Lua (programming language)

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.[6]

Lua's predecessors were the data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).[7] 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.

In The Evolution of Lua, the language's authors wrote:[6]

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.

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 meaning "Sun" in Portuguese, and Lua meaning "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"[6]), 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.[8]

Lua semantics have been increasingly influenced by Scheme over time,[6] 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.

Wikibooks has a book on the topic of: Lua Programming

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. 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[4]—and easily adaptable to a broad range of applications.

A dynamically typed language intended for use as an extension language or scripting language, Lua 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 and 64-bit integers 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.

print("Hello, World!")

print 'Hello, World!'

function factorial(n)
  local x = 1
  for i = 2, n do
    x = x * i
  end
  return x
end

if condition then
	--statement body
end

if condition then
	--statement body
else
	--statement body
end

if condition then
	--statement body
elseif condition then
	--statement body
else -- optional
	--optional default statement body
end

--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

for key, value in pairs(_G) do
  print(key, value)
end

local grid = {
  { 11, 12, 13 },
  { 21, 22, 23 },
  { 31, 32, 33 }
}

for y, row in ipairs(grid) do
  for x, value in ipairs(row) do
    print(x, y, value)
  end
end

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

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 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_table = {} -- Creates a new, empty table

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.

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.

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

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.

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

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

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
})

local Vector = {}
local VectorMeta = { __index = Vector}

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

function Vector.magnitude(self)     -- Another method
  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(vec))       -- Call a method (output: 1)
print(vec.x)                    -- Access a member variable (output: 0)

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y, z)    -- The constructor
  -- Since the function definition uses a colon, 
  -- its first argument is "self" which refers
  -- to "Vector"
  return setmetatable({x = x, y = y, z = z}, self)
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)

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y, z)    -- The constructor
  -- Here, self refers to whatever class's "new"
  -- method we call.  In a derived class, self will
  -- be the derived class; in the Vector class, self
  -- will be Vector
  return setmetatable({x = x, y = y, z = z}, self)
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

-- Example of class inheritance
local VectorMult = {}
VectorMult.__index = VectorMult
setmetatable(VectorMult, Vector) -- Make VectorMult a child of Vector

function VectorMult:multiply(value) 
  self.x = self.x * value
  self.y = self.y * value
  self.z = self.z * value
  return self
end

local vec = VectorMult:new(0, 1, 0) -- Create a vector
print(vec:magnitude())          -- Call a method (output: 1)
print(vec.y)                    -- Access a member variable (output: 1)
vec:multiply(2)                 -- Multiply all components of vector by 2
print(vec.y)                    -- Access member again (output: 2)

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, especially when a JIT compiler is used, 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 5.3.4 is implemented in approximately 24,000 lines of C code.[3][4]

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.[16] Parrot and Android's Dalvik are two other well-known register-based VMs. PCScheme's VM was also register-based.[17]

This example is the bytecode listing of the factorial function defined above (as shown by the luac 5.1 compiler):[18]

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

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.[19] 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 (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.

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_pop(L, 1); // return stack to original state
    lua_close(L); // close Lua state
    return 0;
}

Running this example gives:

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

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[20] 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.

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,[19] just like modules written in Lua itself, or with package.loadlib.[21] When a C library is loaded via require("foo") Lua will look for the function luaopen_foo and call it, which acts as any C function callable from Lua and generally returns a table filled with methods . A growing collection of modules known as rocks are available through a package management system called LuaRocks,[22] in the spirit of CPAN, RubyGems and Python eggs. Prewritten Lua bindings exist for most popular programming languages, including other scripting languages.[23] For C++, there are a number of template-based approaches and some automatic binding generators.

In 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.[24] Notable games which use Lua include Roblox,[25] Garry's Mod, World of Warcraft, Payday 2, Phantasy Star Online 2, Dota 2, Angry Birds Space,[26] Crysis,[27] and many others. Some games that do not natively support Lua programming or scripting, have this functionality added by mods, such as ComputerCraft does for Minecraft. In addition, Lua is also used in non-video game software, such as Adobe Lightroom, Moho, iClone, Aerospike and certain system software in FreeBSD and NetBSD, and is used as a template scripting language on MediaWiki using the Scribunto extension.[28]

In 2003, a poll conducted by GameDev.net showed Lua was the most popular scripting language for game programming.[29] 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.[30]

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, Nginx, a web server, and Wireshark, a network packet analyzer.

Through the Scribunto extension, Lua is available as a server-side scripting language in the MediaWiki software that powers Wikipedia and other wikis.[31] Among its uses are allowing the integration of data from Wikidata into articles,[32] and powering the automated taxobox system.

• Comparison of programming languages

• Ierusalimschy, R. (2013). Programming in Lua (3rd ed.). Lua.org. ISBN 978-85-903798-5-0. (The 1st ed. is available online.)
• Gutschmidt, T. (2003). Game Programming with Python, Lua, and Ruby. Course Technology PTR. ISBN 978-1-59200-077-7.
• Schuytema, P.; Manyen, M. (2005). Game Development with Lua. Charles River Media. ISBN 978-1-58450-404-7.
• Jung, K.; Brown, A. (2007). Beginning Lua Programming. Wrox Press. ISBN 978-0-470-06917-2. Archived from the original on 8 July 2018. Retrieved 7 July 2018.
• Figueiredo, L. H.; Celes, W.; Ierusalimschy, R., eds. (2008). Lua Programming Gems. Lua.org. ISBN 978-85-903798-4-3.
• Takhteyev, Yuri (2012). Coding Places: Software Practice in a South American City. The MIT Press. ISBN 978-0-262-01807-4. Archived from the original on 2 November 2012. Chapters 6 and 7 are dedicated to Lua, while others look at software in Brazil more broadly.
• Varma, Jayant (2012). Learn Lua for iOS Game Development. Apress. ISBN 978-1-4302-4662-6.
• Matheson, Ash (29 April 2003). "An Introduction to Lua". GameDev.net. Archived from the original on 18 December 2012. Retrieved 3 January 2013.
• Fieldhouse, Keith (16 February 2006). "Introducing Lua". ONLamp.com. O'Reilly Media. Archived from the original on 12 March 2006. Retrieved 28 February 2006.
• Streicher, Martin (28 April 2006). "Embeddable scripting with Lua". developerWorks. IBM. Archived from the original on 2 July 2009. Retrieved 7 July 2018.
• Quigley, Joseph (1 June 2007). "A Look at Lua". Linux Journal.
• Hamilton, Naomi (11 September 2008). "The A-Z of Programming Languages: Lua". Computerworld. IDG. Archived from the original on 8 July 2018. Retrieved 7 July 2018. Interview with Roberto Ierusalimschy.
• Ierusalimschy, Roberto; de Figueiredo, Luiz Henrique; Celes, Waldemar (12 May 2011). "Passing a Language through the Eye of a Needle". ACM Queue. 9 (5): 20–29. doi:10.1145/1978862.1983083. S2CID 19484689. How the embeddability of Lua impacted its design.
• Ierusalimschy, Roberto; de Figueiredo, Luiz Henrique; Celes, Waldemar (November 2018). "A Look at the Design of Lua". Communications of the ACM. 61 (11): 114–123. doi:10.1145/3186277. S2CID 53114923.
• Lua papers and theses

• Official website
• Lua Users, Community
• Lua Forum
• LuaDist
• Lua Rocks - Package manager
• Projects in Lua