Lua (programming language)

For use of the Lua programming language in Wikipedia, see Wikipedia:Lua. For other uses, see Lua (disambiguation).
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 (1993)
Stable release
5.3.3 / 6 June 2016 (2016-06-06)
Preview release
5.3.3 RC3 / 30 May 2016 (2016-05-30)
Typing discipline dynamic, strong, duck
Implementation language ANSI C
OS Cross-platform
License MIT License
Filename extensions .lua
Website www.lua.org
Major implementations
Lua, LuaJIT, LLVM-Lua
Dialects
Metalua, Idle, GSL Shell
Influenced by
C++, CLU, Modula, Scheme, SNOBOL
Influenced
Falcon, GameMonkey, Io, JavaScript, Julia, MiniD, Red, Ruby, Squirrel, MoonScript

Lua (/ˈluːə/ LOO-ə, from Portuguese: lua [ˈlu.(w)ɐ] meaning moon) is a lightweight multi-paradigm programming language designed primarily for embedded systems and clients.[1] Lua is cross-platform since it is written in ANSI C,[2] and has a relatively simple C API.[3]

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.

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.[4]

Lua's historical "father and mother" were the data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).[5] 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.

As the language's authors wrote, in The Evolution of Lua:

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.[4]

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"[4]), 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.[6]

Lua semantics have been increasingly influenced by Scheme over time,[4] 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. Both are permissive free software licences and are almost identical.

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 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 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[2] – 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!")

It can also be written as

io.write('Hello World!\n')

or, the example given on the Lua website

io.write("Hello world, from ", _VERSION, "!\n")

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 numeric for loop, and the generic for loop.

--condition = true

while condition do
  --statements
end

repeat
  --statements
until condition

for i = first,last,delta do     --delta may be negative, allowing the for loop to count down or up
  --statements
  --example: print(i)
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
         through oldprint. The new one has only one argument.]]
    oldprint(s == "foo" and "bar" or s)
  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

--This can also be achieved by calling the function in the following way:
print(addto(4)(3))
--[[ This is because we are calling the returned function from `addto(4)' 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 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 structures (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, dictionaries in Python and hashes in Ruby or Perl.

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 any value but nil and NaN. A numeric 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 (See Call by sharing):

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.

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. The easier-to-read
                             --     dot notation is just syntactic sugar.

Quoting the Lua 5.1 Reference Manual:[7]

"The syntax var.Name is just syntactic sugar for var['Name'];"

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}  --  return {["x"] = x, ["y"] = y}
end

Point.set_x = function(point, x)
  point.x = 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).[8]

A two dimensional table:

ExampleTable =
{
     {1,2,3,4},
     {5,6,7,8}
}
print(ExampleTable[1][3]) -- Prints "3"
print(ExampleTable[2][4]) -- Prints "8"

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
                        -- array = { { x = 10, y = 20 }, { x = 30, y = 40 }, { x = 50, y = 60 } };
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[9] 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 , fibs[n] will give the th Fibonacci number using dynamic programming and memoization.

fibs = { 1, 1 }                                -- Initial values for fibs[1] and fibs[2].
setmetatable(fibs, {
  __index = function(values, n)                --[[ __index is a function predefined by Lua,
                                                    it is called if key "n" does not exist. ]]
    values[n] = values[n - 1] + values[n - 2]  -- Calculate and memoize fibs[n].
    return values[n]
  end
})

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:

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y, z)    -- The constructor
  return setmetatable({x = x, y = y, z = z}, Vector)
end

function Vector:magnitude()     -- Another method
  -- Reference the implicit object using self
  return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end

local vec = Vector:new(0, 1, 0) -- Create a vector
print(vec:magnitude())          -- Call a method (output: 1)
print(vec.x)                    -- Access a member variable (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 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 dump function from the string library and the load/loadstring/loadfile functions. Lua version 5.3.3 is implemented in approximately 24,000 lines of C code.[1][2]

