Lua (programming language)

Lua ( ; from meaning "moon") is a lightweight, high-level, multi-paradigm programming language designed primarily for embedded use in applications. Lua is cross-platform, since the interpreter of compiled bytecode is written in ANSI C, and Lua has a relatively simple C API to embed it into applications.
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.
Lua's predecessors were the data-description/configuration languages "SOL" (Simple Object Language) and "DEL" (data-entry language). 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:
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 word, "Lua" meaning "Moon"). Lua syntax for control structures was mostly borrowed from Modula (codice_1, codice_2, codice_3/codice_4), 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"), 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.
Lua semantics have been increasingly influenced by Scheme over time, 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.
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. 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—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.
Syntax.
The classic "Hello, World!" program can be written as follows:
print("Hello, World!")
or as:
print 'Hello, World!'
A comment in Lua starts with a double-hyphen and runs to the end of the line, similar to Ada, Eiffel, Haskell, SQL and VHDL. Multi-line strings and comments are adorned with double square brackets.
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
Control flow.
Lua has four types of loops: the codice_2 loop, the codice_3 loop (similar to a codice_7 loop), the numeric codice_8 loop, and the generic codice_8 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 codice_8 loop:
for key, value in pairs(_G) do
 print(key, value)
end
would iterate over the table codice_11 using the standard iterator function codice_12, until it returns codice_13.
Loops can also be nested (put inside of another loop).
local grid = {
 { 11, 12, 13 },
 { 21, 22, 23 },
for y, row in ipairs(grid) do
 for x, value in ipairs(row) do
 print(x, y, value)
 end
end
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 codice_14 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 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 codice_15 is created every time codice_16 is called, so that each new anonymous function returned will always access its own codice_15 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 associative arrays with addition of automatic numeric key and special syntax.
A table is a collection of key and data pairs, where the data is referenced by key; in other words, it is a hashed heterogeneous associative array.
Tables are created using the codice_18 constructor syntax.
a_table = {} -- Creates a new, empty table
Tables are always passed by reference (see Call by sharing).
A key (index) can be any value except codice_13 and NaN, including functions.
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.
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.
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.
By using a table to store related functions, it can act as a namespace.
Point.new = function(x, y)
end
Point.set_x = function(point, x)
 point.x = x -- point["x"] = x;
end
Tables are automatically assigned a numerical key, enabling them to be used as an array data type. The first automatic index is 1 rather than 0 as it is for many other programming languages (though an explicit index of 0 is allowed).
A numeric key codice_20 is distinct from a string key codice_21.
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 codice_22 is defined to be any integer index codice_23 such that codice_24 is not codice_13 and codice_26 is codice_13; moreover, if codice_28 is codice_13, codice_23 can be zero. For a regular array, with non-nil values from 1 to a given codice_23, its length is exactly that codice_23, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then codice_33 can be any of the indices that directly precedes a codice_13 value (that is, it may consider any such nil value as the end of the array).
ExampleTable =
 {1, 2, 3, 4},
print(ExampleTable[1][3]) -- Prints "3"
print(ExampleTable[2][4]) -- Prints "8"
A table can be 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 is normally slower than using an actual array; however, Lua tables are optimized for use as arrays to help avoid this issue.
Metatables.
Extensible semantics is a key feature of Lua, and the metatable concept allows powerful customization of tables. The following example demonstrates an "infinite" table. For any codice_23, codice_36 will give the codice_23-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 memorize fibs[n].
 return values[n]
 end
Object-oriented programming.
Although Lua does not have a built-in concept of classes, object-oriented programming can be emulated using functions and tables. An object is formed by putting methods and fields in a table. Inheritance (both single and multiple) can be implemented with metatables, delegating nonexistent methods and fields to a parent object.
There is no such concept as "class" with these techniques; rather, prototypes are used, similar to Self or JavaScript. New objects are created either with a factory method (that constructs new objects from scratch) or by cloning an existing object.
Creating a basic vector object:
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)
Here, tells Lua to look for an element in the table if it is not present in the table. , which is equivalent to , first looks in the table for the element. The table does not have a element, but its metatable delegates to the table for the element when it's not found in the table.
Lua provides some syntactic sugar to facilitate object orientation. To declare member functions inside a prototype table, one can use , which is equivalent to . Calling class methods also makes use of the colon: is equivalent to .
