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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>

<head>
<title>Teliva Reference Manual</title>
<link rel="stylesheet" type="text/css" href="lua.css">
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<body>

<hr>
<h1>
Teliva Reference Manual
</h1>

<small>
by Kartik Agaram
</small>

<p>
Based on <a href="http://www.lua.org/">Lua 5.1</a> by Roberto Ierusalimschy,
Luiz Henrique de Figueiredo, Waldemar Celes

<p>
<small>
Copyright &copy; 2006&ndash;2012 Lua.org, PUC-Rio.
Freely available under the terms of the
<a href="http://www.lua.org/license.html">Lua license</a>.
</small>

<hr>
<p>

<a href="contents.html#contents">contents</A>
&middot;
<a href="contents.html#index">index</A>
&middot;
<A HREF="http://www.lua.org/manual/">other versions</A>

<!-- ====================================================================== -->
<p>

<!-- $Id: manual.of,v 1.49.1.2 2012/01/13 20:23:26 roberto Exp $ -->




<h1>1 - <a name="1">Introduction</a></h1>

Teliva is a platform based on Lua for sandboxed software packaged with an
environment for making changes to it. For a more detailed introduction, see
<a href='../README.md'>the Readme</a>.

<p>
Teliva is free software, and is provided as usual with no guarantees, as
stated in its license.

<p>
This manual is based on the Lua manual. Lua features absent in Teliva should
be absent in this manual as well. <span class='teliva'>Features added to
Teliva above and beyond Lua will be described in text like this.</span>

<p>
For a discussion of the decisions behind the design of Lua,
see the technical papers available at Lua's web site.
For a detailed introduction to programming in Lua,
see Roberto's book, <em>Programming in Lua (Second Edition)</em>.



<h1>2 - <a name="2">The Language</a></h1>

<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 notation,
in which
{<em>a</em>}&nbsp;means&nbsp;0 or more <em>a</em>'s, and
[<em>a</em>]&nbsp;means an optional <em>a</em>.
Non-terminals are shown like non-terminal,
keywords are shown like <b>kword</b>,
and other terminal symbols are shown like `<b>=</b>&acute;.
The complete syntax of Lua can be found in <a href="#8">&sect;8</a>
at the end of this manual.



<h2>2.1 - <a name="2.1">Lexical Conventions</a></h2>

<p>
<em>Names</em>
(also called <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 names 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.)
Identifiers are used to name variables and table fields.


<p>
The following <em>keywords</em> are reserved
and cannot be used as names:


<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 names.
As a convention, names starting with an underscore followed by
uppercase letters (such as <a href="#pdf-_VERSION"><code>_VERSION</code></a>)
are reserved for internal global variables used by Lua.


<p>
The following strings denote other tokens:

<pre>
     +     -     *     /     %     ^     #
     ==    ~=    &lt;=    &gt;=    &lt;     &gt;     =
     (     )     {     }     [     ]
     ;     :     ,     .     ..    ...
</pre>

<p>
<em>Literal strings</em>
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
'<code>\a</code>' (bell),
'<code>\b</code>' (backspace),
'<code>\f</code>' (form feed),
'<code>\n</code>' (newline),
'<code>\r</code>' (carriage return),
'<code>\t</code>' (horizontal tab),
'<code>\v</code>' (vertical tab),
'<code>\\</code>' (backslash),
'<code>\"</code>' (quotation mark [double quote]),
and '<code>\'</code>' (apostrophe [single quote]).
Moreover, a backslash followed by a real newline
results in a newline in the string.
A character in a string can also be specified by its numerical value
using the escape sequence <code>\<em>ddd</em></code>,
where <em>ddd</em> is a sequence of up to three decimal digits.
(Note that if a numerical escape is to be followed by a digit,
it must be expressed using exactly three digits.)
Strings in Lua can contain any 8-bit value, including embedded zeros,
which can be specified as '<code>\0</code>'.


<p>
Literal strings can also be defined using a long format
enclosed by <em>long brackets</em>.
We define an <em>opening long bracket of level <em>n</em></em> as an opening
square bracket followed by <em>n</em> equal signs followed by another
opening square bracket.
So, an opening long bracket of level&nbsp;0 is written as <code>[[</code>,
an opening long bracket of level&nbsp;1 is written as <code>[=[</code>,
and so on.
A <em>closing long bracket</em> is defined similarly;
for instance, a closing long bracket of level&nbsp;4 is written as <code>]====]</code>.
A long string starts with an opening long bracket of any level and
ends at the first closing long bracket of the same level.
Literals in this bracketed form can run for several lines,
do not interpret any escape sequences,
and ignore long brackets of any other level.
They can contain anything except a closing bracket of the proper level.


<p>
For convenience,
when the opening long bracket 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&nbsp;97,
newline is coded as&nbsp;10, and '<code>1</code>' is coded as&nbsp;49),
the five literal strings below denote the same string:

<pre>
     a = 'alo\n123"'
     a = "alo\n123\""
     a = '\97lo\10\04923"'
     a = [[alo
     123"]]
     a = [==[
     alo
     123"]==]
</pre>

<p>
A <em>numerical constant</em> can be written with an optional decimal part
and an optional decimal exponent.
Lua also accepts integer hexadecimal constants,
by prefixing them with <code>0x</code>.
Examples of valid numerical constants are

<pre>
     3   3.0   3.1416   314.16e-2   0.31416E1   0xff   0x56
</pre>

<p>
A <em>comment</em> starts with a double hyphen (<code>--</code>)
anywhere outside a string.
If the text immediately after <code>--</code> is not an opening long bracket,
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 closing long bracket.
Long comments are frequently used to disable code temporarily.





<h2>2.2 - <a name="2.2">Values and Types</a></h2>

<p>
Lua is a <em>dynamically typed language</em>.
This 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>
All values in Lua are <em>first-class values</em>.
This means that all values can be stored in variables,
passed as arguments to other functions, and returned as results.


<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;
it usually represents the absence of a useful value.
<em>Boolean</em> is the type of the values <b>false</b> and <b>true</b>.
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;
see file <code>luaconf.h</code>.)
<em>String</em> represents arrays of characters.

Lua is 8-bit clean:
strings can contain any 8-bit character,
including embedded zeros ('<code>\0</code>') (see <a href="#2.1">&sect;2.1</a>).


<p>
<div class='teliva'>The type <em>userdata</em> is opaque to Teliva, and
used by low-level code to manage details of C&nbsp;data stored in Lua
variables. Unlike Lua, Teliva doesn't support creating or managing userdata
since it doesn't permit extension by C libraries. This manual doesn't allude
to userdata further.</div>

<p>
The type <em>thread</em> represents independent threads of execution
and it is used to implement coroutines (see <a href="#2.11">&sect;2.11</a>).
Do not confuse Lua threads with operating-system threads.
Lua supports coroutines on all systems,
even those that do not support threads.


<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>).
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 can 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="#2.5.7">&sect;2.5.7</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 can contain functions.
Thus tables can also carry <em>methods</em> (see <a href="#2.5.9">&sect;2.5.9</a>).


<p>
Tables, functions and threads 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 <a href="#pdf-type"><code>type</code></a> returns a string describing the type
of a given value.



<h3>2.2.1 - <a name="2.2.1">Coercion</a></h3>

<p>
Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
this string to a number, following the usual conversion 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 over how numbers are converted to strings,
use the <code>format</code> function from the string library
(see <a href="#pdf-string.format"><code>string.format</code></a>).







<h2>2.3 - <a name="2.3">Variables</a></h2>

<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 function's formal parameter,
which is a particular kind of local variable):

<pre>
	var ::= Name
</pre><p>
Name denotes identifiers, as defined in <a href="#2.1">&sect;2.1</a>.


<p>
Any variable is assumed to be global unless explicitly declared
as a local (see <a href="#2.4.7">&sect;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="#2.6">&sect;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>&acute; exp `<b>]</b>&acute;
</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="#2.8">&sect;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>
The syntax <code>var.Name</code> is just syntactic sugar for
<code>var["Name"]</code>:

<pre>
	var ::= prefixexp `<b>.</b>&acute; Name
</pre>

<p>
All global variables live as fields in ordinary Lua tables,
called <em>environment tables</em> or simply
<em>environments</em> (see <a href="#2.9">&sect;2.9</a>).
Each function has its own reference to an environment,
so that all global variables in this function
will refer to this environment table.
When a function is created,
it inherits the environment from the function that created it.
To get the environment table of a Lua function,
you call <a href="#pdf-getfenv"><code>getfenv</code></a>.
To replace it,
you call <a href="#pdf-setfenv"><code>setfenv</code></a>.


<p>
An access to a global variable <code>x</code>
is equivalent to <code>_env.x</code>,
which in turn is equivalent to

<pre>
     gettable_event(_env, "x")
</pre><p>
where <code>_env</code> is the environment of the running function.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>gettable_event</code> function.
This function is not defined or callable in Lua.
Similarly, the <code>_env</code> variable is not defined in Lua.
We use them here only for explanatory purposes.)





<h2>2.4 - <a name="2.4">Statements</a></h2>

<p>
Lua supports an almost conventional set of statements,
similar to those in Pascal or C.
This set includes
assignments, control structures, function calls,
and variable declarations.



<h3>2.4.1 - <a name="2.4.1">Chunks</a></h3>

<p>
The unit of execution of Lua is called a <em>chunk</em>.
A chunk is simply a sequence of statements,
which are executed sequentially.
Each statement can be optionally followed by a semicolon:

<pre>
	chunk ::= {stat [`<b>;</b>&acute;]}
</pre><p>
There are no empty statements and thus '<code>;;</code>' is not legal.


<p>
Lua handles a chunk as the body of an anonymous function 
with a variable number of arguments
(see <a href="#2.5.9">&sect;2.5.9</a>).
As such, chunks can define local variables,
receive arguments, and return values.


<p>
A chunk can be stored in a file or in a string inside the host program.
To execute a chunk,
Lua first pre-compiles the chunk into instructions for a virtual machine,
and then it executes the compiled code
with an interpreter for the virtual machine.


<p>
Chunks can also be pre-compiled into binary form;
see program <code>luac</code> for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.






<h3>2.4.2 - <a name="2.4.2">Blocks</a></h3><p>
A block is a list of statements;
syntactically, a block is the same as a chunk:

<pre>
	block ::= chunk
</pre>

<p>
A block can be explicitly delimited to produce a single statement:

<pre>
	stat ::= <b>do</b> block <b>end</b>
</pre><p>
Explicit blocks are useful
to control the scope of variable declarations.
Explicit blocks are also sometimes used to
add a <b>return</b> or <b>break</b> statement in the middle
of another block (see <a href="#2.4.4">&sect;2.4.4</a>).





<h3>2.4.3 - <a name="2.4.3">Assignment</a></h3>

<p>
Lua allows multiple assignments.
Therefore, the syntax for assignment
defines a list of variables on the left side
and a list of expressions on the right side.
The elements in both lists are separated by commas:

<pre>
	stat ::= varlist `<b>=</b>&acute; explist
	varlist ::= var {`<b>,</b>&acute; var}
	explist ::= exp {`<b>,</b>&acute; exp}
</pre><p>
Expressions are discussed in <a href="#2.5">&sect;2.5</a>.


