Lua (programming language)

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Lua
File:Lua-logo-nolabel.svg
Paradigm Multi-paradigm: scripting, imperative, functional, object-oriented, prototype-based
Appeared in 1993
Designed by Roberto Ierusalimschy
Waldemar Celes
Luiz Henrique de Figueiredo
Stable release 5.1.4 (22, 2008 (2008-08-22))
Typing discipline dynamic, weak ("duck")
Major implementations Lua, LuaJIT, LLVM-Lua, LuaCLR, Nua, Lua Alchemy
Dialects Metalua
Influenced by Scheme, SNOBOL, Modula, CLU, C++
Influenced Io, GameMonkey, Squirrel, Falcon, Dao, MiniD
OS Cross-platform
License MIT License
Website www.lua.org

In computing, Lua (pronounced /ˈluː.ə/, LOO) is a lightweight, reflective, imperative and functional programming language, designed as a scripting language with extensible semantics as a primary goal. The name comes from the Portuguese word lua meaning "moon". Lua has a relatively simple C API compared to other scripting languages.

Because both Lua and JavaScript use prototype-based objects and were influenced by Scheme, they feature many common semantics, despite the great differences in syntax. In its design, Lua is also similar to Icon, perhaps due to both of them being influenced by SNOBOL.

Lua has been used in many applications, both commercial and non-commercial, and especially in the video game industry.

Contents

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 motivated by a nationalistic feeling 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. Added to the natural geographical isolation of Brazil from other research and development centers, those reasons led Tecgraf to implement from scratch the basic tools it needed.[1]

Lua's historical 'father and mother' were data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).[2] 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 a continuously growing need for a full programming power to them.

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.[1]

So as a consequence, Lua was born. Lua 1.0 object constructors, being then slightly different from the current light and flexible style, had incorporated data-description syntax of SOL. (Hence the name Lua — sol is Portuguese for sun.) Lua syntax for control structures was mostly borrowed from Modula (if, while, repeat/until), but also had taken influence from CLU (multiple assignments and multiple returns from function calls, as a simpler alternative to reference parameters or explicit pointers), C++ ("neat idea of allowing a local variable to be declared only where we need it"[1]), 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.[3]

Current Lua semantics was gained mainly from Scheme:

Semantically, Lua has many similarities with Scheme, even though these similarities are not immediately clear because the two languages are syntactically very different. The influence of Scheme on Lua has gradually increased during Lua's evolution: initially, Scheme was just a language in the background, but later it became increasingly important as a source of inspiration, especially with the introduction of anonymous functions and full lexical scoping.[1]

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.

Features

Lua is commonly described as a “multi-paradigm” language, providing a small set of general features that can be extended to fit different problem types, rather than providing a more complex and rigid specification to match a single paradigm. Lua, for instance, does not contain explicit support for inheritance, but allows it to be implemented relatively easily 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 powerful techniques from functional programming; and full lexical scoping allows fine-grained information hiding to enforce the principle of least privilege.

In general, Lua strives to provide flexible meta-features that can be extended as needed, rather than supply a feature-set specific to one programming paradigm. As a result, the base language is light — in fact, the full reference interpreter is only about 150kB compiled — and easily adaptable to a broad range of applications.

Lua is a dynamically typed language intended for use as an extension or scripting language, and is compact enough to fit on a variety of host platforms. It supports only a small number of atomic data structures such as boolean values, numbers (double-precision floating point by default), and strings. Typical data structures such as arrays, sets, lists, and records can be represented using Lua’s single native data structure, the table, which is essentially a heterogeneous associative array.

Lua implements a small set of advanced features such as first-class functions, garbage collection, closures, proper tail calls, coercion (automatic conversion between string and number values at run time), coroutines (cooperative multitasking) and dynamic module loading.

By including only a minimum set of data types, Lua attempts to strike a balance between power and size.

Example code

The classic hello world program can be written as follows:

print("Hello World!")


-- A comment in Lua starts with a double-hyphen and runs to the end of the line.
--[[ Multi-line strings & comments
     are adorned with double square brackets.]]

The factorial is an example of a recursive function:

function factorial(n)
  if n == 0 then
    return 1
  else
    return n * factorial(n - 1)
  end
end

The second form of factorial function originates from Lua's short-circuit evaluation of boolean operators, in which Lua will return the value of last operand evaluated in an expression:

function factorial2(n)             
  return n == 0 and 1 or n * factorial2(n - 1)
end

Lua’s treatment of functions as first-class variables 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
    if s == "foo" then
      oldprint("bar")
    else 
      oldprint(s) 
    end
  end
end

Any future calls to ‘print’ will now be routed through the new function, and thanks to 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 makeaddfunc(x)
  -- Return a new function that adds x to the argument
  return function(y)
    -- When we refer to the variable x, which is outside of the current
    -- scope and whose lifetime is longer than that of this anonymous
    -- function, Lua creates a closure.
    return x + y
  end
end
plustwo = makeaddfunc(2)
print(plustwo(5)) -- Prints 7

A new closure for the variable x is created every time makeaddfunc is called, so that the anonymous function returned will always access its own x parameter. The closure is managed by Lua’s garbage collector, just like any other object.

