Lua (programming language)
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Lua ( ; from meaning ''
moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
'') is a
lightweight Lightweight is a weight class in combat sports and rowing. Boxing Professional boxing The lightweight division is over 130 pounds (59 kilograms) and up to 135 pounds (61.2 kilograms) weight class in the sport of boxing. Notable lightweight boxe ...
,
high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as computer scienc ...
,
multi-paradigm programming language Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
designed primarily for embedded use in applications. Lua is
cross-platform In computing, cross-platform software (also called multi-platform software, platform-agnostic software, or platform-independent software) is computer software that is designed to work in several computing platforms. Some cross-platform software r ...
, since the interpreter of
compiled In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
bytecode Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (norma ...
is written in
ANSI C ANSI C, ISO C, and Standard C are successive standards for the C programming language published by the American National Standards Institute (ANSI) and ISO/IEC JTC 1/SC 22/WG 14 of the International Organization for Standardization (ISO) and th ...
, and Lua has a relatively simple C
API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
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 Procedural programming is a programming paradigm, derived from imperative programming, based on the concept of the '' procedure call''. Procedures (a type of routine or subroutine) simply contain a series of computational steps to be carrie ...
languages, but more complicated or
domain-specific Domain specificity is a theoretical position in cognitive science (especially modern cognitive development) that argues that many aspects of cognition are supported by specialized, presumably evolutionarily specified, learning devices. The posit ...
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 In everyday use and in kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quanti ...
, portability, extensibility, and ease-of-use in development.


History

Lua was created in 1993 by
Roberto Ierusalimschy Roberto Ierusalimschy (; born May 21, 1960) is a Brazilian computer scientist, known for creating the Lua programming language. He holds a PhD in Computer Science from the Pontifical Catholic University of Rio de Janeiro where he has an appoint ...
, 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 Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
. From 1977 until 1992, Brazil had a policy of strong
trade barrier Trade barriers are government-induced restrictions on international trade. According to the theory of comparative advantage, trade barriers are detrimental to the world economy and decrease overall economic efficiency. Most trade barriers work o ...
s (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, and ''Lua'' meaning "Moon"). Lua
syntax In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
for control structures was mostly borrowed from
Modula The Modula programming language is a descendant of the Pascal language. It was developed in Switzerland, at ETH Zurich, in the mid-1970s by Niklaus Wirth, the same person who designed Pascal. The main innovation of Modula over Pascal is a modul ...
(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 Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a lis ...
),
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
("neat idea of allowing a
local variable In computer science, a local variable is a variable that is given ''local scope''. A local variable reference in the function or block in which it is declared overrides the same variable name in the larger scope. In programming languages with o ...
to be declared only where we need it"),
SNOBOL SNOBOL ("StriNg Oriented and symBOlic Language") is a series of programming languages developed between 1962 and 1967 at AT&T Bell Laboratories by David J. Farber, Ralph E. Griswold and Ivan P. Polonsky, culminating in SNOBOL4. It was one of ...
and
AWK AWK (''awk'') is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep, it is a filter, and is a standard feature of most Unix-like operating systems. The AWK lang ...
(
associative array In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms an ...
s). 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 A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby union ...
) were a major influence on their decision to develop the table as the primary data structure of Lua. Lua
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy Philosophy (f ...
have been increasingly influenced by Scheme over time, especially with the introduction of anonymous functions and full
lexical scoping In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts ...
. 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 BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD lice ...
. From version 5.0 onwards, Lua has been licensed under the
MIT License The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts only very limited restriction on reuse and has, therefore, high license comp ...
. Both are
permissive free software licences A permissive software license, sometimes also called BSD-like or BSD-style license, is a free-software license which instead of copyleft protections, carries only minimal restrictions on how the software can be used, modified, and redistributed, ...
and are almost identical.