<p>
Before the assignment,
the list of values is <em>adjusted</em> to the length of
the list of variables.
If there are more values than needed,
the excess values are thrown away.
If there are fewer values than needed,
the list is extended with as many  <b>nil</b>'s as needed.
If the list of expressions ends with a function call,
then all values returned by that call enter the list of values,
before the adjustment
(except when the call is enclosed in parentheses; see <a href="#2.5">&sect;2.5</a>).


<p>
The assignment statement first evaluates all its expressions
and only then are the assignments performed.
Thus the code

<pre>
     i = 3
     i, a[i] = i+1, 20
</pre><p>
sets <code>a[3]</code> to 20, without affecting <code>a[4]</code>
because the <code>i</code> in <code>a[i]</code> is evaluated (to 3)
before it is assigned&nbsp;4.
Similarly, the line

<pre>
     x, y = y, x
</pre><p>
exchanges the values of <code>x</code> and <code>y</code>,
and

<pre>
     x, y, z = y, z, x
</pre><p>
cyclically permutes the values of <code>x</code>, <code>y</code>, and <code>z</code>.


<p>
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable <code>t[i] = val</code> is equivalent to
<code>settable_event(t,i,val)</code>.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>settable_event</code> function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)


<p>
An assignment to a global variable <code>x = val</code>
is equivalent to the assignment
<code>_env.x = val</code>,
which in turn is equivalent to

<pre>
     settable_event(_env, "x", val)
</pre><p>
where <code>_env</code> is the environment of the running function.
(The <code>_env</code> variable is not defined in Lua.
We use it here only for explanatory purposes.)





<h3>2.4.4 - <a name="2.4.4">Control Structures</a></h3><p>
The control structures
<b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and
familiar syntax:




<pre>
	stat ::= <b>while</b> exp <b>do</b> block <b>end</b>
	stat ::= <b>repeat</b> block <b>until</b> exp
	stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b>
</pre><p>
Lua also has a <b>for</b> statement, in two flavors (see <a href="#2.4.5">&sect;2.4.5</a>).


<p>
The condition expression of a
control structure can return any value.
Both <b>false</b> and <b>nil</b> are considered false.
All values different from <b>nil</b> and <b>false</b> are considered true
(in particular, the number 0 and the empty string are also true).


<p>
In the <b>repeat</b>&ndash;<b>until</b> loop,
the inner block does not end at the <b>until</b> keyword,
but only after the condition.
So, the condition can refer to local variables
declared inside the loop block.


<p>
The <b>return</b> statement is used to return values
from a function or a chunk (which is just a function).

Functions and chunks can return more than one value,
and so the syntax for the <b>return</b> statement is

<pre>
	stat ::= <b>return</b> [explist]
</pre>

<p>
The <b>break</b> statement is used to terminate the execution of a
<b>while</b>, <b>repeat</b>, or <b>for</b> loop,
skipping to the next statement after the loop:


<pre>
	stat ::= <b>break</b>
</pre><p>
A <b>break</b> ends the innermost enclosing loop.


<p>
The <b>return</b> and <b>break</b>
statements can only be written as the <em>last</em> statement of a block.
If it is really necessary to <b>return</b> or <b>break</b> in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
<code>do return end</code> and <code>do break end</code>,
because now <b>return</b> and <b>break</b> are the last statements in
their (inner) blocks.





<h3>2.4.5 - <a name="2.4.5">For Statement</a></h3>

<p>

The <b>for</b> statement has two forms:
one numeric and one generic.


<p>
The numeric <b>for</b> loop repeats a block of code while a
control variable runs through an arithmetic progression.
It has the following syntax:

<pre>
	stat ::= <b>for</b> Name `<b>=</b>&acute; exp `<b>,</b>&acute; exp [`<b>,</b>&acute; exp] <b>do</b> block <b>end</b>
</pre><p>
The <em>block</em> is repeated for <em>name</em> starting at the value of
the first <em>exp</em>, until it passes the second <em>exp</em> by steps of the
third <em>exp</em>.
More precisely, a <b>for</b> statement like

<pre>
     for v = <em>e1</em>, <em>e2</em>, <em>e3</em> do <em>block</em> end
</pre><p>
is equivalent to the code:

<pre>
     do
       local <em>var</em>, <em>limit</em>, <em>step</em> = tonumber(<em>e1</em>), tonumber(<em>e2</em>), tonumber(<em>e3</em>)
       if not (<em>var</em> and <em>limit</em> and <em>step</em>) then error() end
       while (<em>step</em> &gt; 0 and <em>var</em> &lt;= <em>limit</em>) or (<em>step</em> &lt;= 0 and <em>var</em> &gt;= <em>limit</em>) do
         local v = <em>var</em>
         <em>block</em>
         <em>var</em> = <em>var</em> + <em>step</em>
       end
     end
</pre><p>
Note the following:

<ul>

<li>
All three control expressions are evaluated only once,
before the loop starts.
They must all result in numbers.
</li>

<li>
<code><em>var</em></code>, <code><em>limit</em></code>, and <code><em>step</em></code> are invisible variables.
The names shown here are for explanatory purposes only.
</li>

<li>
If the third expression (the step) is absent,
then a step of&nbsp;1 is used.
</li>

<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>

<li>
The loop variable <code>v</code> is local to the loop;
you cannot use its value after the <b>for</b> ends or is broken.
If you need this value,
assign it to another variable before breaking or exiting the loop.
</li>

</ul>

<p>
The generic <b>for</b> statement works over functions,
called <em>iterators</em>.
On each iteration, the iterator function is called to produce a new value,
stopping when this new value is <b>nil</b>.
The generic <b>for</b> loop has the following syntax:

<pre>
	stat ::= <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b>
	namelist ::= Name {`<b>,</b>&acute; Name}
</pre><p>
A <b>for</b> statement like

<pre>
     for <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> in <em>explist</em> do <em>block</em> end
</pre><p>
is equivalent to the code:

<pre>
     do
       local <em>f</em>, <em>s</em>, <em>var</em> = <em>explist</em>
       while true do
         local <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> = <em>f</em>(<em>s</em>, <em>var</em>)
         <em>var</em> = <em>var_1</em>
         if <em>var</em> == nil then break end
         <em>block</em>
       end
     end
</pre><p>
Note the following:

<ul>

<li>
<code><em>explist</em></code> is evaluated only once.
Its results are an <em>iterator</em> function,
a <em>state</em>,
and an initial value for the first <em>iterator variable</em>.
</li>

<li>
<code><em>f</em></code>, <code><em>s</em></code>, and <code><em>var</em></code> are invisible variables.
The names are here for explanatory purposes only.
</li>

<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>

<li>
The loop variables <code><em>var_i</em></code> are local to the loop;
you cannot use their values after the <b>for</b> ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
</li>

</ul>




<h3>2.4.6 - <a name="2.4.6">Function Calls as Statements</a></h3><p>
To allow possible side-effects,
function calls can be executed as statements:

<pre>
	stat ::= functioncall
</pre><p>
In this case, all returned values are thrown away.
Function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>.





<h3>2.4.7 - <a name="2.4.7">Local Declarations</a></h3><p>
Local variables can be declared anywhere inside a block.
The declaration can include an initial assignment:

<pre>
	stat ::= <b>local</b> namelist [`<b>=</b>&acute; explist]
</pre><p>
If present, an initial assignment has the same semantics
of a multiple assignment (see <a href="#2.4.3">&sect;2.4.3</a>).
Otherwise, all variables are initialized with <b>nil</b>.


<p>
A chunk is also a block (see <a href="#2.4.1">&sect;2.4.1</a>),
and so local variables can be declared in a chunk outside any explicit block.
The scope of such local variables extends until the end of the chunk.


<p>
The visibility rules for local variables are explained in <a href="#2.6">&sect;2.6</a>.







<h2>2.5 - <a name="2.5">Expressions</a></h2>

<p>
The basic expressions in Lua are the following:

<pre>
	exp ::= prefixexp
	exp ::= <b>nil</b> | <b>false</b> | <b>true</b>
	exp ::= Number
	exp ::= String
	exp ::= function
	exp ::= tableconstructor
	exp ::= `<b>...</b>&acute;
	exp ::= exp binop exp
	exp ::= unop exp
	prefixexp ::= var | functioncall | `<b>(</b>&acute; exp `<b>)</b>&acute;
</pre>

<p>
Numbers and literal strings are explained in <a href="#2.1">&sect;2.1</a>;
variables are explained in <a href="#2.3">&sect;2.3</a>;
function definitions are explained in <a href="#2.5.9">&sect;2.5.9</a>;
function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>;
table constructors are explained in <a href="#2.5.7">&sect;2.5.7</a>.
Vararg expressions,
denoted by three dots ('<code>...</code>'), can only be used when
directly inside a vararg function;
they are explained in <a href="#2.5.9">&sect;2.5.9</a>.


<p>
Binary operators comprise arithmetic operators (see <a href="#2.5.1">&sect;2.5.1</a>),
relational operators (see <a href="#2.5.2">&sect;2.5.2</a>), logical operators (see <a href="#2.5.3">&sect;2.5.3</a>),
and the concatenation operator (see <a href="#2.5.4">&sect;2.5.4</a>).
Unary operators comprise the unary minus (see <a href="#2.5.1">&sect;2.5.1</a>),
the unary <b>not</b> (see <a href="#2.5.3">&sect;2.5.3</a>),
and the unary <em>length operator</em> (see <a href="#2.5.5">&sect;2.5.5</a>).


<p>
Both function calls and vararg expressions can result in multiple values.
If an expression is used as a statement
(only possible for function calls (see <a href="#2.4.6">&sect;2.4.6</a>)),
then its return list is adjusted to zero elements,
thus discarding all returned values.
If an expression is used as the last (or the only) element
of a list of expressions,
then no adjustment is made
(unless the call is enclosed in parentheses).
In all other contexts,
Lua adjusts the result list to one element,
discarding all values except the first one.


<p>
Here are some examples:

<pre>
     f()                -- adjusted to 0 results
     g(f(), x)          -- f() is adjusted to 1 result
     g(x, f())          -- g gets x plus all results from f()
     a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)
     a,b = ...          -- a gets the first vararg parameter, b gets
                        -- the second (both a and b can get nil if there
                        -- is no corresponding vararg parameter)
     
     a,b,c = x, f()     -- f() is adjusted to 2 results
     a,b,c = f()        -- f() is adjusted to 3 results
     return f()         -- returns all results from f()
     return ...         -- returns all received vararg parameters
     return x,y,f()     -- returns x, y, and all results from f()
     {f()}              -- creates a list with all results from f()
     {...}              -- creates a list with all vararg parameters
     {f(), nil}         -- f() is adjusted to 1 result
</pre>

<p>
Any expression enclosed in parentheses always results in only one value.
Thus,
<code>(f(x,y,z))</code> is always a single value,
even if <code>f</code> returns several values.
(The value of <code>(f(x,y,z))</code> is the first value returned by <code>f</code>
or <b>nil</b> if <code>f</code> does not return any values.)



