Lua (programming language)

Lua 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 originated 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, believing that Brazil could and should produce its own hardware and software. In that atmosphere, Tecgraf's clients could not afford, either politically or financially, to buy customized software from abroad; under the market reserve, clients would have to go through a complicated bureaucratic process to prove their needs couldn't be met by Brazilian companies. 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:

"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 (, ,  / ), 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.

As 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, with or without parentheses:

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:

Control flow
Lua has one type of conditional test:  with optional   and   execution control constructs.

The generic  statement requires all three keywords:

The  keyword may be added with an accompanying statement block to control execution when the   condition evaluates to  :

Execution may also be controlled according to multiple conditions using the  keywords:

Lua has four types of conditional loops: the  loop, the   loop (similar to a   loop), the numeric   loop and the generic   loop.

This generic  loop would iterate over the table   using the standard iterator function , until it returns  :

Loops can also be nested (put inside of another loop).

Functions
Lua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified: Any future calls to  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: A new closure for the variable  is created every time   is called, so that each new anonymous function returned will always access its own   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  constructor syntax.

Tables are always passed by reference (see Call by sharing).

A key (index) can be any value except  and NaN, including functions.

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.

By using a table to store related functions, it can act as a namespace.

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  is distinct from a string key.

The length of a table  is defined to be any integer index   such that   is not   and   is  ; moreover, if   is ,   can be zero. For a regular array, with non-nil values from 1 to a given, its length is exactly that  , the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then  can be any of the indices that directly precedes a   value (that is, it may consider any such nil value as the end of the array).

A table can be an array of objects.

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,   will give the  -th Fibonacci number using dynamic programming and memoization.

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:

Here, setmetatable tells Lua to look for an element in the Vector table if it is not present in the vec table. vec.magnitude, which is equivalent to vec["magnitude"], first looks in the vec table for the magnitude element. The vec table does not have a magnitude element, but its metatable delegates to the Vector table for the magnitude element when it's not found in the vec table.

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).

That in mind, here is a corresponding class with : syntactic sugar:

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.

Lua also supports multiple inheritance; __index can either be a function or a table. Operator overloading can also be done; Lua metatables can have elements such as __add, __sub 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  function from the string library and the   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  5.1 compiler):

function  (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  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:

Running this example gives:

The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At  prior to Lua 5.2 is the globals table,   from within Lua, which is the main namespace. There is also a registry located at  where C programs can store Lua values for later retrieval.

Modules
Besides standard library (core) modules it is possible to write extensions 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, just like modules written in Lua itself, or with. When a C library is loaded via Lua will look for the function   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, 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, mainly due to its perceived easiness to embed, fast execution, and short learning curve. Notable games which use Lua include Roblox, Garry's Mod,  World of Warcraft, Payday 2, Phantasy Star Online 2, Dota 2, Crysis, 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.

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, ScyllaDB, a Wide-column store, 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. Among its uses are allowing the integration of data from Wikidata into articles, and powering the.

Languages that compile to Lua

 * MoonScript is a dynamic, whitespace-sensitive scripting language inspired by CoffeeScript, which is compiled into Lua. This means that instead of using  and   (or   and  ) to delimit sections of code it uses line breaks and indentation style.  A notable usage of MoonScript is the video game distribution website Itch.io.
 * Haxe supports compilation to a Lua target, supporting Lua 5.1-5.3 as well as LuaJIT 2.0 and 2.1.
 * Fennel, a Lisp dialect that targets Lua.
 * Urn, a Lisp dialect that is built on Lua.
 * Amulet, an ML-like functional language, whose compiler outputs Lua files.

Dialects
In addition, the Lua users community provides some power patches on top of the reference C implementation.
 * LuaJIT, a just-in-time compiler of Lua 5.1.
 * Luau from Roblox, a Lua 5.1 language with gradual typing and ergonomic additions.
 * Ravi, a JIT-enabled Lua 5.3 language with optional static typing. JIT is guided by type information.
 * Shine, a fork of LuaJIT with many extensions, including a module system and a macro system.
 * Glua, a modified version embedded into the game Garry's Mod as its scripting language.
 * Teal, a statically typed Lua dialect written in Lua.