WikiPortallus: Lua Programming Language.html

Some of its closest relatives include Icon for its design and Python for its ease of use by non-programmers. Perhaps because both languages were influenced by Scheme, Lua and JavaScript feature many common semantics, despite the great differences in syntax.

Apart from games, Lua has been used in many applications, both commercial and non-commercial. See the Applications section for a detailed list.

History

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.

Lua's historical 'father and mother' were data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).¹ 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."²

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"²), 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".²

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.

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

Example code

function factorial(n)
  if n == 0 then
    return 1
  else
    return n * factorial(n - 1)
  end
end                                     
 
function factorial2(n)             -- Shorter equivalent of the above
  return n==0 and 1 or n*factorial2(n - 1)
end

The second form of factorial function originates from Lua's short-circuit evaluation of boolean operators.

Lua’s treatment of functions as first-class variables is shown in the following example, where the print function’s behavior is modified:

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.

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 shorter 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

n

, fibs[n] will give the

n

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.
    namen = namen - 1 + namen - 2        -- Calculate and memoize fibs[n].
    return namen
  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".

-- 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
a_table"x" = 20    -- The value in the table has been changed to 20.
print(a_table"x")  -- Prints 20.
-- Prints 20, because a_table and b_table both refer to the same table.
print(b_table"x")

Table 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:

Table as namespace

Table 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 programming languages (though an explicit index of 0 is allowed).

array = { "a", "b", "c", "d" }   -- Indices are assigned automatically.
print(array2)                  -- Prints "b". Automatic indexing in Lua starts at 1.
print(#array)                    -- Prints 4.  # is the length operator for tables and strings.
array0 = "z"                   -- Zero is a legal index.
print(#array)                    -- Still prints 4, as Lua arrays are 1-based.

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(array2.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, you 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).

Vector = { } -- Create a table to hold the class methods
function Vector:new(x, y, z) -- The constructor
  local object = { x = x, y = y, z = z }
  setmetatable(object, {
    -- Overload the index event so that fields not present within the object are
    -- looked up in the prototype Vector table
    __index = Vector
  })
  return object
end
-- Declare another member function, to determine the
-- magnitude of the vector
function Vector:mag()
  -- Reference the implicit object using self
  return math.sqrt(self.x * self.x + self.y * self.y + self.z * self.z)
end
vec = Vector:new(0, 1, 0)   -- Create a vector
print(vec:mag())            -- Call a member function using ":"
print(vec.x)                -- Access a member variable using "."

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 VM to have a wide use.⁴ Parrot (currently in development) is an another well-known register-based VM.

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

There is also a free, third-party just-in-time compiler for the latest version (5.1) of Lua called LuaJIT. It is very small (under 32kB of additional code) and can often improve the performance of a Lua program significantly.⁶

LLVM-Lua is a compiler that converts Lua bytecode to LLVM IR code. It supports both just-in-time and ahead-of-time compilation. In contrast to LuaJIT, it's not limited to just x86; it compiles Lua scripts to native (to any CPU architecture supported by LLVM) executables.

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.

The Lua API is fairly straightforward because its unique 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.

Examples

Here is a simple example that lets you enter words in array named buff and checks for errors:

#ifdef __cplusplus
extern "C"{ /* add libraries */
    #include <stdio.h>
    #include "lua.h"
    #include "lauxlib.h"
    #include "lualib.h"
}  
#endif
 
int main(void)
{
    char buff256; /* create arrey */
    int error; /* create a variable to represent error(s) */
    lua_State *L = luaL_newstate(); /* Opens a new Lua state */
    luaL_openlibs(L) /* Opens Lua Libraries */
 
    while(fgets(buff, sizeof(buff), stdin)) /* While loop that runs when your entering words */
    {
       error = luaL_loadbuffer(L,buff,strlen(buff), "line") || lua_pcall(L,0,0,0); /* basic checks for error */
       if(error) /* if there is a error */
       {
           fprint(srderr, "%s", lua_tostring(L,-1)); /* print error */
           lua_pop(L,1); /* Removes the error variable form the stack. The reason why it is in the first slot is because its the latest thing we did(the line above it */ 
       }
    }
    lua_close(L); /* Close Lua state */
    return 0;
}
 
#ifdef __cplusplus
}
#endif /* ends C API */

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 modules

It is possible to write extension modules using the Lua API. Extension modules are shared objects which can be used to extend the functionality of the interpreter by providing native facilities to Lua scripts. Lua scripts may load extension modules using require.⁷ A growing collection of modules known as rocks are available through a package management system called LuaRocks, in the spirit of RubyGems.

Development

Distributions

Similar to Linux kernel, the official Lua distribution provides C sources only. Similar to GNU/Linux distributions, there are many third-party distributions of Lua.

Only major or for some reason noticeable distributions are listed here. For a complete list, see Lua Distributions and Lua Implementations on lua-users.org wiki.

Generic

Domain-specific

These are specialized 'Lua scripting environments' — domain-specific, platform-generic development tools which contain or can substitute a reference Lua interpreter.

Platform-specific

Desktop platforms

Mobile platforms

Embedded systems

Bindings to other languages

Miscellaneous

Many other bindings, wrappers, libraries, Lua distributions, IDEs and tools are listed in the Lua Addons directory of lua-users.org wiki. They can also be found on LuaForge and be browsed as Lua rocks.