<h3>2.5.1 - <a name="2.5.1">Arithmetic Operators</a></h3><p>
Lua supports the usual arithmetic operators:
the binary <code>+</code> (addition),
<code>-</code> (subtraction), <code>*</code> (multiplication),
<code>/</code> (division), <code>%</code> (modulo), and <code>^</code> (exponentiation);
and unary <code>-</code> (negation).
If the operands are numbers, or strings that can be converted to
numbers (see <a href="#2.2.1">&sect;2.2.1</a>),
then all operations have the usual meaning.
Exponentiation works for any exponent.
For instance, <code>x^(-0.5)</code> computes the inverse of the square root of <code>x</code>.
Modulo is defined as

<pre>
     a % b == a - math.floor(a/b)*b
</pre><p>
That is, it is the remainder of a division that rounds
the quotient towards minus infinity.





<h3>2.5.2 - <a name="2.5.2">Relational Operators</a></h3><p>
The relational operators in Lua are

<pre>
     ==    ~=    &lt;     &gt;     &lt;=    &gt;=
</pre><p>
These operators always result in <b>false</b> or <b>true</b>.


<p>
Equality (<code>==</code>) first compares the type of its operands.
If the types are different, then the result is <b>false</b>.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Objects (tables, threads, and functions)
are compared by <em>reference</em>:
two objects are considered equal only if they are the <em>same</em> object.
Every time you create a new object
(a table, thread, or function),
this new object is different from any previously existing object.


<p>
You can change the way that Lua compares tables by using the "eq" metamethod
(see <a href="#2.8">&sect;2.8</a>).


<p>
The conversion rules of <a href="#2.2.1">&sect;2.2.1</a>
<em>do not</em> apply to equality comparisons.
Thus, <code>"0"==0</code> evaluates to <b>false</b>,
and <code>t[0]</code> and <code>t["0"]</code> denote different
entries in a table.


<p>
The operator <code>~=</code> is exactly the negation of equality (<code>==</code>).


<p>
The order operators work as follows.
If both arguments are numbers, then they are compared as such.
Otherwise, if both arguments are strings,
then their values are compared according to the current locale.
Otherwise, Lua tries to call the "lt" or the "le"
metamethod (see <a href="#2.8">&sect;2.8</a>).
A comparison <code>a &gt; b</code> is translated to <code>b &lt; a</code>
and <code>a &gt;= b</code> is translated to <code>b &lt;= a</code>.





<h3>2.5.3 - <a name="2.5.3">Logical Operators</a></h3><p>
The logical operators in Lua are
<b>and</b>, <b>or</b>, and <b>not</b>.
Like the control structures (see <a href="#2.4.4">&sect;2.4.4</a>),
all logical operators consider both <b>false</b> and <b>nil</b> as false
and anything else as true.


<p>
The negation operator <b>not</b> always returns <b>false</b> or <b>true</b>.
The conjunction operator <b>and</b> returns its first argument
if this value is <b>false</b> or <b>nil</b>;
otherwise, <b>and</b> returns its second argument.
The disjunction operator <b>or</b> returns its first argument
if this value is different from <b>nil</b> and <b>false</b>;
otherwise, <b>or</b> returns its second argument.
Both <b>and</b> and <b>or</b> use short-cut evaluation;
that is,
the second operand is evaluated only if necessary.
Here are some examples:

<pre>
     10 or 20            --&gt; 10
     10 or error()       --&gt; 10
     nil or "a"          --&gt; "a"
     nil and 10          --&gt; nil
     false and error()   --&gt; false
     false and nil       --&gt; false
     false or nil        --&gt; nil
     10 and 20           --&gt; 20
</pre><p>
(In this manual,
<code>--&gt;</code> indicates the result of the preceding expression.)





<h3>2.5.4 - <a name="2.5.4">Concatenation</a></h3><p>
The string concatenation operator in Lua is
denoted by two dots ('<code>..</code>').
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in <a href="#2.2.1">&sect;2.2.1</a>.
Otherwise, the "concat" metamethod is called (see <a href="#2.8">&sect;2.8</a>).





<h3>2.5.5 - <a name="2.5.5">The Length Operator</a></h3>

<p>
The length operator is denoted by the unary operator <code>#</code>.
The length of a string is its number of bytes
(that is, the usual meaning of string length when each
character is one byte).


<p>
The length of a table <code>t</code> is defined to be any
integer index <code>n</code>
such that <code>t[n]</code> is not <b>nil</b> and <code>t[n+1]</code> is <b>nil</b>;
moreover, if <code>t[1]</code> is <b>nil</b>, <code>n</code> can be zero.
For a regular array, with non-nil values from 1 to a given <code>n</code>,
its length is exactly that <code>n</code>,
the index of its last value.
If the array has "holes"
(that is, <b>nil</b> values between other non-nil values),
then <code>#t</code> can be any of the indices that
directly precedes a <b>nil</b> value
(that is, it may consider any such <b>nil</b> value as the end of
the array). 





<h3>2.5.6 - <a name="2.5.6">Precedence</a></h3><p>
Operator precedence in Lua follows the table below,
from lower to higher priority:

<pre>
     or
     and
     &lt;     &gt;     &lt;=    &gt;=    ~=    ==
     ..
     +     -
     *     /     %
     not   #     - (unary)
     ^
</pre><p>
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation ('<code>..</code>') and exponentiation ('<code>^</code>')
operators are right associative.
All other binary operators are left associative.





<h3>2.5.7 - <a name="2.5.7">Table Constructors</a></h3><p>
Table constructors are expressions that create tables.
Every time a constructor is evaluated, a new table is created.
A constructor can be used to create an empty table
or to create a table and initialize some of its fields.
The general syntax for constructors is

<pre>
	tableconstructor ::= `<b>{</b>&acute; [fieldlist] `<b>}</b>&acute;
	fieldlist ::= field {fieldsep field} [fieldsep]
	field ::= `<b>[</b>&acute; exp `<b>]</b>&acute; `<b>=</b>&acute; exp | Name `<b>=</b>&acute; exp | exp
	fieldsep ::= `<b>,</b>&acute; | `<b>;</b>&acute;
</pre>

<p>
Each field of the form <code>[exp1] = exp2</code> adds to the new table an entry
with key <code>exp1</code> and value <code>exp2</code>.
A field of the form <code>name = exp</code> is equivalent to
<code>["name"] = exp</code>.
Finally, fields of the form <code>exp</code> are equivalent to
<code>[i] = exp</code>, where <code>i</code> are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,

<pre>
     a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
</pre><p>
is equivalent to

<pre>
     do
       local t = {}
       t[f(1)] = g
       t[1] = "x"         -- 1st exp
       t[2] = "y"         -- 2nd exp
       t.x = 1            -- t["x"] = 1
       t[3] = f(x)        -- 3rd exp
       t[30] = 23
       t[4] = 45          -- 4th exp
       a = t
     end
</pre>

<p>
If the last field in the list has the form <code>exp</code>
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
(see <a href="#2.5.8">&sect;2.5.8</a>).
To avoid this,
enclose the function call or the vararg expression
in parentheses (see <a href="#2.5">&sect;2.5</a>).


<p>
The field list can have an optional trailing separator,
as a convenience for machine-generated code.





<h3>2.5.8 - <a name="2.5.8">Function Calls</a></h3><p>
A function call in Lua has the following syntax:

<pre>
	functioncall ::= prefixexp args
</pre><p>
In a function call,
first prefixexp and args are evaluated.
If the value of prefixexp has type <em>function</em>,
then this function is called
with the given arguments.
Otherwise, the prefixexp "call" metamethod is called,
having as first parameter the value of prefixexp,
followed by the original call arguments
(see <a href="#2.8">&sect;2.8</a>).


<p>
The form

<pre>
	functioncall ::= prefixexp `<b>:</b>&acute; Name args
</pre><p>
can be used to call "methods".
A call <code>v:name(<em>args</em>)</code>
is syntactic sugar for <code>v.name(v,<em>args</em>)</code>,
except that <code>v</code> is evaluated only once.


<p>
Arguments have the following syntax:

<pre>
	args ::= `<b>(</b>&acute; [explist] `<b>)</b>&acute;
	args ::= tableconstructor
	args ::= String
</pre><p>
All argument expressions are evaluated before the call.
A call of the form <code>f{<em>fields</em>}</code> is
syntactic sugar for <code>f({<em>fields</em>})</code>;
that is, the argument list is a single new table.
A call of the form <code>f'<em>string</em>'</code>
(or <code>f"<em>string</em>"</code> or <code>f[[<em>string</em>]]</code>)
is syntactic sugar for <code>f('<em>string</em>')</code>;
that is, the argument list is a single literal string.


<p>
As an exception to the free-format syntax of Lua,
you cannot put a line break before the '<code>(</code>' in a function call.
This restriction avoids some ambiguities in the language.
If you write

<pre>
     a = f
     (g).x(a)
</pre><p>
Lua would see that as a single statement, <code>a = f(g).x(a)</code>.
So, if you want two statements, you must add a semi-colon between them.
If you actually want to call <code>f</code>,
you must remove the line break before <code>(g)</code>.


<p>
A call of the form <code>return</code> <em>functioncall</em> is called
a <em>tail call</em>.
Lua implements <em>proper tail calls</em>
(or <em>proper tail recursion</em>):
in a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the <b>return</b> has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
So, none of the following examples are tail calls:

<pre>
     return (f(x))        -- results adjusted to 1
     return 2 * f(x)
     return x, f(x)       -- additional results
     f(x); return         -- results discarded
     return x or f(x)     -- results adjusted to 1
</pre>




<h3>2.5.9 - <a name="2.5.9">Function Definitions</a></h3>

<p>
The syntax for function definition is

<pre>
	function ::= <b>function</b> funcbody
	funcbody ::= `<b>(</b>&acute; [parlist] `<b>)</b>&acute; block <b>end</b>
</pre>

<p>
The following syntactic sugar simplifies function definitions:

<pre>
	stat ::= <b>function</b> funcname funcbody
	stat ::= <b>local</b> <b>function</b> Name funcbody
	funcname ::= Name {`<b>.</b>&acute; Name} [`<b>:</b>&acute; Name]
</pre><p>
The statement

<pre>
     function f () <em>body</em> end
</pre><p>
translates to

<pre>
     f = function () <em>body</em> end
</pre><p>
The statement

<pre>
     function t.a.b.c.f () <em>body</em> end
</pre><p>
translates to

<pre>
     t.a.b.c.f = function () <em>body</em> end
</pre><p>
The statement

<pre>
     local function f () <em>body</em> end
</pre><p>
translates to

<pre>
     local f; f = function () <em>body</em> end
</pre><p>
<em>not</em> to

<pre>
     local f = function () <em>body</em> end
</pre><p>
(This only makes a difference when the body of the function
contains references to <code>f</code>.)