Features

Lua is commonly described as a "
multi-paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
" 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 Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
, but allows it to be implemented with
metatable {{For, the object prototype mechanism in Lua, Lua (programming language)#Object-oriented programming A metatable is the section of a database or other data holding structure that is designated to hold data that will act as source code or metadata. ...
s. Similarly, Lua allows programmers to implement
namespace In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...
s, classes, and other related features using its single table implementation;
first-class function In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values from ...
s allow the employment of many techniques from
functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
; and full
lexical scoping In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts ...
allows fine-grained
information hiding In computer science, information hiding is the principle of segregation of the ''design decisions'' in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decisio ...
to enforce the
principle of least privilege In information security, computer science, and other fields, the principle of least privilege (PoLP), also known as the principle of minimal privilege (PoMP) or the principle of least authority (PoLA), requires that in a particular abstraction la ...
. 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 Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
—the full reference interpreter is only about 247  kB compiled—and easily adaptable to a broad range of applications. A
dynamically typed In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
language intended for use as an extension language or
scripting language A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled. A scripting ...
, 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 In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can b ...
and 64-bit
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s by default), and
strings String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...
. Typical data structures such as
arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
, sets,
lists A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...
, and
records A record, recording or records may refer to: An item or collection of data Computing * Record (computer science), a data structure ** Record, or row (database), a set of fields in a database related to one entity ** Boot sector or boot record, ...
can be represented using Lua's single native data structure, the table, which is essentially a heterogeneous
associative array In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms an ...
. Lua implements a small set of advanced features such as
first-class function In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values from ...
s,
garbage collection Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
, closures, proper tail calls, coercion (automatic conversion between string and number values at run time),
coroutine Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
s (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 Comment may refer to: * Comment (linguistics) or rheme, that which is said about the topic (theme) of a sentence * Bernard Comment (born 1960), Swiss writer and publisher Computing * Comment (computer programming), explanatory text or informa ...
in Lua starts with a double-hyphen and runs to the end of the line, similar to
Ada Ada may refer to: Places Africa * Ada Foah, a town in Ghana * Ada (Ghana parliament constituency) * Ada, Osun, a town in Nigeria Asia * Ada, Urmia, a village in West Azerbaijan Province, Iran * Ada, Karaman, a village in Karaman Province, ...
,
Eiffel Eiffel may refer to: Places * Eiffel Peak, a summit in Alberta, Canada * Champ de Mars – Tour Eiffel station, Paris, France; a transit station Structures * Eiffel Tower, in Paris, France, designed by Gustave Eiffel * Eiffel Bridge, Ungheni, M ...
,
Haskell Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lan ...
, SQL and
VHDL The VHSIC Hardware Description Language (VHDL) is a hardware description language (HDL) that can model the behavior and structure of digital systems at multiple levels of abstraction, ranging from the system level down to that of logic gate ...
. 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 one type of conditional test: if then end with optional else and elseif then execution control constructs. The generic if then end statement requires all three keywords: if condition then --statement body end The else keyword may be added with an accompanying statement block to control execution when the if condition evaluates to false: if condition then --statement body else --statement body end Execution may also be controlled according to multiple conditions using the elseif then keywords: if condition then --statement body elseif condition then --statement body else -- optional --optional default statement body end Lua has four types of conditional 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. Loops can also be
nested ''Nested'' is the seventh studio album by Bronx-born singer, songwriter and pianist Laura Nyro, released in 1978 on Columbia Records. Following on from her extensive tour to promote 1976's ''Smile'', which resulted in the 1977 live album '' Seas ...
(put inside of another loop). local grid = for y, row in pairs(grid) do for x, value in pairs(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 print will now be routed through the new function, and because of Lua's
lexical scoping In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts ...
, 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 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 computer science, a composite data type or compound data type is any data type which can be constructed in a program using the programming language's primitive data types and other composite types. It is sometimes called a structure or aggreg ...
) 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. a_table = -- Creates a new, empty table Tables are always passed by reference (see
Call by sharing In a programming language, an evaluation strategy is a set of rules for evaluating expressions. The term is often used to refer to the more specific notion of a ''parameter-passing strategy'' that defines the kind of value that is passed to the f ...
). A key (index) can be any value except nil and
NaN Nan or NAN may refer to: Places China * Nan County, Yiyang, Hunan, China * Nan Commandery, historical commandery in Hubei, China Thailand * Nan Province ** Nan, Thailand, the administrative capital of Nan Province * Nan River People Given name ...
, including functions. a_table = -- 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 String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...
as keys. Because such use is very common, Lua features a special syntax for accessing such fields. point = -- 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 = Point.new = function(x, y) return -- return 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 In computer science, array is a data type that represents a collection of ''elements'' (value (computer science), values or variable (computer science), variables), each selected by one or more indices (identifying keys) that can be computed at R ...
. 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 1 is distinct from a string key "1". array = -- Indices are assigned automatically. print(array -- Prints "b". Automatic indexing in Lua starts at 1. print(#array) -- Prints 4. # is the length operator for tables and strings. array = "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 /code> is not nil and t +1/code> is nil; moreover, if t /code> 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). ExampleTable = print(ExampleTable 3]) -- Prints "3" print(ExampleTable 4]) -- Prints "8" A table can be an array of objects. function Point(x, y) -- "Point" object constructor return -- Creates and returns a new object (table) end array = -- Creates array of points -- array = ; print(array 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 {{For, the object prototype mechanism in Lua, Lua (programming language)#Object-oriented programming A metatable is the section of a database or other data holding structure that is designated to hold data that will act as source code or metadata. ...
concept allows powerful customization of tables. The following example demonstrates an "infinite" table. For any n, fibs /code> will give the n-th
Fibonacci number In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...
using
dynamic programming Dynamic programming is both a mathematical optimization method and a computer programming method. The method was developed by Richard Bellman in the 1950s and has found applications in numerous fields, from aerospace engineering to economics. ...
and memoization. fibs = -- Initial values for fibs and fibs setmetatable(fibs, )


