Lua (programming language)

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Lua
Paradigm: Multi-paradigm
Appeared in: 1993
Designed by: Roberto Ierusalimschy

Waldemar Celes

Luiz Henrique de Figueiredo
Latest release: 5.1.1 / June 9th, 2006
Influenced by: Scheme, Icon
Influenced: Io
OS: Cross-platform
License: MIT License
Website: www.lua.org

In computing, the Lua (pronounced 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

[edit] 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 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.

[edit] 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.

Lua has been used in many commercial applications, such as Far Cry, Garry's Mod, World of Warcraft and Adobe Photoshop Lightroom, as well as non-commercial applications, such as Angband and its variants. (See the Applications section for a more detailed list.) A ported version of Lua has been used to program homebrew for the Playstation Portable and then Nintendo DS.

Some of its closest relatives include Icon for its design and Python for its ease of use by non-programmers. In a paper 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]

[edit] Features

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 programing 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.

[edit] 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.

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
})

[edit] Tables

Tables are the most important 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.

[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 array

By using a numerical key, the table resembles an array data type.

A simple array of the strings:

array = { "a", "b", "c", "d" }   -- Indexes are assigned automatically
print(array[2])                  -- Prints "b". Automatic indexing in Lua starts at 1.

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 and objects, 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. 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 "."

[edit] 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.

There is also a third-party just-in-time Lua run-time, called LuaJIT, for the new 5.1 version.

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

[edit] Applications

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

[edit] Games

[edit] Other applications

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

[edit] Books

[edit] External links

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