<p>
A function definition is an executable expression,
whose value has type <em>function</em>.
When Lua pre-compiles a chunk,
all its function bodies are pre-compiled too.
Then, whenever Lua executes the function definition,
the function is <em>instantiated</em> (or <em>closed</em>).
This function instance (or <em>closure</em>)
is the final value of the expression.
Different instances of the same function
can refer to different  external local variables
and can have different environment tables.


<p>
Parameters act as local variables that are
initialized with the argument values:

<pre>
	parlist ::= namelist [`<b>,</b>&acute; `<b>...</b>&acute;] | `<b>...</b>&acute;
</pre><p>
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters,
unless the function is a variadic or <em>vararg function</em>,
which is
indicated by three dots ('<code>...</code>') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a <em>vararg expression</em>,
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless that last expression is enclosed in parentheses).


<p>
As an example, consider the following definitions:

<pre>
     function f(a, b) end
     function g(a, b, ...) end
     function r() return 1,2,3 end
</pre><p>
Then, we have the following mapping from arguments to parameters and
to the vararg expression:

<pre>
     CALL            PARAMETERS
     
     f(3)             a=3, b=nil
     f(3, 4)          a=3, b=4
     f(3, 4, 5)       a=3, b=4
     f(r(), 10)       a=1, b=10
     f(r())           a=1, b=2
     
     g(3)             a=3, b=nil, ... --&gt;  (nothing)
     g(3, 4)          a=3, b=4,   ... --&gt;  (nothing)
     g(3, 4, 5, 8)    a=3, b=4,   ... --&gt;  5  8
     g(5, r())        a=5, b=1,   ... --&gt;  2  3
</pre>

<p>
Results are returned using the <b>return</b> statement (see <a href="#2.4.4">&sect;2.4.4</a>).
If control reaches the end of a function
without encountering a <b>return</b> statement,
then the function returns with no results.


<p>
The <em>colon</em> syntax
is used for defining <em>methods</em>,
that is, functions that have an implicit extra parameter <code>self</code>.
Thus, the statement

<pre>
     function t.a.b.c:f (<em>params</em>) <em>body</em> end
</pre><p>
is syntactic sugar for

<pre>
     t.a.b.c.f = function (self, <em>params</em>) <em>body</em> end
</pre>






<h2>2.6 - <a name="2.6">Visibility Rules</a></h2>

<p>

Lua is a lexically scoped language.
The scope of variables begins at the first statement <em>after</em>
their declaration and lasts until the end of the innermost block that
includes the declaration.
Consider the following example:

<pre>
     x = 10                -- global variable
     do                    -- new block
       local x = x         -- new 'x', with value 10
       print(x)            --&gt; 10
       x = x+1
       do                  -- another block
         local x = x+1     -- another 'x'
         print(x)          --&gt; 12
       end
       print(x)            --&gt; 11
     end
     print(x)              --&gt; 10  (the global one)
</pre>

<p>
Notice that, in a declaration like <code>local x = x</code>,
the new <code>x</code> being declared is not in scope yet,
and so the second <code>x</code> refers to the outside variable.


<p>
Because of the lexical scoping rules,
local variables can be freely accessed by functions
defined inside their scope.
A local variable used by an inner function is called
an <em>upvalue</em>, or <em>external local variable</em>,
inside the inner function.


<p>
Notice that each execution of a <b>local</b> statement
defines new local variables.
Consider the following example:

<pre>
     a = {}
     local x = 20
     for i=1,10 do
       local y = 0
       a[i] = function () y=y+1; return x+y end
     end
</pre><p>
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different <code>y</code> variable,
while all of them share the same <code>x</code>.





<h2>2.7 - <a name="2.7">Error Handling</a></h2>

<p>
<div class='teliva'>
Errors returned by Teliva code should display on screen in red. Please report
crashes or other behavior.
</div>


<p>
Lua code can explicitly generate an error by calling the
<a href="#pdf-error"><code>error</code></a> function.
If you need to catch errors in Lua,
you can use the <a href="#pdf-pcall"><code>pcall</code></a> function.





<h2>2.8 - <a name="2.8">Metatables</a></h2>

<p>
Every value in Lua can have a <em>metatable</em>.
This <em>metatable</em> is an ordinary Lua table
that defines the behavior of the original value
under certain special operations.
You can change several aspects of the behavior
of operations over a value by setting specific fields in its metatable.
For instance, when a non-numeric value is the operand of an addition,
Lua checks for a function in the field <code>"__add"</code> in its metatable.
If it finds one,
Lua calls this function to perform the addition.


<p>
We call the keys in a metatable <em>events</em>
and the values <em>metamethods</em>.
In the previous example, the event is <code>"add"</code> 
and the metamethod is the function that performs the addition.


<p>
You can query the metatable of any value
through the <a href="#pdf-getmetatable"><code>getmetatable</code></a> function.


<p>
You can replace the metatable of tables
through the <a href="#pdf-setmetatable"><code>setmetatable</code></a>
function.


<p>
Tables have individual metatables
(although multiple tables can share their metatables).
Values of all other types share one single metatable per type;
that is, there is one single metatable for all numbers,
one for all strings, etc.


<p>
A metatable controls how an object behaves in arithmetic operations,
order comparisons, concatenation, length operation, and indexing.
For each of these operations Lua associates a specific key
called an <em>event</em>.
When Lua performs one of these operations over a value,
it checks whether this value has a metatable with the corresponding event.
If so, the value associated with that key (the metamethod)
controls how Lua will perform the operation.


<p>
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores, '<code>__</code>';
for instance, the key for operation "add" is the
string <code>"__add"</code>.
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes the operation.


<p>
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter
and it is much more efficient than this simulation.
All functions used in these descriptions
(<a href="#pdf-rawget"><code>rawget</code></a>, <a href="#pdf-tonumber"><code>tonumber</code></a>, etc.)
are described in <a href="#5.1">&sect;5.1</a>.
In particular, to retrieve the metamethod of a given object,
we use the expression

<pre>
     metatable(obj)[event]
</pre><p>
This should be read as

<pre>
     rawget(getmetatable(obj) or {}, event)
</pre><p>

That is, the access to a metamethod does not invoke other metamethods,
and the access to objects with no metatables does not fail
(it simply results in <b>nil</b>).



<ul>

<li><b>"add":</b>
the <code>+</code> operation.



<p>
The function <code>getbinhandler</code> below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.

<pre>
     function getbinhandler (op1, op2, event)
       return metatable(op1)[event] or metatable(op2)[event]
     end
</pre><p>
By using this function,
the behavior of the <code>op1 + op2</code> is

<pre>
     function add_event (op1, op2)
       local o1, o2 = tonumber(op1), tonumber(op2)
       if o1 and o2 then  -- both operands are numeric?
         return o1 + o2   -- '+' here is the primitive 'add'
       else  -- at least one of the operands is not numeric
         local h = getbinhandler(op1, op2, "__add")
         if h then
           -- call the handler with both operands
           return (h(op1, op2))
         else  -- no handler available: default behavior
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
</li>

<li><b>"sub":</b>
the <code>-</code> operation.

Behavior similar to the "add" operation.
</li>

<li><b>"mul":</b>
the <code>*</code> operation.

Behavior similar to the "add" operation.
</li>

<li><b>"div":</b>
the <code>/</code> operation.

Behavior similar to the "add" operation.
</li>

<li><b>"mod":</b>
the <code>%</code> operation.

Behavior similar to the "add" operation,
with the operation
<code>o1 - floor(o1/o2)*o2</code> as the primitive operation.
</li>

<li><b>"pow":</b>
the <code>^</code> (exponentiation) operation.

Behavior similar to the "add" operation,
with the function <code>pow</code> (from the C&nbsp;math library)
as the primitive operation.
</li>

<li><b>"unm":</b>
the unary <code>-</code> operation.


<pre>
     function unm_event (op)
       local o = tonumber(op)
       if o then  -- operand is numeric?
         return -o  -- '-' here is the primitive 'unm'
       else  -- the operand is not numeric.
         -- Try to get a handler from the operand
         local h = metatable(op).__unm
         if h then
           -- call the handler with the operand
           return (h(op))
         else  -- no handler available: default behavior
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
</li>

<li><b>"concat":</b>
the <code>..</code> (concatenation) operation.


<pre>
     function concat_event (op1, op2)
       if (type(op1) == "string" or type(op1) == "number") and
          (type(op2) == "string" or type(op2) == "number") then
         return op1 .. op2  -- primitive string concatenation
       else
         local h = getbinhandler(op1, op2, "__concat")
         if h then
           return (h(op1, op2))
         else
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
</li>

<li><b>"len":</b>
the <code>#</code> operation.


<pre>
     function len_event (op)
       if type(op) == "string" then
         return strlen(op)         -- primitive string length
       elseif type(op) == "table" then
         return #op                -- primitive table length
       else
         local h = metatable(op).__len
         if h then
           -- call the handler with the operand
           return (h(op))
         else  -- no handler available: default behavior
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
See <a href="#2.5.5">&sect;2.5.5</a> for a description of the length of a table.
</li>

<li><b>"eq":</b>
the <code>==</code> operation.

The function <code>getcomphandler</code> defines how Lua chooses a metamethod
for comparison operators.
A metamethod only is selected when both objects
being compared have the same type
and the same metamethod for the selected operation.

<pre>
     function getcomphandler (op1, op2, event)
       if type(op1) ~= type(op2) then return nil end
       local mm1 = metatable(op1)[event]
       local mm2 = metatable(op2)[event]
       if mm1 == mm2 then return mm1 else return nil end
     end
</pre><p>
The "eq" event is defined as follows:

<pre>
     function eq_event (op1, op2)
       if type(op1) ~= type(op2) then  -- different types?
         return false   -- different objects
       end
       if op1 == op2 then   -- primitive equal?
         return true   -- objects are equal
       end
       -- try metamethod
       local h = getcomphandler(op1, op2, "__eq")
       if h then
         return (h(op1, op2))
       else
         return false
       end
     end
</pre><p>
<code>a ~= b</code> is equivalent to <code>not (a == b)</code>.
</li>

<li><b>"lt":</b>
the <code>&lt;</code> operation.


<pre>
     function lt_event (op1, op2)
       if type(op1) == "number" and type(op2) == "number" then
         return op1 &lt; op2   -- numeric comparison
       elseif type(op1) == "string" and type(op2) == "string" then
         return op1 &lt; op2   -- lexicographic comparison
       else
         local h = getcomphandler(op1, op2, "__lt")
         if h then
           return (h(op1, op2))
         else
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
<code>a &gt; b</code> is equivalent to <code>b &lt; a</code>.
</li>

<li><b>"le":</b>
the <code>&lt;=</code> operation.