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.[10] 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):[11]

function <factorial.lua:1,7> (9 instructions, 36 bytes at 0x8063c60)
1 param, 6 slots, 0 upvalues, 6 locals, 2 constants, 0 functions
	1	[2]	LOADK    	1 -1	; 1
	2	[3]	LOADK    	2 -2	; 2
	3	[3]	MOVE     	3 0
	4	[3]	LOADK    	4 -1	; 1
	5	[3]	FORPREP  	2 1	; to 7
	6	[4]	MUL      	1 1 5
	7	[3]	FORLOOP  	2 -2	; to 6
	8	[6]	RETURN   	1 2
	9	[7]	RETURN   	0 1

C API

Lua 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.[12]

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.

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 top (most recently pushed value), while positive indices indicate offsets from the bottom (oldest value).

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 read from the stack.

Example

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

#include <stdio.h>
#include <lua.h> //Lua main library (lua_*)
#include <lauxlib.h> //Lua auxiliary library (luaL_*)

int main(void)
{
    //create a Lua state
    lua_State *L = luaL_newstate();

    //load and execute a string
    if (luaL_dostring(L, "function foo (x,y) return x+y end")) {
        lua_close(L);
        return -1;
    }

    //push value of global "foo" (the function defined above)
    //to the stack, followed by integers 5 and 3
    lua_getglobal(L, "foo");
    lua_pushinteger(L, 5);
    lua_pushinteger(L, 3);
    lua_call(L, 2, 1); //call a function with two arguments and one return value
    printf("Result: %d\n", lua_tointeger(L, -1)); //print integer value of item at stack top
    lua_close(L); //close Lua state
    return 0;
}

Running this example gives:

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

Special tables

The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At LUA_GLOBALSINDEX prior to Lua 5.2[13] 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,[12] just like modules written in Lua itself.

A growing collection of modules known as rocks are available through a package management system called LuaRocks,[14] in the spirit of CPAN, RubyGems and Python Eggs. Other modules can be found through the Lua Addons directory of the lua-users.org wiki.[15]

Prewritten Lua bindings exist for most popular programming languages, including other scripting languages.[16] For C++, there are a number of template-based approaches and some automatic binding generators.

Applications

Video games

In video game development, Lua is widely used as a scripting language by game programmers, perhaps due to its perceived easiness to embed, fast execution, and short learning curve.[17]

In 2003, a poll conducted by GameDev.net showed Lua as the most popular scripting language for game programming.[18] 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.[19]

Other

Other applications using Lua include:

