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Paradigm Designed by Roberto IerusalimschyWaldemar CelesLuiz Henrique de Figueiredo First appeared 1993; 24 years ago (1993) Stable release 5.3.4 / 30 January 2017 (2017-01-30) | ||
Lua (/ˈluːə/ LOO-ə, from Portuguese: lua [ˈlu.(w)ɐ] meaning moon) is a lightweight multi-paradigm programming language designed primarily for embedded systems and clients. Lua is cross-platform, since it is written in ANSI C, and has a relatively simple C API.
Contents
- History
- Features
- Example code
- Loops
- Functions
- Tables
- As record
- As namespace
- As array
- Metatables
- Object oriented programming
- Internals
- C API
- Stack
- Example
- Special tables
- Extension and binding
- Video games
- Other
- References
Lua was originally designed in 1993 as a language for extending software applications to meet the increasing demand for customization at the time. It provided the basic facilities of most procedural programming languages, but more complicated or domain-specific features were not included; rather, it included mechanisms for extending the language, allowing programmers to implement such features. As Lua was intended to be a general embeddable extension language, the designers of Lua focused on improving its speed, portability, extensibility, and ease-of-use in development.
History
Lua was created in 1993 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, members of the Computer Graphics Technology Group (Tecgraf) at the Pontifical Catholic University of Rio de Janeiro, in Brazil.
From 1977 until 1992, Brazil had a policy of strong trade barriers (called a market reserve) for computer hardware and software. In that atmosphere, Tecgraf's clients could not afford, either politically or financially, to buy customized software from abroad. Those reasons led Tecgraf to implement the basic tools it needed from scratch.
Lua's historical "father and mother" 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.
As the language's authors wrote in The Evolution of Lua:
In 1993, the only real contender was Tcl, which had been explicitly designed to be embedded into applications. However, Tcl had unfamiliar syntax, did not offer good support for data description, and ran only on Unix platforms. We did not consider LISP or Scheme because of their unfriendly syntax. Python was still in its infancy. In the free, do-it-yourself atmosphere that then reigned in Tecgraf, it was quite natural that we should try to develop our own scripting language ... Because many potential users of the language were not professional programmers, the language should avoid cryptic syntax and semantics. The implementation of the new language should be highly portable, because Tecgraf's clients had a very diverse collection of computer platforms. Finally, since we expected that other Tecgraf products would also need to embed a scripting language, the new language should follow the example of SOL and be provided as a library with a C API.
Lua 1.0 was designed in such a way that its object constructors, being then slightly different from the current light and flexible style, incorporated the data-description syntax of SOL (hence the name Lua: Sol is also the Portuguese word for "Sun", Lua being the one for "Moon"). 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"), SNOBOL and AWK (associative arrays). In an article published in Dr. Dobb's Journal, Lua's creators also state that LISP and Scheme with their single, ubiquitous data-structure mechanism (the list) were a major influence on their decision to develop the table as the primary data structure of Lua.
Lua semantics have been increasingly influenced by Scheme over time, especially with the introduction of anonymous functions and full lexical scoping.
Versions of Lua prior to version 5.0 were released under a license similar to the BSD license. From version 5.0 onwards, Lua has been licensed under the MIT License. Both are permissive free software licences and are almost identical.
Features
Lua is commonly described as a "multi-paradigm" language, providing a small set of general features that can be extended to fit different problem types, 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 with metatables. Similarly, Lua allows programmers to implement namespaces, classes, and other related features using its single table implementation; first-class functions allow the employment of many techniques from functional programming; and full lexical scoping allows fine-grained information hiding to enforce the principle of least privilege.
In general, Lua strives to provide simple, flexible meta-features that can be extended as needed, rather than supply a feature-set specific to one programming paradigm. As a result, the base language is light – the full reference interpreter is only about 180 kB 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 minimal 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:
It can also be written as
or, the example given on the Lua website,
Comments use the following syntax, similar to that of Ada, Eiffel, Haskell, SQL and VHDL:
The factorial function is implemented as a function in this example:
Loops
Lua has four types of loops: the while loop, the repeat loop (similar to a do while loop), the numeric for loop, and the generic for loop.
The generic for loop:
would iterate over the table _G using the standard iterator function pairs, until it returns nil.
Functions
Lua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified:
Any future calls to print will now be routed through the new function, and because of Lua's lexical scoping, the old print function will only be accessible by the new, modified print.
Lua also supports closures, as demonstrated below:
A new closure for the variable 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 Lua and are the foundation of all user-created types. They are conceptually similar to associative arrays in PHP, dictionaries in Python and hashes in Ruby or Perl.
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. A key (index) can be any value except nil and NaN. A numeric key 1 is considered distinct from a string key "1".
Tables are created using the {} constructor syntax:
Tables are always passed by reference (see Call by sharing):
As record
A table is often used as structure (or record) by using strings as keys. Because such use is very common, Lua features a special syntax for accessing such fields. Example:
Quoting the Lua 5.1 Reference Manual:
"The syntax var.Name is just syntactic sugar for var['Name'];"
As namespace
By using a table to store related functions, it can act as a namespace.
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:
The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] 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).
A two-dimensional table:
An array of objects:
Using a hash map to emulate an array normally is slower than using an actual array; however, Lua tables are optimized for use as arrays to help avoid this issue.
Metatables
Extensible semantics is a key feature of Lua, and the metatable concept allows Lua's tables to be customized in powerful 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.
Object-oriented programming
Although Lua does not have a built-in concept of classes, object-oriented programming can be achieved 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 using the metatable mechanism, telling the object to look up 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).
Creating a basic vector object:
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 invisible 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. 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 5.3.3 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. Perl's Parrot 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 the luac 5.1 compiler):
function <factorial.lua:1,7> (9 instructions, 36 bytes at 0x8063c60)1 param, 6 slots, 0 upvalues, 6 locals, 2 constants, 0 functions 1 [2] LOADK 1 -1 ; 1 2 [3] LOADK 2 -2 ; 2 3 [3] MOVE 3 0 4 [3] LOADK 4 -1 ; 1 5 [3] FORPREP 2 1 ; to 7 6 [4] MUL 1 1 5 7 [3] FORLOOP 2 -2 ; to 6 8 [6] RETURN 1 2 9 [7] RETURN 0 1C API
Lua is intended to be embedded into other applications, and provides a C API for this purpose. The API is divided into two parts: the Lua core and the Lua auxiliary library.
The Lua API's design eliminates the need for manual reference management in C code, unlike Python's API. The API, like the language, is minimalistic. Advanced functionality is provided by the auxiliary library, which consists largely of preprocessor macros which assist with complex table operations.
Stack
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.
Example
Here is an example of calling a Lua function from C:
Running this example gives:
$ cc -o example example.c -llua$ ./exampleResult: 8Special tables
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. 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, just like modules written in Lua itself.
A growing collection of modules known as rocks are available through a package management system called LuaRocks, in the spirit of CPAN, RubyGems and Python Eggs.
Prewritten Lua bindings exist for most popular programming languages, including other scripting languages. For C++, there are a number of template-based approaches and some automatic binding generators.
Video games
In video game development, Lua is widely used as a scripting language by game programmers, perhaps due to its perceived easiness to embed, fast execution, and short learning curve.
In 2003, a poll conducted by GameDev.net showed Lua as 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.
Other
Other applications using Lua include: