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<title>Lua 5.0 Reference Manual</title>
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Lua 5.0 Reference Manual
</h1>
by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes
<p>
<small>
<a HREF="http://www.lua.org/copyright.html">Copyright</a>
© 2003 TeCGraf, PUC-Rio. All rights reserved.
</small>
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<p>
<p>
<!-- ====================================================================== -->
<a name="intro"><h1>1 - Introduction</h1></a>
<p>Lua is an extension programming language designed to support
general procedural programming with data description
facilities.
It also offers good suport for object-oriented programming,
functional programming, and data-driven programming.
Lua is intended to be used as a powerful, light-weight
configuration language for any program that needs one.
Lua is implemented as a library, written in <em>clean</em> C
(that is, in the common subset of ANSI C and C++).
<p>Being an extension language, Lua has no notion of a "main" program:
it only works <em>embedded</em> in a host client,
called the <em>embedding program</em> or simply the <em>host</em>.
This host program can invoke functions to execute a piece of Lua code,
can write and read Lua variables,
and can register C functions to be called by Lua code.
Through the use of C functions, Lua can be augmented to cope with
a wide range of different domains,
thus creating customized programming languages sharing a syntactical framework.
<p>The Lua distribuition includes a stand-alone embedding program,
<code>lua</code>, that uses the Lua library to offer a complete Lua interpreter.
<p>Lua is free software,
and is provided as usual with no guarantees,
as stated in its copyright notice.
The implementation described in this manual is available
at Lua's official web site, <code>www.lua.org</code>.
<p>Like any other reference manual,
this document is dry in places.
For a discussion of the decisions behind the design of Lua,
see the papers below,
which are available at Lua's web site.
<ul>
<li>
R. Ierusalimschy, L. H. de Figueiredo, and W. Celes.
Lua---an extensible extension language.
<em>Software: Practice & Experience</em> <b>26</b> #6 (1996) 635-652.
<li>
L. H. de Figueiredo, R. Ierusalimschy, and W. Celes.
The design and implementation of a language for extending applications.
<em>Proceedings of XXI Brazilian Seminar on Software and Hardware</em> (1994) 273-283.
<li>
L. H. de Figueiredo, R. Ierusalimschy, and W. Celes.
Lua: an extensible embedded language.
<em>Dr. Dobb's Journal</em> <b>21</b> #12 (Dec 1996) 26-33.
<li>
R. Ierusalimschy, L. H. de Figueiredo, and W. Celes.
The evolution of an extension language: a history of Lua,
<em>Proceedings of V Brazilian Symposium on Programming Languages</em> (2001) B-14-B-28.
</ul>
<p>Lua means "moon" in Portuguese and is pronounced LOO-ah.
<p>
<a name="language"><h1>2 - The Language</h1></a>
<p>This section describes the lexis, the syntax, and the semantics of Lua.
In other words,
this section describes
which tokens are valid,
how they can be combined,
and what their combinations mean.
<p>The language constructs will be explained using the usual extended BNF,
in which
{<em>a</em>} means 0 or more <em>a</em>'s, and
[<em>a</em>] means an optional <em>a</em>.
Non-terminals are shown in <em>italics</em>,
keywords are shown in <b>bold</b>,
and other terminal symbols are shown in <code>typewriter</code> font,
enclosed in single quotes.
<p><a name="lexical"><h2>2.1 - Lexical Conventions</h2></a>
<p><em>Identifiers</em> in Lua can be any string of letters,
digits, and underscores,
not beginning with a digit.
This coincides with the definition of identifiers in most languages.
(The definition of letter depends on the current locale:
any character considered alphabetic by the current locale
can be used in an identifier.)
<p>The following <em>keywords</em> are reserved
and cannot be used as identifiers:
<PRE>
and break do else elseif
end false for function if
in local nil not or
repeat return then true until
while
</PRE>
<p>Lua is a case-sensitive language:
<code>and</code> is a reserved word, but <code>And</code> and <code>AND</code>
are two different, valid identifiers.
As a convention, identifiers starting with an underscore followed by
uppercase letters (such as <code>_VERSION</code>)
are reserved for internal variables used by Lua.