See also

References

  1. 1 2 Ierusalimschy, Roberto; de Figueiredo, Luiz Henrique; Filho, Waldemar Celes (June 1996). "Lua—An Extensible Extension Language". Software: Practice and Experience. 26 (6): 635–652. doi:10.1002/(SICI)1097-024X(199606)26:6<635::AID-SPE26>3.0.CO;2-P. Retrieved 24 October 2015.
  2. 1 2 3 "About Lua". Lua.org. Retrieved 2011-08-11.
  3. ↑ Yuri Takhteyev (21 April 2013). "From Brazil to Wikipedia". Foreign Affairs. Retrieved 25 April 2013.
  4. 1 2 3 4 Ierusalimschy, R.; Figueiredo, L. H.; Celes, W. (2007). "The evolution of Lua" (PDF). Proc. of ACM HOPL III. pp. 2–1–2–26. doi:10.1145/1238844.1238846. ISBN 978-1-59593-766-7.
  5. ↑ "The evolution of an extension language: a history of Lua". 2001. Retrieved 2008-12-18.
  6. ↑ Figueiredo, L. H.; Ierusalimschy, R.; Celes, W. (December 1996). "Lua: an Extensible Embedded Language. A few metamechanisms replace a host of features". Dr. Dobb's Journal. 21 (12). pp. 26–33.
  7. ↑ "Lua 5.1 Reference Manual". 2014. Retrieved 2014-02-27.
  8. ↑ "Lua 5.1 Reference Manual". 2012. Retrieved 2012-10-16.
  9. ↑ "Lua 5.1 Source Code". 2006. Retrieved 2011-03-24.
  10. ↑ Ierusalimschy, R.; Figueiredo, L. H.; Celes, W. (2005). "The implementation of Lua 5.0". J. Of Universal Comp. Sci. 11 (7): 1159–1176.
  11. ↑ Kein-Hong Man (2006). "A No-Frills Introduction to Lua 5.1 VM Instructions" (PDF).
  12. 1 2 "Lua 5.2 Reference Manual". Lua.org. Retrieved 2012-10-23.
  13. ↑ "Changes in the API – Lua 5.2 Reference Manual". Lua.org. Retrieved 2014-05-09.
  14. ↑ "LuaRocks". LuaRocks wiki. Retrieved 2009-05-24.
  15. ↑ "Lua Addons". Lua-users wiki. Retrieved 2009-05-24.
  16. ↑ "Binding Code To Lua". Lua-users wiki. Retrieved 2009-05-24.
  17. ↑ Why is Lua considered a game language? at the Wayback Machine (archived 20 August 2013)
  18. ↑ Poll Results at the Wayback Machine (archived 7 December 2003)
  19. ↑ Front Line Award Winners Announced at the Wayback Machine (archived 15 June 2013)
  20. ↑ "how to setup a web server". wiki.netbsd.org. Retrieved 2016-10-28.
  21. ↑ "bozohttpd - NetBSD Manual Pages". netbsd.gw.com. Retrieved 2016-10-28.
  22. ↑ "Using Lua with darktable".
  23. ↑ Zetter, Kim (28 May 2012). "Meet 'Flame,' The Massive Spy Malware Infiltrating Iranian Computers". Wired News.
  24. ↑ "Algorithm discovery by protein folding game players".
  25. ↑ http://blog.haproxy.com/2015/10/14/whats-new-in-haproxy-1-6/
  26. ↑ "pbLua Scriptable Operating Systems with Lua".
  27. ↑ "LuaTeX". luatex.org. Retrieved 21 April 2015.
  28. ↑ "LuCI". Retrieved 2 July 2015.
  29. ↑ Technology report, Wikipedia Signpost (30 January 2012)
  30. ↑ "Public Software Group e. V. – LiquidFeedback Frontend". public-software-group.org. Public Software Group. Retrieved 3 April 2015.
  31. ↑ "LUA(4) Man Page". netbsd.gw.com. Retrieved 2015-04-21.
  32. ↑ "NPF Scripting with Lua EuroBSDCon 2014" (PDF).
  33. ↑ "Scriptable Operating Systems with Lua" (PDF). Dynamic Languages Symposium 2014.
  34. ↑ "HttpLuaModule". Wiki.nginx.org. Retrieved 2013-07-17.
  35. ↑ "Nmap Scripting Engine". Retrieved 2010-04-10.
  36. ↑ Huang R. "NodeMCU devkit". Github. Retrieved 3 April 2015.
  37. ↑ "Know Your SBCs" (PDF). Retrieved 2014-03-08.
  38. ↑ "Redis Lua scripting".
  39. ↑ "Lua for RPM".
  40. ↑ https://support.sas.com/documentation/cdl/en/proc/68954/HTML/default/viewer.htm#p0lqta2cbq9b44n12h28nil7a093.htm
  41. ↑ "Lua in Snort 3.0". Retrieved 2010-04-10.
  42. ↑ "VMOD Lua for Varnish 3.0". Retrieved 2016-02-29.
  43. ↑ "Vim documentation: if_lua". Retrieved 2011-08-17.
  44. ↑ "Lua in Wireshark". Retrieved 2010-04-10.

Further reading

Books

Articles

External links

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