LuaSearch is a website that intends to provide a centralized online interface for browsing Lua modules and documentation (both in POD and HTML formats), similar to Perl's search.cpan.org.

Applications

Lua, as a compiled binary, is small. Coupled with it being relatively fast and having the liberal MIT license, it has gained a following among game developers.

Games

Other applications

References

^ "The evolution of an extension language: a history of Lua" (2001). Retrieved on 18 December 2008.
^ ^a ^b ^c Ierusalimschy, R.; Figueiredo, L. H.; Celes, W. (2007), "The evolution of Lua", Proc. of ACM HOPL III, pp. 2-1–2-26, doi:10.1145/1238844.1238846, ISBN 978-1-59593-766-X, http://www.tecgraf.puc-rio.br/~lhf/ftp/doc/hopl.pdf
^ Figueiredo, L. H.; Ierusalimschy, R.; Celes, W. (Dec 1996), "Lua: an Extensible Embedded Language. A few metamechanisms replace a host of features", Dr. Dobb's Journal 21 (12): 26–33, http://www.lua.org/ddj.html
^ Ierusalimschy, R.; Figueiredo, L. H.; Celes, W. (2005), "The implementation of Lua 5.0", J. of Universal Comp. Sci. 11 (7): 1159-1176, http://www.jucs.org/jucs_11_7/the_implementation_of_lua/jucs_11_7_1159_1176_defigueiredo.html
^ Kein-Hong Man (2006), A No-Frills Introduction to Lua 5.1 VM Instructions, http://luaforge.net/docman/view.php/83/98/ANoFrillsIntroToLua51VMInstructions.pdf
^ LuaJIT Performance, http://luajit.org/luajit_performance.html, retrieved on 20 December 2008
^ Lua 5.1 Reference Manual: Modules
^ TIOBE Programming Community Index, http://www.tiobe.com/content/paperinfo/tpci/index.html, retrieved on 22 December 2008
^ ^a ^b Mascarenhas, F.; Ierusalimschy, R. (2004), "LuaInterface: scripting .NET CLR with Lua", J. of Universal Comp. Sci. 10 (7): 892-909, http://www.jucs.org/jucs_10_7/luainterface_scripting_the_.net
^ ^a ^b Mascarenhas, F.; Ierusalimschy, R. (2005), "Running Lua scripts on the CLR through bytecode translation", J. of Universal Comp. Sci. 11 (7): 1275-1290, http://www.jucs.org/jucs_11_7/running_lua_scripts_on
^ ^a ^b Mascarenhas, F.; Ierusalimschy, R. (2008), "Efficient compilation of Lua for the CLR", Proc. of 2008 ACM Symposium on Applied Computing, pp. 217-221, doi:10.1145/1363686.1363743, ISBN 978-1-59593-753-7
^ Joel Pobar (8 July 2007), "Lua on the DLR", Joel Pobar's weblog, http://callvirt.net/blog/entry.aspx?entryid=745a1eb7-bd8e-45bf-8bbf-7b391e08799a, retrieved on 23 December 2008
^ Robert Stehwien (2 December 2008), "Lua ActionScript Alchemy", Blogspot, http://arcanearcade.blogspot.com/2008/12/lua-actionscript-alchemy.html, retrieved on 23 December 2008
^ ^a ^b Mike Pall (11 November 2007), "Lua on cell phones", lua-l archive, http://lua-users.org/lists/lua-l/2007-11/msg00248.html, retrieved on 23 December 2008
^ "BREW Mobile Platform Overview", BREW 2008 Developers Conference, Qualcomm, May 2008, http://brew.qualcomm.com/brew_bnry/pdf/brew_2008/TECH-103.pdf, retrieved on 27 December 2008
^ "Lunatic Python", lua-users.org wiki, http://lua-users.org/en/LunaticPython, retrieved on 28 December 2008
^ "Lua", Tcl.tk wiki, http://wiki.tcl.tk/2255, retrieved on 28 December 2008
^ Paul Du Bois, "Psychonauts and Lua: a case study", Lua Workshop 2005, http://www.lua.org/wshop05.html#DuBois, retrieved on 28 December 2008

Ierusalimschy, R.; Figueiredo, L. H.; Celes, W. (2006), Lua 5.1 Reference Manual, Lua.org, ISBN 85-903798-3-3, http://www.lua.org/manual/5.1/ (paper book)
Ierusalimschy, R. (2006), Programming in Lua (2nd ed.), Lua.org, ISBN 85-903798-2-5, http://www.inf.puc-rio.br/~roberto/pil2/ (1st ed. is available online)
Figueiredo, L. H.; Celes, W.; Ierusalimschy, R., eds. (2008), Lua Programming Gems, Lua.org, ISBN 978-85-903798-4-3, http://www.lua.org/gems/
Gutschmidt, T. (2003), Game Programming with Python, Lua, and Ruby, Course Technology PTR, ISBN 1592000770
Schuytema, P.; Manyen, M. (2005), Game Development with Lua, Charles River Media, ISBN 1-58450-404-8
Jung, K.; Brown, A. (2007), Beginning Lua Programming, Wrox, ISBN 0-470-06917-1, http://www.wrox.com/WileyCDA/WroxTitle/productCd-0470069171.html
Lua papers and theses

Lua Programming Language.html

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