<p>The following strings denote other tokens:
<PRE>
+ - * / ^ =
~= <= >= < > ==
( ) { } [ ]
; : , . .. ...
</PRE>
<p><em>Literal strings</em>
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
<ul>
<li><b><code>\a</code></b> --- bell
<li><b><code>\b</code></b> --- backspace
<li><b><code>\f</code></b> --- form feed
<li><b><code>\n</code></b> --- newline
<li><b><code>\r</code></b> --- carriage return
<li><b><code>\t</code></b> --- horizontal tab
<li><b><code>\v</code></b> --- vertical tab
<li><b><code>\\</code></b> --- backslash
<li><b><code>\"</code></b> --- quotation mark
<li><b><code>\'</code></b> --- apostrophe
<li><b><code>\[</code></b> --- left square bracket
<li><b><code>\]</code></b> --- right square bracket
</ul>
Moreover, a `<code>\</code><em>newline</em>´
(that is, a backslash followed by a real newline)
results in a newline in the string.
A character in a string may also be specified by its numerical value
using the escape sequence `<code>\</code><em>ddd</em>´,
where <em>ddd</em> is a sequence of up to three decimal digits.
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as `<code>\0</code>´.
<p>Literal strings can also be delimited by matching double square brackets
<code>[[</code> · · · <code>]]</code>.
Literals in this bracketed form may run for several lines,
may contain nested <code>[[</code> · · · <code>]]</code> pairs,
and do not interpret any escape sequences.
For convenience,
when the opening `<code>[[</code>´ is immediately followed by a newline,
the newline is not included in the string.
As an example, in a system using ASCII
(in which `<code>a</code>´ is coded as 97,
newline is coded as 10, and `<code>1</code>´ is coded as 49),
the four literals below denote the same string:
<PRE>
(1) "alo\n123\""
(2) '\97lo\10\04923"'
(3) [[alo
123"]]
(4) [[
alo
123"]]
</PRE>
<p><em>Numerical constants</em> may be written with an optional decimal part
and an optional decimal exponent.
Examples of valid numerical constants are
<PRE>
3 3.0 3.1416 314.16e-2 0.31416E1
</PRE>
<p><em>Comments</em> start anywhere outside a string with a
double hyphen (<code>--</code>).
If the text immediately after <code>--</code> is different from <code>[[</code>,
the comment is a <em>short comment</em>,
which runs until the end of the line.
Otherwise, it is a <em>long comment</em>,
which runs until the corresponding <code>]]</code>.
Long comments may run for several lines
and may contain nested <code>[[</code> · · · <code>]]</code> pairs.
<p>For convenience,
the first line of a chunk is skipped if it starts with <code>#</code>.
This facility allows the use of Lua as a script interpreter
in Unix systems (see <a href="#lua-sa">6</a>).
<p><a name="TypesSec"><h2>2.2 - Values and Types</h2></a>
<p>Lua is a <em>dynamically typed language</em>.
That means that
variables do not have types; only values do.
There are no type definitions in the language.
All values carry their own type.
<p>There are eight basic types in Lua:
<em>nil</em>, <em>boolean</em>, <em>number</em>,
<em>string</em>, <em>function</em>, <em>userdata</em>, <em>thread</em>, and <em>table</em>.
<em>Nil</em> is the type of the value <B>nil</B>,
whose main property is to be different from any other value;
usually it represents the absence of a useful value.
<em>Boolean</em> is the type of the values <B>false</B> and <B>true</B>.
In Lua, both <B>nil</B> and <B>false</B> make a condition false;
any other value makes it true.
<em>Number</em> represents real (double-precision floating-point) numbers.
(It is easy to build Lua interpreters that use other
internal representations for numbers,
such as single-precision float or long integers.)
<em>String</em> represents arrays of characters.
Lua is 8-bit clean:
Strings may contain any 8-bit character,
including embedded zeros (<code>'\0'</code>) (see <a href="#lexical">2.1</a>).
<p>Functions are <em>first-class values</em> in Lua.