<pre>
     function le_event (op1, op2)
       if type(op1) == "number" and type(op2) == "number" then
         return op1 &lt;= op2   -- numeric comparison
       elseif type(op1) == "string" and type(op2) == "string" then
         return op1 &lt;= op2   -- lexicographic comparison
       else
         local h = getcomphandler(op1, op2, "__le")
         if h then
           return (h(op1, op2))
         else
           h = getcomphandler(op1, op2, "__lt")
           if h then
             return not h(op2, op1)
           else
             error(&middot;&middot;&middot;)
           end
         end
       end
     end
</pre><p>
<code>a &gt;= b</code> is equivalent to <code>b &lt;= a</code>.
Note that, in the absence of a "le" metamethod,
Lua tries the "lt", assuming that <code>a &lt;= b</code> is
equivalent to <code>not (b &lt; a)</code>.
</li>

<li><b>"index":</b>
The indexing access <code>table[key]</code>.


<pre>
     function gettable_event (table, key)
       local h
       if type(table) == "table" then
         local v = rawget(table, key)
         if v ~= nil then return v end
         h = metatable(table).__index
         if h == nil then return nil end
       else
         h = metatable(table).__index
         if h == nil then
           error(&middot;&middot;&middot;)
         end
       end
       if type(h) == "function" then
         return (h(table, key))     -- call the handler
       else return h[key]           -- or repeat operation on it
       end
     end
</pre><p>
</li>

<li><b>"newindex":</b>
The indexing assignment <code>table[key] = value</code>.


<pre>
     function settable_event (table, key, value)
       local h
       if type(table) == "table" then
         local v = rawget(table, key)
         if v ~= nil then rawset(table, key, value); return end
         h = metatable(table).__newindex
         if h == nil then rawset(table, key, value); return end
       else
         h = metatable(table).__newindex
         if h == nil then
           error(&middot;&middot;&middot;)
         end
       end
       if type(h) == "function" then
         h(table, key,value)           -- call the handler
       else h[key] = value             -- or repeat operation on it
       end
     end
</pre><p>
</li>

<li><b>"call":</b>
called when Lua calls a value.


<pre>
     function function_event (func, ...)
       if type(func) == "function" then
         return func(...)   -- primitive call
       else
         local h = metatable(func).__call
         if h then
           return h(func, ...)
         else
           error(&middot;&middot;&middot;)
         end
       end
     end
</pre><p>
</li>

</ul>




<h2>2.9 - <a name="2.9">Environments</a></h2>

<p>
Besides metatables, objects of types thread and function
have another table associated with them, called their
<em>environment</em>.
Like metatables, environments are regular tables and
multiple objects can share the same environment.


<p>
Threads are created sharing the environment of the creating thread.
Nested Lua functions are created sharing the environment of
the creating Lua function.


<p>
Environments associated with threads are called
<em>global environments</em>.
They are used as the default environment for threads.


<p>
Environments associated with Lua functions are used to resolve
all accesses to global variables within the function (see <a href="#2.3">&sect;2.3</a>).
They are used as the default environment for nested Lua functions
created by the function.


<p>
You can change the environment of a Lua function or the
running thread by calling <a href="#pdf-setfenv"><code>setfenv</code></a>.
You can get the environment of a Lua function or the running thread
by calling <a href="#pdf-getfenv"><code>getfenv</code></a>.





<h2>2.10 - <a name="2.10">Garbage Collection</a></h2>

<p>
Lua performs automatic memory management.
This means that
you have to worry neither about allocating memory for new objects
nor about freeing it when the objects are no longer needed.
Lua manages memory automatically by running
a <em>garbage collector</em> from time to time
to collect all <em>dead objects</em>
(that is, objects that are no longer accessible from Lua).
All memory used by Lua is subject to automatic management:
tables, functions, threads, strings, etc.


<p>
Lua implements an incremental mark-and-sweep collector.
It uses two numbers to control its garbage-collection cycles:
the <em>garbage-collector pause</em> and
the <em>garbage-collector step multiplier</em>.
Both use percentage points as units
(so that a value of 100 means an internal value of 1).


<p>
The garbage-collector pause
controls how long the collector waits before starting a new cycle.
Larger values make the collector less aggressive.
Values smaller than 100 mean the collector will not wait to
start a new cycle.
A value of 200 means that the collector waits for the total memory in use
to double before starting a new cycle.


<p>
The step multiplier
controls the relative speed of the collector relative to
memory allocation.
Larger values make the collector more aggressive but also increase
the size of each incremental step.
Values smaller than 100 make the collector too slow and
can result in the collector never finishing a cycle.
The default, 200, means that the collector runs at "twice"
the speed of memory allocation.


<p>
<div class='teliva'>Modifying these parameters requires changes to Teliva's C
souces.</div>



<h3>2.10.2 - <a name="2.10.2">Weak Tables</a></h3>

<p>
A <em>weak table</em> is a table whose elements are
<em>weak references</em>.
A weak reference is ignored by the garbage collector.
In other words,
if the only references to an object are weak references,
then the garbage collector will collect this object.


<p>
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but prevents the collection of its values.
A table with both weak keys and weak values allows the collection of
both keys and values.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is controlled by the
<code>__mode</code> field of its metatable.
If the <code>__mode</code> field is a string containing the character&nbsp;'<code>k</code>',
the keys in the table are weak.
If <code>__mode</code> contains '<code>v</code>',
the values in the table are weak.


<p>
After you use a table as a metatable,
you should not change the value of its <code>__mode</code> field.
Otherwise, the weak behavior of the tables controlled by this
metatable is undefined.







<h2>2.11 - <a name="2.11">Coroutines</a></h2>

<p>
Lua supports coroutines,
also called <em>collaborative multithreading</em>.
A coroutine in Lua represents an independent thread of execution.
Unlike threads in multithread systems, however,
a coroutine only suspends its execution by explicitly calling
a yield function.


<p>
You create a coroutine with a call to <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>.
Its sole argument is a function
that is the main function of the coroutine.
The <code>create</code> function only creates a new coroutine and
returns a handle to it (an object of type <em>thread</em>);
it does not start the coroutine execution.


<p>
When you first call <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
passing as its first argument
a thread returned by <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>,
the coroutine starts its execution,
at the first line of its main function.
Extra arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> are passed on
to the coroutine main function.
After the coroutine starts running,
it runs until it terminates or <em>yields</em>.


<p>
A coroutine can terminate its execution in two ways:
normally, when its main function returns
(explicitly or implicitly, after the last instruction);
and abnormally, if there is an unprotected error.
In the first case, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>true</b>,
plus any values returned by the coroutine main function.
In case of errors, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>false</b>
plus an error message.


<p>
A coroutine yields by calling <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>.
When a coroutine yields,
the corresponding <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns immediately,
even if the yield happens inside nested function calls
(that is, not in the main function,
but in a function directly or indirectly called by the main function).
In the case of a yield, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> also returns <b>true</b>,
plus any values passed to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>.
The next time you resume the same coroutine,
it continues its execution from the point where it yielded,
with the call to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a> returning any extra
arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>.


<p>
Like <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>,
the <a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> function also creates a coroutine,
but instead of returning the coroutine itself,
it returns a function that, when called, resumes the coroutine.
Any arguments passed to this function
go as extra arguments to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>.
<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> returns all the values returned by <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
except the first one (the boolean error code).
Unlike <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> does not catch errors;
any error is propagated to the caller.


<p>
As an example,
consider the following code:

<pre>
     function foo (a)
       print("foo", a)
       return coroutine.yield(2*a)
     end
     
     co = coroutine.create(function (a,b)
           print("co-body", a, b)
           local r = foo(a+1)
           print("co-body", r)
           local r, s = coroutine.yield(a+b, a-b)
           print("co-body", r, s)
           return b, "end"
     end)
            
     print("main", coroutine.resume(co, 1, 10))
     print("main", coroutine.resume(co, "r"))
     print("main", coroutine.resume(co, "x", "y"))
     print("main", coroutine.resume(co, "x", "y"))
</pre><p>
When you run it, it produces the following output:

<pre>
     co-body 1       10
     foo     2
     
     main    true    4
     co-body r
     main    true    11      -9
     co-body x       y
     main    true    10      end
     main    false   cannot resume dead coroutine
</pre>




<h1>3 - <a name="3">C API</a></h1>

<div class='teliva'>Unlike Lua, Teliva isn't intended to be modified at the C
level. However, forks of Teliva are encouraged.</div>


<h1>5 - <a name="5">Standard Libraries</a></h1>

<p>
The standard Lua libraries provide useful functions
that are implemented directly in C.
Some of these functions provide essential services to the language
(e.g., <a href="#pdf-type"><code>type</code></a> and <a href="#pdf-getmetatable"><code>getmetatable</code></a>);
others provide access to "outside" services (e.g., I/O);
and others could be implemented in Lua itself,
but are quite useful or have critical performance requirements that
deserve an implementation in C (e.g., <a href="#pdf-table.sort"><code>table.sort</code></a>).


<p>
Currently, Lua has the following standard libraries:

<ul>

<li>basic library, which includes the coroutine sub-library;</li>

<li>package library;</li>

<li>string manipulation;</li>

<li>table manipulation;</li>

<li>mathematical functions (sin, log, etc.);</li>

<li>input and output;</li>

<li>operating system facilities;</li>

<li>debug facilities.</li>

</ul><p>
Except for the basic and package libraries,
each library provides all its functions as fields of a global table
or as methods of its objects.


<h2>5.1 - <a name="5.1">Basic Functions</a></h2>

<p>
The basic library provides some core functions to Lua.
If you do not include this library in your application,
you should check carefully whether you need to provide 
implementations for some of its facilities.


<p>
<hr><h3><a name="pdf-assert"><code>assert (v [, message])</code></a></h3>
Issues an  error when
the value of its argument <code>v</code> is false (i.e., <b>nil</b> or <b>false</b>);
otherwise, returns all its arguments.
<code>message</code> is an error message;
when absent, it defaults to "assertion failed!"




<p>
<hr><h3><a name="pdf-collectgarbage"><code>collectgarbage ([opt [, arg]])</code></a></h3>


<p>
This function is a generic interface to the garbage collector.
It performs different functions according to its first argument, <code>opt</code>:

<ul>

<li><b>"collect":</b>
performs a full garbage-collection cycle.
This is the default option.
</li>

<li><b>"stop":</b>
stops the garbage collector.
</li>

<li><b>"restart":</b>
restarts the garbage collector.
</li>

<li><b>"count":</b>
returns the total memory in use by Lua (in Kbytes).
</li>

<li><b>"step":</b>
performs a garbage-collection step.
The step "size" is controlled by <code>arg</code>
(larger values mean more steps) in a non-specified way.
If you want to control the step size
you must experimentally tune the value of <code>arg</code>.
Returns <b>true</b> if the step finished a collection cycle.
</li>

<li><b>"setpause":</b>
sets <code>arg</code> as the new value for the <em>pause</em> of
the collector (see <a href="#2.10">&sect;2.10</a>).
Returns the previous value for <em>pause</em>.
</li>

<li><b>"setstepmul":</b>
sets <code>arg</code> as the new value for the <em>step multiplier</em> of
the collector (see <a href="#2.10">&sect;2.10</a>).
Returns the previous value for <em>step</em>.
</li>

</ul>




<p>
<hr><h3><a name="pdf-error"><code>error (message [, level])</code></a></h3>
Terminates the last protected function called
and returns <code>message</code> as the error message.
Function <code>error</code> never returns.