That in mind, here is a corresponding class with syntactic sugar:
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)
Inheritance.
Lua supports using metatables to give Lua class inheritance. In this example, we allow vectors to have their values multiplied by a constant in a derived class.
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
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 also supports multiple inheritance; can either be a function or a table. Operator overloading can also be done; Lua metatables can have elements such as , , and so on.
Implementation.
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 codice_38 function from the string library and the codice_39 functions. Lua version 5.3.4 is implemented in approximately 24,000 lines of C code.
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. Parrot and Android's Dalvik are two other well-known register-based VMs. PCScheme's VM was also register-based.
This example is the bytecode listing of the factorial function defined above (as shown by the codice_40 5.1 compiler):
 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. 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 codice_41 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:
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:
The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At codice_42 prior to Lua 5.2 is the globals table, codice_11 from within Lua, which is the main namespace. There is also a registry located at codice_44 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. From the Lua side, such a module appears as a namespace table holding its functions and variables. Lua scripts may load extension modules using codice_45, just like modules written in Lua itself. A growing collection of modules known as "rocks" are available through a package management system called LuaRocks, in the spirit of CPAN, RubyGems and Python eggs. Prewritten Lua bindings exist for most popular programming languages, including other scripting languages. For C++, there are a number of template-based approaches and some automatic binding generators.
Applications.
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. One of the notable gaming platforms is Roblox in which their own dialect, Luau, is used for scripting quick development of games. Another is World of Warcraft which also uses a scaled down version of Lua. 
In 2003, a poll conducted by GameDev.net showed Lua was the most popular scripting language for game programming. 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.
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, and Nginx, a web server.
Through the Scribunto extension, Lua is available as a server-side scripting language in the MediaWiki software that powers Wikipedia and other wikis. Among its uses are allowing the integration of data from Wikidata into articles, and powering the .
Derived languages.
Dialects.
In addition, the Lua users community provides some "power patches" on top of the reference C implementation.
LuaJIT.
LuaJIT is a just in time compiler for Lua. It has been used for embedding or for general purposes. In version 2.0 of LuaJIT, the project has been rewritten for better optimizations for performance.
History.
The LuaJIT project has started in 2005 by developer Mike Pall, released under the MIT open source license. The latest release, 2.0.5 is released in 2017. Since then, the project is not currently maintained by developers other than contributors.
Installation.
LuaJIT is open source and the project has to be compiled in order for it to be used. The repository will have to be download with Git or other methods of downloading repositories. Then it is compiled with any C compiler, usually with GNU make, but other options are available. Finally, the LuaJIT executable and the Lua 5.1 DLL have to be in the same directory in order for the LuaJIT compiler to be used.
There is a guide on using the LuaJIT compiler which includes command line options.
Performance.
When compared to other Lua run-times, LuaJIT is often the fastest Lua compiler.
Platforms.
LuaJIT can be used in:
It can be compiled using either GCC, Clang, or MSVC.
Examples.
The FFi Library can be used to call C functions and use C data structures from Lua Code. There is a guide provided from LuaJIT about using this library. As such, there are multiple LuaJIT bindings to C libraries that use the FFI Library. This example would call a C function, printf from pure Lua code and will output Hello world!.
local ffi = require("ffi")
ffi.cdef
int printf(const char *fmt, ...);
ffi.C.printf("Hello world!\n")
The LuaJIT compiler has also added some extensions to Lua's C API. This example written in C++ would be used for debugging purposes.
// Catch C++ exceptions and convert them to Lua error messages.
// Customize as needed for your own exception classes.
static int wrap_exceptions(lua_State *L, lua_CFunction f)
 try {
 return f(L); // Call wrapped function and return result.
 } catch (const char *s) { // Catch and convert exceptions.
 lua_pushstring(L, s);
 } catch (std::exception& e) {
 lua_pushstring(L, e.what());
 } catch (...) {
 lua_pushliteral(L, "caught (...)");
 return lua_error(L); // Rethrow as a Lua error.
static int myinit(lua_State *L)
 ...
 // Define wrapper function and enable it.
 lua_pushlightuserdata(L, (void *)wrap_exceptions);
 luaJIT_setmode(L, -1, LUAJIT_MODE_WRAPCFUNC|LUAJIT_MODE_ON);
 lua_pop(L, 1);