That means that functions can be stored in variables,
passed as arguments to other functions, and returned as results.
Lua can call (and manipulate) functions written in Lua and
functions written in C
(see <a href="#functioncall">2.5.7</a>).
<p>The type <em>userdata</em> is provided to allow arbitrary C data to
be stored in Lua variables.
This type corresponds to a block of raw memory
and has no pre-defined operations in Lua,
except assignment and identity test.
However, by using <em>metatables</em>,
the programmer can define operations for userdata values
(see <a href="#metatable">2.8</a>).
Userdata values cannot be created or modified in Lua,
only through the C API.
This guarantees the integrity of data owned by the host program.
<p>The type <em>thread</em> represents independent threads of execution
and it is used to implement coroutines.
<p>The type <em>table</em> implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except <B>nil</B>).
Moreover,
tables can be <em>heterogeneous</em>,
that is, they can contain values of all types (except <B>nil</B>).
Tables are the sole data structuring mechanism in Lua;
they may be used to represent ordinary arrays,
symbol tables, sets, records, graphs, trees, etc.
To represent records, Lua uses the field name as an index.
The language supports this representation by
providing <code>a.name</code> as syntactic sugar for <code>a["name"]</code>.
There are several convenient ways to create tables in Lua
(see <a href="#tableconstructor">2.5.6</a>).
<p>Like indices,
the value of a table field can be of any type (except <B>nil</B>).
In particular,
because functions are first class values,
table fields may contain functions.
Thus tables may also carry <em>methods</em> (see <a href="#func-def">2.5.8</a>).
<p>Tables, functions, and userdata values are <em>objects</em>:
variables do not actually <em>contain</em> these values,
only <em>references</em> to them.
Assignment, parameter passing, and function returns
always manipulate references to such values;
these operations do not imply any kind of copy.
<p>The library function <code>type</code> returns a string describing the type
of a given value (see <a href="#pdf-type">5.1</a>).
<p><a name="coercion"><h3>2.2.1 - Coercion</h3></a>
<p>Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
that string to a number, following the usual rules.
Conversely, whenever a number is used where a string is expected,
the number is converted to a string, in a reasonable format.
For complete control of how numbers are converted to strings,
use the <code>format</code> function from the string library (see <a href="#format">5.3</a>).
<p><a name="variables"><h2>2.3 - Variables</h2></a>
<p>Variables are places that store values.
There are three kinds of variables in Lua:
global variables, local variables, and table fields.
<p>A single name can denote a global variable or a local variable
(or a formal parameter of a function,
which is a particular form of local variable):
<pre>
var ::= Name
</pre>
Variables are assumed to be global unless explicitly declared local
(see <a href="#localvar">2.4.7</a>).
Local variables are <em>lexically scoped</em>:
Local variables can be freely accessed by functions
defined inside their scope (see <a href="#visibility">2.6</a>).
<p>Before the first assignment to a variable, its value is <B>nil</B>.
<p>Square brackets are used to index a table:
<pre>
var ::= prefixexp `<b>[</b>´ exp `<b>]</b>´
</pre>
The first expression (<em>prefixexp</em>)should result in a table value;
the second expression (<em>exp</em>)
identifies a specific entry inside that table.
The expression denoting the table to be indexed has a restricted syntax;
see <a href="#expressions">2.5</a> for details.
<p>The syntax <code>var.NAME</code> is just syntactic sugar for
<code>var["NAME"]</code>:
<pre>
var ::= prefixexp `<b>.</b>´ Name
</pre>
<p>The meaning of accesses to global variables
and table fields can be changed via metatables.
An access to an indexed variable <code>t[i]</code> is equivalent to
a call <code>gettable_event(t,i)</code>.
(See <a href="#metatable">2.8</a> for a complete description of the
<code>gettable_event</code> function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
<p>All global variables live as fields in ordinary Lua tables,
called <em>environment tables</em> or simply <em>environments</em>.
Functions written in C and exported to Lua (<em>C functions</em>)
all share a common <em>global environment</em>.
Each function written in Lua (a <em>Lua function</em>)
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