<p>
Usually, <code>error</code> adds some information about the error position
at the beginning of the message.
The <code>level</code> argument specifies how to get the error position.
With level&nbsp;1 (the default), the error position is where the
<code>error</code> function was called.
Level&nbsp;2 points the error to where the function
that called <code>error</code> was called; and so on.
Passing a level&nbsp;0 avoids the addition of error position information
to the message.




<p>
<hr><h3><a name="pdf-_G"><code>_G</code></a></h3>
A global variable (not a function) that
holds the global environment (that is, <code>_G._G = _G</code>).
Lua itself does not use this variable;
changing its value does not affect any environment,
nor vice-versa.
(Use <a href="#pdf-setfenv"><code>setfenv</code></a> to change environments.)




<p>
<hr><h3><a name="pdf-getfenv"><code>getfenv ([f])</code></a></h3>
Returns the current environment in use by the function.
<code>f</code> can be a Lua function or a number
that specifies the function at that stack level:
Level&nbsp;1 is the function calling <code>getfenv</code>.
If the given function is not a Lua function,
or if <code>f</code> is 0,
<code>getfenv</code> returns the global environment.
The default for <code>f</code> is 1.




<p>
<hr><h3><a name="pdf-getmetatable"><code>getmetatable (object)</code></a></h3>


<p>
If <code>object</code> does not have a metatable, returns <b>nil</b>.
Otherwise,
if the object's metatable has a <code>"__metatable"</code> field,
returns the associated value.
Otherwise, returns the metatable of the given object.




<p>
<hr><h3><a name="pdf-ipairs"><code>ipairs (t)</code></a></h3>


<p>
Returns three values: an iterator function, the table <code>t</code>, and 0,
so that the construction

<pre>
     for i,v in ipairs(t) do <em>body</em> end
</pre><p>
will iterate over the pairs (<code>1,t[1]</code>), (<code>2,t[2]</code>), &middot;&middot;&middot;,
up to the first integer key absent from the table.


<p>
<hr><h3><a name="pdf-next"><code>next (table [, index])</code></a></h3>


<p>
Allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
<code>next</code> returns the next index of the table
and its associated value.
When called with <b>nil</b> as its second argument,
<code>next</code> returns an initial index
and its associated value.
When called with the last index,
or with <b>nil</b> in an empty table,
<code>next</code> returns <b>nil</b>.
If the second argument is absent, then it is interpreted as <b>nil</b>.
In particular,
you can use <code>next(t)</code> to check whether a table is empty.


<p>
The order in which the indices are enumerated is not specified,
<em>even for numeric indices</em>.
(To traverse a table in numeric order,
use a numerical <b>for</b> or the <a href="#pdf-ipairs"><code>ipairs</code></a> function.)


<p>
The behavior of <code>next</code> is <em>undefined</em> if,
during the traversal,
you assign any value to a non-existent field in the table.
You may however modify existing fields.
In particular, you may clear existing fields.




<p>
<hr><h3><a name="pdf-pairs"><code>pairs (t)</code></a></h3>


<p>
Returns three values: the <a href="#pdf-next"><code>next</code></a> function, the table <code>t</code>, and <b>nil</b>,
so that the construction

<pre>
     for k,v in pairs(t) do <em>body</em> end
</pre><p>
will iterate over all key&ndash;value pairs of table <code>t</code>.


<p>
See function <a href="#pdf-next"><code>next</code></a> for the caveats of modifying
the table during its traversal.




<p>
<hr><h3><a name="pdf-print"><code>print (&middot;&middot;&middot;)</code></a></h3>
Receives any number of arguments,
and prints their values to <code>stdout</code>,
using the <a href="#pdf-tostring"><code>tostring</code></a> function to convert them to strings.
<code>print</code> is not intended for formatted output,
but only as a quick way to show a value,
typically for debugging.
For formatted output, use <a href="#pdf-string.format"><code>string.format</code></a>.




<p>
<hr><h3><a name="pdf-rawequal"><code>rawequal (v1, v2)</code></a></h3>
Checks whether <code>v1</code> is equal to <code>v2</code>,
without invoking any metamethod.
Returns a boolean.




<p>
<hr><h3><a name="pdf-rawget"><code>rawget (table, index)</code></a></h3>
Gets the real value of <code>table[index]</code>,
without invoking any metamethod.
<code>table</code> must be a table;
<code>index</code> may be any value.




<p>
<hr><h3><a name="pdf-rawset"><code>rawset (table, index, value)</code></a></h3>
Sets the real value of <code>table[index]</code> to <code>value</code>,
without invoking any metamethod.
<code>table</code> must be a table,
<code>index</code> any value different from <b>nil</b>,
and <code>value</code> any Lua value.


<p>
This function returns <code>table</code>.




<p>
<hr><h3><a name="pdf-select"><code>select (index, &middot;&middot;&middot;)</code></a></h3>


<p>
If <code>index</code> is a number,
returns all arguments after argument number <code>index</code>.
Otherwise, <code>index</code> must be the string <code>"#"</code>,
and <code>select</code> returns the total number of extra arguments it received.




<p>
<hr><h3><a name="pdf-setfenv"><code>setfenv (f, table)</code></a></h3>


<p>
Sets the environment to be used by the given function.
<code>f</code> can be a Lua function or a number
that specifies the function at that stack level:
Level&nbsp;1 is the function calling <code>setfenv</code>.
<code>setfenv</code> returns the given function.


<p>
As a special case, when <code>f</code> is 0 <code>setfenv</code> changes
the environment of the running thread.
In this case, <code>setfenv</code> returns no values.




<p>
<hr><h3><a name="pdf-setmetatable"><code>setmetatable (table, metatable)</code></a></h3>


<p>
Sets the metatable for the given table.
(You cannot change the metatable of other types.)
If <code>metatable</code> is <b>nil</b>,
removes the metatable of the given table.
If the original metatable has a <code>"__metatable"</code> field,
raises an error.


<p>
This function returns <code>table</code>.




<p>
<hr><h3><a name="pdf-tonumber"><code>tonumber (e [, base])</code></a></h3>
Tries to convert its argument to a number.
If the argument is already a number or a string convertible
to a number, then <code>tonumber</code> returns this number;
otherwise, it returns <b>nil</b>.


<p>
An optional argument specifies the base to interpret the numeral.
The base may be any integer between 2 and 36, inclusive.
In bases above&nbsp;10, the letter '<code>A</code>' (in either upper or lower case)
represents&nbsp;10, '<code>B</code>' represents&nbsp;11, and so forth,
with '<code>Z</code>' representing 35.
In base 10 (the default), the number can have a decimal part,
as well as an optional exponent part (see <a href="#2.1">&sect;2.1</a>).
In other bases, only unsigned integers are accepted.




<p>
<hr><h3><a name="pdf-tostring"><code>tostring (e)</code></a></h3>
Receives an argument of any type and
converts it to a string in a reasonable format.
For complete control of how numbers are converted,
use <a href="#pdf-string.format"><code>string.format</code></a>.


<p>
If the metatable of <code>e</code> has a <code>"__tostring"</code> field,
then <code>tostring</code> calls the corresponding value
with <code>e</code> as argument,
and uses the result of the call as its result.




<p>
<hr><h3><a name="pdf-type"><code>type (v)</code></a></h3>
Returns the type of its only argument, coded as a string.
The possible results of this function are
"<code>nil</code>" (a string, not the value <b>nil</b>),
"<code>number</code>",
"<code>string</code>",
"<code>boolean</code>",
"<code>table</code>",
"<code>function</code>",
"<code>thread</code>",
and "<code>userdata</code>".




<p>
<hr><h3><a name="pdf-unpack"><code>unpack (list [, i [, j]])</code></a></h3>
Returns the elements from the given table.
This function is equivalent to

<pre>
     return list[i], list[i+1], &middot;&middot;&middot;, list[j]
</pre><p>
except that the above code can be written only for a fixed number
of elements.
By default, <code>i</code> is&nbsp;1 and <code>j</code> is the length of the list,
as defined by the length operator (see <a href="#2.5.5">&sect;2.5.5</a>).




<p>
<hr><h3><a name="pdf-_VERSION"><code>_VERSION</code></a></h3>
A global variable (not a function) that
holds a string containing the current interpreter version.
The current contents of this variable is "<code>Lua 5.1</code>".




<h2>5.2 - <a name="5.2">Coroutine Manipulation</a></h2>

<p>
The operations related to coroutines comprise a sub-library of
the basic library and come inside the table <a name="pdf-coroutine"><code>coroutine</code></a>.
See <a href="#2.11">&sect;2.11</a> for a general description of coroutines.


<p>
<hr><h3><a name="pdf-coroutine.create"><code>coroutine.create (f)</code></a></h3>


<p>
Creates a new coroutine, with body <code>f</code>.
<code>f</code> must be a Lua function.
Returns this new coroutine,
an object with type <code>"thread"</code>.




<p>
<hr><h3><a name="pdf-coroutine.resume"><code>coroutine.resume (co [, val1, &middot;&middot;&middot;])</code></a></h3>


<p>
Starts or continues the execution of coroutine <code>co</code>.
The first time you resume a coroutine,
it starts running its body.
The values <code>val1</code>, &middot;&middot;&middot; are passed
as the arguments to the body function.
If the coroutine has yielded,
<code>resume</code> restarts it;
the values <code>val1</code>, &middot;&middot;&middot; are passed
as the results from the yield.


<p>
If the coroutine runs without any errors,
<code>resume</code> returns <b>true</b> plus any values passed to <code>yield</code>
(if the coroutine yields) or any values returned by the body function
(if the coroutine terminates).
If there is any error,
<code>resume</code> returns <b>false</b> plus the error message.




<p>
<hr><h3><a name="pdf-coroutine.running"><code>coroutine.running ()</code></a></h3>


<p>
Returns the running coroutine,
or <b>nil</b> when called by the main thread.




<p>
<hr><h3><a name="pdf-coroutine.status"><code>coroutine.status (co)</code></a></h3>


<p>
Returns the status of coroutine <code>co</code>, as a string:
<code>"running"</code>,
if the coroutine is running (that is, it called <code>status</code>);
<code>"suspended"</code>, if the coroutine is suspended in a call to <code>yield</code>,
or if it has not started running yet;
<code>"normal"</code> if the coroutine is active but not running
(that is, it has resumed another coroutine);
and <code>"dead"</code> if the coroutine has finished its body function,
or if it has stopped with an error.




<p>
<hr><h3><a name="pdf-coroutine.wrap"><code>coroutine.wrap (f)</code></a></h3>


<p>
Creates a new coroutine, with body <code>f</code>.
<code>f</code> must be a Lua function.
Returns a function that resumes the coroutine each time it is called.
Any arguments passed to the function behave as the
extra arguments to <code>resume</code>.
Returns the same values returned by <code>resume</code>,
except the first boolean.
In case of error, propagates the error.




