The name is derived from the Portuguese word for moon.

Contents

Philosophy

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 fallbacks. 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 featureset 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.

History

Lua was created in 1993 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, members of the Computer Graphics Technology Group at the Pontifical University of Rio de Janeiro in Brazil. 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.

Lua has been used in many commercial applications (e.g., in LucasArts' Escape from Monkey Island adventure game and robot control software) as well as non-commercial applications (like Angband and its variants). Some of its closest relatives include Icon for its design and Python for its ease of use by non-programmers.

Features

Lua is 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, hash tables, lists, and records can be represented using Lua's single native data structure, the table, which is essentially a heterogeneous map. Namespaces and objects can also be created using tables. By including only a minimum of data types, Lua attempts to strike a balance between power and size.

Lua's semantics can be extended and modified by redefining certain built-in functions in metatables. Furthermore, Lua supports advanced features such as higher-order functions and garbage collection. By combining many of its features, it is possible to write object-oriented programs in Lua.

Example code

The classic hello world program can be written as follows: 

print "Hello, world!"

The factorial is an example of a recursive function: 

function factorial(n)
    if n == 0 then
        return 1
    end
    
    return n * factorial(n - 1)
end

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
    print = function (s)
        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.

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. 

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

Internals

Lua programs are not interpreted directly, but are compiled to 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 performance or reduce the memory footprint of the host environment by leaving out the compiler.

This example is the bytecode listing of the factorial function described above (in Lua 5.0): 

function <factorial.lua:1> (10 instructions, 40 bytes at 00326DA0)
1 param, 3 stacks, 0 upvalues, 1 local, 3 constants, 0 functions
    1   [2] EQ          0 0 250 ; compare value to 0
    2   [2] JMP         0 2     ; to line 5
    3   [3] LOADK       1 1     ; 1
    4   [3] RETURN      1 2 0
    5   [6] GETGLOBAL   1 2     ; fact
    6   [6] SUB         2 0 251 ; - 1
    7   [6] CALL        1 2 2
    8   [6] MUL         1 0 1
    9   [6] RETURN      1 2 0
    10  [7] RETURN      0 1 0

Applications

Lua features prominently in many games, such as Far Cry, a first-person shooter, World of Warcraft, a massively multiplayer online role-playing game, where users are able to customize its user interface, character animation and world appearance via Lua, and Bioware's Baldur's Gate series and MDK2 video games, where it is used as a module scripting language. It also appears in some open-source games such as Daimonin and the roguelikes ToME and H-World. Lua will soon be implemented in the popular Half-Life 2 mod, Garry's Mod, where players can script things such as custom gamemodes, weapons, HUD's, and probably much much more. Examples include a kick/ban gun, an ascii snake game, A RPG with player lock and laser tracking.

Therescript, used to drive the vehicles and animations in There, is Lua plus some application-specific functions.

The window manager Ion uses Lua for customization and extensibility.

A list of projects known to use Lua is located here.

Books

External links

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