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Lua (programming language)

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Lua
File:Lua logo.svg
ParadigmMulti-paradigm
Designed byRoberto Ierusalimschy

Waldemar Celes

Luiz Henrique de Figueiredo
First appeared1993
Stable release
5.1.2 / April 2nd, 2007
OSCross-platform
LicenseMIT License
Websitewww.lua.org
Influenced by
Scheme, Icon
Influenced
Io, Squirrel, Dao

In computing, the Lua (Template:PronEng LOO-ah) 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.

History

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

Some of its closest relatives include Icon for its design and Python for its ease of use by non-programmers. 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.[1]

Lua has been used in many applications, both commercial and non-commercial. See the Applications section for a detailed list. Lua has been used to program homebrew applications for the PlayStation Portable and the Nintendo DS.[citation needed]

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, hash tables, lists, and records can be represented using Lua's single native data structure, the table, which is essentially a heterogeneous map.

Lua has no built-in support for namespaces and object-oriented programming. Instead, metatable and metamethods are used to extend the language to support both programming paradigms in an elegant and straight-forward manner.

Lua implements a small set of advanced features such as higher-order functions, garbage collection, first-class functions, 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!"

The factorial is an example of a recursive function: 

function factorial(n)
  if n == 0 then
    return 1
  end
  return n * factorial(n - 1)     -- A comment in Lua starts with a double-hyphen
end                               -- and runs to the end of the line

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 old print
  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. 

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
})

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: 

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

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

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

Object-oriented programming

Although Lua does not have a built-in concept of classes, the language is powerful enough to easily implement them using two language features: first-class functions and tables. By simply 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 Self programming language 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 function
  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
function Vector:mag()  -- Declare another member function, to determine the magnitude of the vector
  -- Reference the implicit object using self
  return math.sqrt(self.x * self.x + self.y * self.y + self.z * self.z)
end
local 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 "."

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 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 5.1.1): 

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. It's very small (under 32kB of additional code) and can often improve the performance of a Lua program significantly. [2]

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 [3], and the Lua auxiliary library [4].

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.

The Lua 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.

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 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 [5].

Bindings to Other Languages

  • LuaInterface [6] for CLR based languages.
  • Tao.Lua [7] for .Net and Mono
  • LuaJava [8] for Java.
  • Kahlua [9] for J2ME (CLDC 1.1). This is not a binding, but a reimplementation of the Lua virtual machine.

Applications

Lua, as a compiled binary, is very small by code standards. Coupled with it being relatively fast and having a very lenient license, it has gained a following among game developers for providing a viable scripting interface.

Games

Other applications

  • Celestia uses Lua to expand its capabilities without recompiling its source code
  • Multimedia Fusion Developer 2 has a Lua extension that allows 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.
  • The Aegisub subtitles manipulation program uses Lua in its automation module, to generate advanced effects, such as karaoke.
  • Intellipool Network Monitor uses Lua for customization and extensibility.
  • Lua Player is a port designed to run on Sony Computer Entertainment's PlayStation Portable to allow entry-level programming.
  • CMUcam uses Lua for customization and extensibility as part of the CMUcam 3 scripter
  • lighttpd uses Lua for its 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.
  • MySQL Proxy provides Lua scripting hooks.
  • 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.
  • Electro a 3D engine developed by EVL Electronic Visualization Labs (from the University of Illinois at Chicago) is a visualization and game development environment that uses Lua as its core development language ([10]).
  • A new extended version of the TeX typesetting system using Lua as its embedded scripting lanuage is currently under development and reached Beta status in August 2007 (LuaTeX).

A list of projects known to use Lua is located at Lua.org.

Books

Trivia

  • The historic "SCUMM bar" from the Monkey Island games is renovated and renamed to the "Lua bar" during Escape from Monkey Island, as a reference to the fact that the game was the first in the series to be coded in Lua (previous games used the SCUMM engine).
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