<p>
<hr><h3><a name="pdf-coroutine.yield"><code>coroutine.yield (&middot;&middot;&middot;)</code></a></h3>


<p>
Suspends the execution of the calling coroutine.
The coroutine cannot be running a primitive implemented in
C, a metamethod, or an iterator.
Any arguments to <code>yield</code> are passed as extra results to <code>resume</code>.







<h2>5.4 - <a name="5.4">String Manipulation</a></h2>

<p>
This library provides generic functions for string manipulation,
such as finding and extracting substrings, and pattern matching.
When indexing a string in Lua, the first character is at position&nbsp;1
(not at&nbsp;0, as in C).
Indices are allowed to be negative and are interpreted as indexing backwards,
from the end of the string.
Thus, the last character is at position -1, and so on.


<p>
The string library provides all its functions inside the table
<a name="pdf-string"><code>string</code></a>.
It also sets a metatable for strings
where the <code>__index</code> field points to the <code>string</code> table.
Therefore, you can use the string functions in object-oriented style.
For instance, <code>string.byte(s, i)</code>
can be written as <code>s:byte(i)</code>.


<p>
The string library assumes one-byte character encodings.


<p>
<hr><h3><a name="pdf-string.byte"><code>string.byte (s [, i [, j]])</code></a></h3>
Returns the internal numerical codes of the characters <code>s[i]</code>,
<code>s[i+1]</code>, &middot;&middot;&middot;, <code>s[j]</code>.
The default value for <code>i</code> is&nbsp;1;
the default value for <code>j</code> is&nbsp;<code>i</code>.


<p>
Note that numerical codes are not necessarily portable across platforms.




<p>
<hr><h3><a name="pdf-string.char"><code>string.char (&middot;&middot;&middot;)</code></a></h3>
Receives zero or more integers.
Returns a string with length equal to the number of arguments,
in which each character has the internal numerical code equal
to its corresponding argument.


<p>
Note that numerical codes are not necessarily portable across platforms.



<p>
<hr><h3><a name="pdf-string.find"><code>string.find (s, pattern [, init [, plain]])</code></a></h3>
Looks for the first match of
<code>pattern</code> in the string <code>s</code>.
If it finds a match, then <code>find</code> returns the indices of&nbsp;<code>s</code>
where this occurrence starts and ends;
otherwise, it returns <b>nil</b>.
A third, optional numerical argument <code>init</code> specifies
where to start the search;
its default value is&nbsp;1 and can be negative.
A value of <b>true</b> as a fourth, optional argument <code>plain</code>
turns off the pattern matching facilities,
so the function does a plain "find substring" operation,
with no characters in <code>pattern</code> being considered "magic".
Note that if <code>plain</code> is given, then <code>init</code> must be given as well.


<p>
If the pattern has captures,
then in a successful match
the captured values are also returned,
after the two indices.




<p>
<hr><h3><a name="pdf-string.format"><code>string.format (formatstring, &middot;&middot;&middot;)</code></a></h3>
Returns a formatted version of its variable number of arguments
following the description given in its first argument (which must be a string).
The format string follows the same rules as the <code>printf</code> family of
standard C&nbsp;functions.
The only differences are that the options/modifiers
<code>*</code>, <code>l</code>, <code>L</code>, <code>n</code>, <code>p</code>,
and <code>h</code> are not supported
and that there is an extra option, <code>q</code>.
The <code>q</code> option formats a string in a form suitable to be safely read
back by the Lua interpreter:
the string is written between double quotes,
and all double quotes, newlines, embedded zeros,
and backslashes in the string
are correctly escaped when written.
For instance, the call

<pre>
     string.format('%q', 'a string with "quotes" and \n new line')
</pre><p>
will produce the string:

<pre>
     "a string with \"quotes\" and \
      new line"
</pre>

<p>
The options <code>c</code>, <code>d</code>, <code>E</code>, <code>e</code>, <code>f</code>,
<code>g</code>, <code>G</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>X</code>, and <code>x</code> all
expect a number as argument,
whereas <code>q</code> and <code>s</code> expect a string.


<p>
This function does not accept string values
containing embedded zeros,
except as arguments to the <code>q</code> option.




<p>
<hr><h3><a name="pdf-string.gmatch"><code>string.gmatch (s, pattern)</code></a></h3>
Returns an iterator function that,
each time it is called,
returns the next captures from <code>pattern</code> over string <code>s</code>.
If <code>pattern</code> specifies no captures,
then the whole match is produced in each call.


<p>
As an example, the following loop

<pre>
     s = "hello world from Lua"
     for w in string.gmatch(s, "%a+") do
       print(w)
     end
</pre><p>
will iterate over all the words from string <code>s</code>,
printing one per line.
The next example collects all pairs <code>key=value</code> from the
given string into a table:

<pre>
     t = {}
     s = "from=world, to=Lua"
     for k, v in string.gmatch(s, "(%w+)=(%w+)") do
       t[k] = v
     end
</pre>

<p>
For this function, a '<code>^</code>' at the start of a pattern does not
work as an anchor, as this would prevent the iteration.




<p>
<hr><h3><a name="pdf-string.gsub"><code>string.gsub (s, pattern, repl [, n])</code></a></h3>
Returns a copy of <code>s</code>
in which all (or the first <code>n</code>, if given)
occurrences of the <code>pattern</code> have been
replaced by a replacement string specified by <code>repl</code>,
which can be a string, a table, or a function.
<code>gsub</code> also returns, as its second value,
the total number of matches that occurred.


<p>
If <code>repl</code> is a string, then its value is used for replacement.
The character&nbsp;<code>%</code> works as an escape character:
any sequence in <code>repl</code> of the form <code>%<em>n</em></code>,
with <em>n</em> between 1 and 9,
stands for the value of the <em>n</em>-th captured substring (see below).
The sequence <code>%0</code> stands for the whole match.
The sequence <code>%%</code> stands for a single&nbsp;<code>%</code>.


<p>
If <code>repl</code> is a table, then the table is queried for every match,
using the first capture as the key;
if the pattern specifies no captures,
then the whole match is used as the key.


<p>
If <code>repl</code> is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order;
if the pattern specifies no captures,
then the whole match is passed as a sole argument.


<p>
If the value returned by the table query or by the function call
is a string or a number,
then it is used as the replacement string;
otherwise, if it is <b>false</b> or <b>nil</b>,
then there is no replacement
(that is, the original match is kept in the string).


<p>
Here are some examples:

<pre>
     x = string.gsub("hello world", "(%w+)", "%1 %1")
     --&gt; x="hello hello world world"
     
     x = string.gsub("hello world", "%w+", "%0 %0", 1)
     --&gt; x="hello hello world"
     
     x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
     --&gt; x="world hello Lua from"
     
     x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
     --&gt; x="home = /home/roberto, user = roberto"
     
     local t = {name="lua", version="5.1"}
     x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
     --&gt; x="lua-5.1.tar.gz"
</pre>



<p>
<hr><h3><a name="pdf-string.len"><code>string.len (s)</code></a></h3>
Receives a string and returns its length.
The empty string <code>""</code> has length 0.
Embedded zeros are counted,
so <code>"a\000bc\000"</code> has length 5.




<p>
<hr><h3><a name="pdf-string.lower"><code>string.lower (s)</code></a></h3>
Receives a string and returns a copy of this string with all
uppercase letters changed to lowercase.
All other characters are left unchanged.
The definition of what an uppercase letter is depends on the current locale.




<p>
<hr><h3><a name="pdf-string.match"><code>string.match (s, pattern [, init])</code></a></h3>
Looks for the first <em>match</em> of
<code>pattern</code> in the string <code>s</code>.
If it finds one, then <code>match</code> returns
the captures from the pattern;
otherwise it returns <b>nil</b>.
If <code>pattern</code> specifies no captures,
then the whole match is returned.
A third, optional numerical argument <code>init</code> specifies
where to start the search;
its default value is&nbsp;1 and can be negative.




<p>
<hr><h3><a name="pdf-string.rep"><code>string.rep (s, n)</code></a></h3>
Returns a string that is the concatenation of <code>n</code> copies of
the string <code>s</code>.




<p>
<hr><h3><a name="pdf-string.reverse"><code>string.reverse (s)</code></a></h3>
Returns a string that is the string <code>s</code> reversed.




<p>
<hr><h3><a name="pdf-string.sub"><code>string.sub (s, i [, j])</code></a></h3>
Returns the substring of <code>s</code> that
starts at <code>i</code>  and continues until <code>j</code>;
<code>i</code> and <code>j</code> can be negative.
If <code>j</code> is absent, then it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call <code>string.sub(s,1,j)</code> returns a prefix of <code>s</code>
with length <code>j</code>,
and <code>string.sub(s, -i)</code> returns a suffix of <code>s</code>
with length <code>i</code>.




<p>
<hr><h3><a name="pdf-string.upper"><code>string.upper (s)</code></a></h3>
Receives a string and returns a copy of this string with all
lowercase letters changed to uppercase.
All other characters are left unchanged.
The definition of what a lowercase letter is depends on the current locale.



<h3>5.4.1 - <a name="5.4.1">Patterns</a></h3>


<h4>Character Class:</h4><p>
A <em>character class</em> is used to represent a set of characters.
The following combinations are allowed in describing a character class:

<ul>

<li><b><em>x</em>:</b>
(where <em>x</em> is not one of the <em>magic characters</em>
<code>^$()%.[]*+-?</code>)
represents the character <em>x</em> itself.
</li>

<li><b><code>.</code>:</b> (a dot) represents all characters.</li>

<li><b><code>%a</code>:</b> represents all letters.</li>

<li><b><code>%c</code>:</b> represents all control characters.</li>

<li><b><code>%d</code>:</b> represents all digits.</li>

<li><b><code>%l</code>:</b> represents all lowercase letters.</li>

<li><b><code>%p</code>:</b> represents all punctuation characters.</li>

<li><b><code>%s</code>:</b> represents all space characters.</li>

<li><b><code>%u</code>:</b> represents all uppercase letters.</li>

<li><b><code>%w</code>:</b> represents all alphanumeric characters.</li>

<li><b><code>%x</code>:</b> represents all hexadecimal digits.</li>

<li><b><code>%z</code>:</b> represents the character with representation 0.</li>

<li><b><code>%<em>x</em></code>:</b> (where <em>x</em> is any non-alphanumeric character)
represents the character <em>x</em>.
This is the standard way to escape the magic characters.
Any punctuation character (even the non magic)
can be preceded by a '<code>%</code>'
when used to represent itself in a pattern.
</li>

<li><b><code>[<em>set</em>]</code>:</b>
represents the class which is the union of all
characters in <em>set</em>.
A range of characters can be specified by
separating the end characters of the range with a '<code>-</code>'.
All classes <code>%</code><em>x</em> described above can also be used as
components in <em>set</em>.
All other characters in <em>set</em> represent themselves.
For example, <code>[%w_]</code> (or <code>[_%w]</code>)
represents all alphanumeric characters plus the underscore,
<code>[0-7]</code> represents the octal digits,
and <code>[0-7%l%-]</code> represents the octal digits plus
the lowercase letters plus the '<code>-</code>' character.