Object-oriented programming

Although Lua does not have a built-in concept of classes,
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
can be emulated using functions and tables. An object is formed by putting methods and fields in a table.
Inheritance Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
(both single and multiple) can be implemented with
metatable {{For, the object prototype mechanism in Lua, Lua (programming language)#Object-oriented programming A metatable is the section of a database or other data holding structure that is designated to hold data that will act as source code or metadata. ...
s, delegating nonexistent methods and fields to a parent object. There is no such concept as "class" with these techniques; rather,
prototypes A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and software programming. A prototype is generally used to ...
are used, similar to
Self The self is an individual as the object of that individual’s own reflective consciousness. Since the ''self'' is a reference by a subject to the same subject, this reference is necessarily subjective. The sense of having a self—or ''selfhoo ...
or
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
. New objects are created either with a
factory method In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by cr ...
(that constructs new objects from scratch) or by cloning an existing object. Creating a basic
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
object: local Vector = local VectorMeta = function Vector.new(x, y, z) -- The constructor return setmetatable(, 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 In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
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: 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(, 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. 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(, 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 also supports
multiple inheritance Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object or ...
; can either be a function or a table.
Operator overloading In computer programming, operator overloading, sometimes termed ''operator ad hoc polymorphism'', is a specific case of polymorphism, where different operators have different implementations depending on their arguments. Operator overloading i ...
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 In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
into bytecode, which is then run on the Lua
virtual machine In computing, a virtual machine (VM) is the virtualization/emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized hardw ...
. The compilation process is typically invisible to the user and is performed during run-time, especially when a
JIT compiler In computing, just-in-time (JIT) compilation (also dynamic translation or run-time compilations) is a way of executing computer code that involves compilation during execution of a program (at run time) rather than before execution. This may cons ...
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. 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 Parrots, also known as psittacines (), are birds of the roughly 398 species in 92 genera comprising the order Psittaciformes (), found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoide ...
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 luac 5.1 compiler): function (9 instructions, 36 bytes at 0x8063c60) 1 param, 6 slots, 0 upvalues, 6 locals, 2 constants, 0 functions 1 LOADK 1 -1 ; 1 2 LOADK 2 -2 ; 2 3 MOVE 3 0 4 LOADK 4 -1 ; 1 5 FORPREP 2 1 ; to 7 6 MUL 1 1 5 7 FORLOOP 2 -2 ; to 6 8 RETURN 1 2 9 RETURN 0 1