Extensible semantics is a key feature of Lua, and the “metatable” concept allows Lua’s tables to be customized in powerful and unique ways. The following example demonstrates an “infinite” table. For any <math>n</math>, fibs[n] will give the <math>n</math>th Fibonacci number using dynamic programming and memoization.

fibs = { 1, 1 }                                -- Initial values for fibs[1] and fibs[2].
setmetatable(fibs, {                           -- Give fibs some magic behavior.
  __index = function(name, n)                  -- Call this function if fibs[n] does not exist.
    name[n] = name[n - 1] + name[n - 2]        -- Calculate and memorize fibs[n].
    return name[n]
  end
})

Tables

Tables are the most important data structure (and, by design, the only complex data structure) in Lua, and are the foundation of all user-created types.

The table is a collection of key and data pairs (known also as hashed heterogeneous associative array), where the data is referenced by key. The key (index) can be of any data type except nil. An integer key of 1 is considered distinct from a string key of "1".

Tables are created using the {} constructor syntax:

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

Tables are always passed by reference:

-- Creates a new table, with one associated entry. The string x mapping to
-- the number 10.
a_table = {x = 10}
-- Prints the value associated with the string key,
-- in this case 10.
print(a_table["x"])
b_table = a_table
b_table["x"] = 20    -- The value in the table has been changed to 20.
print(b_table["x"])  -- Prints 20.
-- Prints 20, because a_table and b_table both refer to the same table.
print(a_table["x"])

As structure

Tables are often used as structures (or objects) by using strings as keys. Because such use is very common, Lua features a special syntax for accessing such fields. Example:

point = { x = 10, y = 20 }   -- Create new table
print(point["x"])            -- Prints 10
print(point.x)               -- Has exactly the same meaning as line above

As namespace

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

Point = {}
Point.new = function (x, y)
  return {x = x, y = y}
end
 
Point.set_x = function (point, x)
  point.x = x
end

As array

By using a numerical key, the table resembles an array data type. Lua arrays are 1-based: the first index is 1 rather than 0 as it is for many other programming languages (though an explicit index of 0 is allowed).

A simple array of strings:

array = { "a", "b", "c", "d" }   -- Indices are assigned automatically.
print(array[2])                  -- Prints "b". Automatic indexing in Lua starts at 1.
print(#array)                    -- Prints 4.  # is the length operator for tables and strings.
array[0] = "z"                   -- Zero is a legal index.
print(#array)                    -- Still prints 4, as Lua arrays are 1-based.

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
print(array[2].y)                                         -- Prints 40

Object-oriented programming

Although Lua does not have a built-in concept of classes, they can be implemented using two language features: first-class functions and tables. By placing functions and related data into a table, an object is formed. Inheritance (both single and multiple) can be implemented via the "metatable" mechanism, telling the object to lookup nonexistent methods and fields in parent object(s).

There is no such concept as "class" with these techniques, rather "prototypes" are used as in the programming languages Self or JavaScript. New objects are created either with a factory method (that constructs new objects from scratch) or by cloning an existing object.

Lua provides some syntactic sugar to facilitate object orientation. To declare member functions inside a prototype table, one can use function table:func(args), which is equivalent to function table.func(self, args). Calling class methods also makes use of the colon: object:func(args) is equivalent to object.func(object, args).

Internals

Lua programs are not interpreted directly from the textual Lua file, but are compiled into bytecode which is then run on the Lua virtual machine. The compilation process is typically transparent to the user and is performed during run-time, but it can be done offline in order to increase loading performance or reduce the memory footprint of the host environment by leaving out the compiler.

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 the first register-based pure VM to have a wide use.[4] Parrot, currently in development, and Android's Dalvik are two other well-known register-based VMs.

This example is the bytecode listing of the factorial function defined above (as shown by luac 5.1.1 compiler):[5]

function <factorial.lua:1,6> (10 instructions, 40 bytes at 003D5818)
1 param, 3 slots, 0 upvalues, 1 local, 3 constants, 0 functions
        1       [2]     EQ              0 0 -1  ; - 0
        2       [2]     JMP             2       ; to 5
        3       [3]     LOADK           1 -2    ; 1
        4       [3]     RETURN          1 2
        5       [5]     GETGLOBAL       1 -3    ; factorial
        6       [5]     SUB             2 0 -2  ; - 1
        7       [5]     CALL            1 2 2
        8       [5]     MUL             1 0 1
        9       [5]     RETURN          1 2
        10      [6]     RETURN          0 1

C API

Lua is intended to be embedded into other applications, and accordingly it provides a robust, easy to use C API. The API is divided into two parts: the Lua core and the Lua auxiliary library.[6]

The Lua API is fairly straightforward because its 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 make complex table operations more palatable.

Stack

The Lua API makes extensive use of a global stack which is used to pass parameters to and from Lua and C functions. 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 last element), while positive indices indicate offsets from the bottom.

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 popped from the stack.

Special tables

The C API also provides several special tables, located at various “pseudo-indices” in the Lua stack. At LUA_GLOBALSINDEX is the globals table, _G from within Lua, which is the main namespace. There is also a registry located at LUA_REGISTRYINDEX where C programs can store Lua values for later retrieval.

Extension and binding

It is possible to write extension modules using the Lua API. Extension modules are shared objects which can be used to extend the functionality of the interpreter by providing native facilities to Lua scripts. From the Lua side, such a module appears as a namespace table holding its functions and variables. Lua scripts may load extension modules using require.[6]

A growing collection of modules known as rocks are available through a package management system called LuaRocks,[7] in the spirit of RubyGems. Other sources include LuaForge and the Lua Addons directory of lua-users.org wiki.[8]

Prewritten Lua bindings exist for most popular programming languages, including other scripting languages.[9] For C/C++, there are a number of template-based approaches and some automatic binding generators such as tolua, tolua++[10], SWIG, Luabind, LuaPlus and Luna.

Applications

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