C API

Lua is intended to be embedded into other applications, and provides a C
API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
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 Reference management software, citation management software, or bibliographic management software is software for scholars and authors to use for recording and utilising bibliographic citations (references) as well as managing project references ...
in C code, unlike
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
's API. The API, like the language, is minimalistic. Advanced functionality is provided by the auxiliary library, which consists largely of
preprocessor In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...
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 #include // Lua main library (lua_*) #include // Lua auxiliary library (luaL_*) int main(void) 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 is the globals table, _G from within Lua, which is the main
namespace In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...
. There is also a registry located at LUA_REGISTRYINDEX 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 require, just like modules written in Lua itself, or with package.loadlib. 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 A package manager or package-management system is a collection of software tools that automates the process of installing, upgrading, configuring, and removing computer programs for a computer in a consistent manner. A package manager deals w ...
called
LuaRocks LuaRocks is a package manager for the Lua programming language that provides a standard format for distributing Lua modules (in a self-contained format called a "rock"), a tool designed to easily manage the installation of rocks, and a server fo ...
, in the spirit of
CPAN The Comprehensive Perl Archive Network (CPAN) is a repository of over 250,000 software modules and accompanying documentation for 39,000 distributions, written in the Perl programming language by over 12,000 contributors. ''CPAN'' can denote eit ...
,
RubyGems RubyGems is a package manager for the Ruby programming language that provides a standard format for distributing Ruby programs and libraries (in a self-contained format called a "gem"), a tool designed to easily manage the installation of gems ...
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 Video game development (or gamedev) is the process of developing a video game. The effort is undertaken by a developer, ranging from a single person to an international team dispersed across the globe. Development of traditional commercial PC ...
, Lua is widely used as a
scripting language A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled. A scripting ...
by
programmers A computer programmer, sometimes referred to as a software developer, a software engineer, a programmer or a coder, is a person who creates computer programs — often for larger computer software. A programmer is someone who writes/creates ...
, mainly due to its perceived easiness to embed, fast execution, and short
learning curve A learning curve is a graphical representation of the relationship between how proficient people are at a task and the amount of experience they have. Proficiency (measured on the vertical axis) usually increases with increased experience (the ...
. Notable games which use Lua include ''
Roblox ''Roblox'' () is an online game platform and game creation system developed by Roblox Corporation that allows users to program games and play games created by other users. Created by David Baszucki and Erik Cassel in 2004 and released i ...
'', ''
Garry's Mod ''Garry's Mod'' is a 2006 sandbox game developed by Facepunch Studios and published by Valve. The base game mode of ''Garry's Mod'' has no set objectives and provides the player with a world in which to freely manipulate objects. Other ga ...
'', ''
World of Warcraft ''World of Warcraft'' (''WoW'') is a massively multiplayer online role-playing game (MMORPG) released in 2004 by Blizzard Entertainment. Set in the ''Warcraft'' fantasy universe, ''World of Warcraft'' takes place within the world of Azeroth ...
'', ''
Payday 2 ''Payday 2'' is a cooperative first-person shooter video game developed by Overkill Software and published by 505 Games. The game is a sequel to 2011's '' Payday: The Heist''. It was released in August 2013 for Windows, PlayStation 3 and Xbox ...
'', ''
Phantasy Star Online 2 is a free-to-play online action role-playing game in the ''Phantasy Star'' series, developed and published by Sega. It was created as a successor to ''Phantasy Star Online'' and '' Phantasy Star Universe'', ''Phantasy Star Online 2'' features ...
'', '' Dota 2'', '' Angry Birds Space'', ''
Crysis ''Crysis'' is a first-person shooter video game series created by Crytek. The series revolves around a group of military protagonists with " nanosuits", technologically advanced suits of armor that give them enhanced physical strength, speed, ...
'', 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 ''Minecraft'' is a sandbox game developed by Mojang Studios. The game was created by Markus "Notch" Persson in the Java (programming language), Java programming language. Following several early private testing versions, it was first made pub ...
''. In addition, Lua is also used in non-video game software, such as
Adobe Lightroom Adobe Lightroom (officially Adobe Photoshop Lightroom) is a piece of image organization and image manipulation software developed by Adobe Inc. as part of the Creative Cloud subscription family. It is supported on Windows, macOS, iOS, Android, ...
, Moho,
iClone iClone is a real-time 3D animation and rendering software program. Real-time playback is enabled by using a 3D videogame engine for instant on-screen rendering. Other functionality includes: full facial and skeletal animation of human and an ...
, Aerospike and certain system software in
FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
and NetBSD, and is used as a template scripting language on
MediaWiki MediaWiki is a free and open-source wiki software. It is used on Wikipedia and almost all other Wikimedia websites, including Wiktionary, Wikimedia Commons and Wikidata; these sites define a large part of the requirement set for MediaWi ...
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 LuaTeX is a TeX-based computer typesetting system which started as a version of pdfTeX with a Lua scripting engine embedded. After some experiments it was adopted by the TeX Live distribution as a successor to pdfTeX (itself an extension of ε- ...
, an implementation of the
TeX Tex may refer to: People and fictional characters * Tex (nickname), a list of people and fictional characters with the nickname * Joe Tex (1933–1982), stage name of American soul singer Joseph Arrington Jr. Entertainment * ''Tex'', the Italian ...
type-setting language, Redis, a key-value database,
Neovim Vim (;
"Vim is pronounced as one word, like Jim, not vi-ai-em. It's written with a capital, since it's a name, again like Jim."
, a text editor,
Nginx Nginx (pronounced "engine x" ) is a web server that can also be used as a reverse proxy, load balancer, mail proxy and HTTP cache. The software was created by Igor Sysoev and publicly released in 2004. Nginx is free and open-source software ...
, a
web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiate ...
, and
Wireshark Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software and communications protocol development, and education. Originally named Ethereal, the project was renamed Wireshark in May 2006 d ...
, a network packet analyzer. Through the Scribunto extension, Lua is available as a server-side scripting language in the
MediaWiki MediaWiki is a free and open-source wiki software. It is used on Wikipedia and almost all other Wikimedia websites, including Wiktionary, Wikimedia Commons and Wikidata; these sites define a large part of the requirement set for MediaWi ...
software that powers
Wikipedia Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system. Wikipedia is the largest and most-read refer ...
and other wikis. Among its uses are allowing the integration of data from
Wikidata Wikidata is a collaboratively edited multilingual knowledge graph hosted by the Wikimedia Foundation. It is a common source of open data that Wikimedia projects such as Wikipedia, and anyone else, can use under the CC0 public domain license ...
into articles, and powering the .


Derived languages


Languages that compile to Lua

* MoonScript is a
dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyna ...
, whitespace-sensitive
scripting language A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled. A scripting ...
inspired by
CoffeeScript CoffeeScript is a programming language that compiles to JavaScript. It adds syntactic sugar inspired by Ruby, Python, and Haskell in an effort to enhance JavaScript's brevity and readability. Specific additional features include list comprehe ...
, which is compiled into Lua. This means that instead of using do and end (or ) to delimit sections of code it uses line breaks and
indentation style In computer programming, an indentation style is a convention governing the indentation of blocks of code to convey program structure. This article largely addresses the free-form languages, such as C and its descendants, but can be (and ofte ...
. A notable usage of MoonScript is the video game distribution website Itch.io. *
Haxe Haxe is an open source high-level cross-platform programming language and compiler that can produce applications and source code, for many different computing platforms from one code-base. It is free and open-source software, released under the ...
supports compilation to a Lua target, supporting Lua 5.1-5.3 as well as
LuaJIT LuaJIT is a Tracing just-in-time compilation, tracing just in time compiler for the Lua (programming language), Lua programming language. History The LuaJIT project was started in 2005 by developer Mike Pall, released under the MIT open sourc ...
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 In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
, whose compiler outputs Lua files.


Dialects

*
LuaJIT LuaJIT is a Tracing just-in-time compilation, tracing just in time compiler for the Lua (programming language), Lua programming language. History The LuaJIT project was started in 2005 by developer Mike Pall, released under the MIT open sourc ...
* Luau from
Roblox ''Roblox'' () is an online game platform and game creation system developed by Roblox Corporation that allows users to program games and play games created by other users. Created by David Baszucki and Erik Cassel in 2004 and released i ...
, Lua 5.1 language with
gradual typing Gradual typing is a type system in which some variable (computer science), variables and expressions may be given types and the correctness of the typing is checked at compile time (which is static typing) and some expressions may be left untyped ...
and ergonomic additions. * Ravi, JIT-enabled Lua 5.3 language with optional static typing. JIT is guided by type information. * Shine, a fork of
LuaJIT LuaJIT is a Tracing just-in-time compilation, tracing just in time compiler for the Lua (programming language), Lua programming language. History The LuaJIT project was started in 2005 by developer Mike Pall, released under the MIT open sourc ...
with many extensions, including a module system and a macro system. In addition, the Lua users community provides some ''power patches'' on top of the reference C implementation.


See also

*
Comparison of programming languages Programming languages are used for controlling the behavior of a machine (often a computer). Like natural languages, programming languages follow the rules for syntax and semantics. There are thousands of programming languages and new ones are c ...


References


Further reading

* (The 1st ed. is availabl
online
) * * * * * Chapters 6 and 7 are dedicated to Lua, while others look at software in Brazil more broadly. * * * * * * Interview with Roberto Ierusalimschy. * How the embeddability of Lua impacted its design. *


External links

*
Lua Users
Community
Lua Forum

LuaDist

Lua Rocks - Package manager

Projects in Lua
{{Portal bar, Free and open-source software, Computer programming Articles with example C code Brazilian inventions Cross-platform free software Cross-platform software Dynamic programming languages Dynamically typed programming languages Embedded systems Free compilers and interpreters Free computer libraries Free software programmed in C Object-oriented programming languages Pontifical Catholic University of Rio de Janeiro Programming languages Programming languages created in 1993 Prototype-based programming languages Register-based virtual machines Scripting languages Software using the MIT license