<p>
The interaction between ranges and classes is not defined.
Therefore, patterns like <code>[%a-z]</code> or <code>[a-%%]</code>
have no meaning.
</li>

<li><b><code>[^<em>set</em>]</code>:</b>
represents the complement of <em>set</em>,
where <em>set</em> is interpreted as above.
</li>

</ul><p>
For all classes represented by single letters (<code>%a</code>, <code>%c</code>, etc.),
the corresponding uppercase letter represents the complement of the class.
For instance, <code>%S</code> represents all non-space characters.


<p>
The definitions of letter, space, and other character groups
depend on the current locale.
In particular, the class <code>[a-z]</code> may not be equivalent to <code>%l</code>.





<h4>Pattern Item:</h4><p>
A <em>pattern item</em> can be

<ul>

<li>
a single character class,
which matches any single character in the class;
</li>

<li>
a single character class followed by '<code>*</code>',
which matches 0 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
</li>

<li>
a single character class followed by '<code>+</code>',
which matches 1 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
</li>

<li>
a single character class followed by '<code>-</code>',
which also matches 0 or more repetitions of characters in the class.
Unlike '<code>*</code>',
these repetition items will always match the <em>shortest</em> possible sequence;
</li>

<li>
a single character class followed by '<code>?</code>',
which matches 0 or 1 occurrence of a character in the class;
</li>

<li>
<code>%<em>n</em></code>, for <em>n</em> between 1 and 9;
such item matches a substring equal to the <em>n</em>-th captured string
(see below);
</li>

<li>
<code>%b<em>xy</em></code>, where <em>x</em> and <em>y</em> are two distinct characters;
such item matches strings that start with&nbsp;<em>x</em>, end with&nbsp;<em>y</em>,
and where the <em>x</em> and <em>y</em> are <em>balanced</em>.
This means that, if one reads the string from left to right,
counting <em>+1</em> for an <em>x</em> and <em>-1</em> for a <em>y</em>,
the ending <em>y</em> is the first <em>y</em> where the count reaches 0.
For instance, the item <code>%b()</code> matches expressions with
balanced parentheses.
</li>

</ul>




<h4>Pattern:</h4><p>
A <em>pattern</em> is a sequence of pattern items.
A '<code>^</code>' at the beginning of a pattern anchors the match at the
beginning of the subject string.
A '<code>$</code>' at the end of a pattern anchors the match at the
end of the subject string.
At other positions,
'<code>^</code>' and '<code>$</code>' have no special meaning and represent themselves.





<h4>Captures:</h4><p>
A pattern can contain sub-patterns enclosed in parentheses;
they describe <em>captures</em>.
When a match succeeds, the substrings of the subject string
that match captures are stored (<em>captured</em>) for future use.
Captures are numbered according to their left parentheses.
For instance, in the pattern <code>"(a*(.)%w(%s*))"</code>,
the part of the string matching <code>"a*(.)%w(%s*)"</code> is
stored as the first capture (and therefore has number&nbsp;1);
the character matching "<code>.</code>" is captured with number&nbsp;2,
and the part matching "<code>%s*</code>" has number&nbsp;3.


<p>
As a special case, the empty capture <code>()</code> captures
the current string position (a number).
For instance, if we apply the pattern <code>"()aa()"</code> on the
string <code>"flaaap"</code>, there will be two captures: 3&nbsp;and&nbsp;5.


<p>
A pattern cannot contain embedded zeros.  Use <code>%z</code> instead.











<h2>5.5 - <a name="5.5">Table Manipulation</a></h2><p>
This library provides generic functions for table manipulation.
It provides all its functions inside the table <a name="pdf-table"><code>table</code></a>.


<p>
Most functions in the table library assume that the table
represents an array or a list.
For these functions, when we talk about the "length" of a table
we mean the result of the length operator.


<p>
<hr><h3><a name="pdf-table.concat"><code>table.concat (table [, sep [, i [, j]]])</code></a></h3>
Given an array where all elements are strings or numbers,
returns <code>table[i]..sep..table[i+1] &middot;&middot;&middot; sep..table[j]</code>.
The default value for <code>sep</code> is the empty string,
the default for <code>i</code> is 1,
and the default for <code>j</code> is the length of the table.
If <code>i</code> is greater than <code>j</code>, returns the empty string.




<p>
<hr><h3><a name="pdf-table.insert"><code>table.insert (table, [pos,] value)</code></a></h3>


<p>
Inserts element <code>value</code> at position <code>pos</code> in <code>table</code>,
shifting up other elements to open space, if necessary.
The default value for <code>pos</code> is <code>n+1</code>,
where <code>n</code> is the length of the table (see <a href="#2.5.5">&sect;2.5.5</a>),
so that a call <code>table.insert(t,x)</code> inserts <code>x</code> at the end
of table <code>t</code>.




<p>
<hr><h3><a name="pdf-table.maxn"><code>table.maxn (table)</code></a></h3>


<p>
Returns the largest positive numerical index of the given table,
or zero if the table has no positive numerical indices.
(To do its job this function does a linear traversal of
the whole table.) 




<p>
<hr><h3><a name="pdf-table.remove"><code>table.remove (table [, pos])</code></a></h3>


<p>
Removes from <code>table</code> the element at position <code>pos</code>,
shifting down other elements to close the space, if necessary.
Returns the value of the removed element.
The default value for <code>pos</code> is <code>n</code>,
where <code>n</code> is the length of the table,
so that a call <code>table.remove(t)</code> removes the last element
of table <code>t</code>.




<p>
<hr><h3><a name="pdf-table.sort"><code>table.sort (table [, comp])</code></a></h3>
Sorts table elements in a given order, <em>in-place</em>,
from <code>table[1]</code> to <code>table[n]</code>,
where <code>n</code> is the length of the table.
If <code>comp</code> is given,
then it must be a function that receives two table elements,
and returns true
when the first is less than the second
(so that <code>not comp(a[i+1],a[i])</code> will be true after the sort).
If <code>comp</code> is not given,
then the standard Lua operator <code>&lt;</code> is used instead.


<p>
The sort algorithm is not stable;
that is, elements considered equal by the given order
may have their relative positions changed by the sort.







<h2>5.6 - <a name="5.6">Mathematical Functions</a></h2>

<p>
This library is an interface to the standard C&nbsp;math library.
It provides all its functions inside the table <a name="pdf-math"><code>math</code></a>.


<p>
<hr><h3><a name="pdf-math.abs"><code>math.abs (x)</code></a></h3>


<p>
Returns the absolute value of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.acos"><code>math.acos (x)</code></a></h3>


<p>
Returns the arc cosine of <code>x</code> (in radians).




<p>
<hr><h3><a name="pdf-math.asin"><code>math.asin (x)</code></a></h3>


<p>
Returns the arc sine of <code>x</code> (in radians).




<p>
<hr><h3><a name="pdf-math.atan"><code>math.atan (x)</code></a></h3>


<p>
Returns the arc tangent of <code>x</code> (in radians).




<p>
<hr><h3><a name="pdf-math.atan2"><code>math.atan2 (y, x)</code></a></h3>


<p>
Returns the arc tangent of <code>y/x</code> (in radians),
but uses the signs of both parameters to find the
quadrant of the result.
(It also handles correctly the case of <code>x</code> being zero.)




<p>
<hr><h3><a name="pdf-math.ceil"><code>math.ceil (x)</code></a></h3>


<p>
Returns the smallest integer larger than or equal to <code>x</code>.




<p>
<hr><h3><a name="pdf-math.cos"><code>math.cos (x)</code></a></h3>


<p>
Returns the cosine of <code>x</code> (assumed to be in radians).




<p>
<hr><h3><a name="pdf-math.cosh"><code>math.cosh (x)</code></a></h3>


<p>
Returns the hyperbolic cosine of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.deg"><code>math.deg (x)</code></a></h3>


<p>
Returns the angle <code>x</code> (given in radians) in degrees.




<p>
<hr><h3><a name="pdf-math.exp"><code>math.exp (x)</code></a></h3>


<p>
Returns the value <em>e<sup>x</sup></em>.




<p>
<hr><h3><a name="pdf-math.floor"><code>math.floor (x)</code></a></h3>


<p>
Returns the largest integer smaller than or equal to <code>x</code>.




<p>
<hr><h3><a name="pdf-math.fmod"><code>math.fmod (x, y)</code></a></h3>


<p>
Returns the remainder of the division of <code>x</code> by <code>y</code>
that rounds the quotient towards zero.




<p>
<hr><h3><a name="pdf-math.frexp"><code>math.frexp (x)</code></a></h3>


<p>
Returns <code>m</code> and <code>e</code> such that <em>x = m2<sup>e</sup></em>,
<code>e</code> is an integer and the absolute value of <code>m</code> is
in the range <em>[0.5, 1)</em>
(or zero when <code>x</code> is zero).




<p>
<hr><h3><a name="pdf-math.huge"><code>math.huge</code></a></h3>


<p>
The value <code>HUGE_VAL</code>,
a value larger than or equal to any other numerical value.




<p>
<hr><h3><a name="pdf-math.ldexp"><code>math.ldexp (m, e)</code></a></h3>


<p>
Returns <em>m2<sup>e</sup></em> (<code>e</code> should be an integer).




<p>
<hr><h3><a name="pdf-math.log"><code>math.log (x)</code></a></h3>


<p>
Returns the natural logarithm of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.log10"><code>math.log10 (x)</code></a></h3>


<p>
Returns the base-10 logarithm of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.max"><code>math.max (x, &middot;&middot;&middot;)</code></a></h3>


<p>
Returns the maximum value among its arguments.




<p>
<hr><h3><a name="pdf-math.min"><code>math.min (x, &middot;&middot;&middot;)</code></a></h3>


<p>
Returns the minimum value among its arguments.




<p>
<hr><h3><a name="pdf-math.modf"><code>math.modf (x)</code></a></h3>


<p>
Returns two numbers,
the integral part of <code>x</code> and the fractional part of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.pi"><code>math.pi</code></a></h3>


<p>
The value of <em>pi</em>.




<p>
<hr><h3><a name="pdf-math.pow"><code>math.pow (x, y)</code></a></h3>


<p>
Returns <em>x<sup>y</sup></em>.
(You can also use the expression <code>x^y</code> to compute this value.)




<p>
<hr><h3><a name="pdf-math.rad"><code>math.rad (x)</code></a></h3>


<p>
Returns the angle <code>x</code> (given in degrees) in radians.




<p>
<hr><h3><a name="pdf-math.random"><code>math.random ([m [, n]])</code></a></h3>


<p>
This function is an interface to the simple
pseudo-random generator function <code>rand</code> provided by ANSI&nbsp;C.
(No guarantees can be given for its statistical properties.)


<p>
When called without arguments,
returns a uniform pseudo-random real number
in the range <em>[0,1)</em>.  
When called with an integer number <code>m</code>,
<code>math.random</code> returns
a uniform pseudo-random integer in the range <em>[1, m]</em>.
When called with two integer numbers <code>m</code> and <code>n</code>,
<code>math.random</code> returns a uniform pseudo-random
integer in the range <em>[m, n]</em>.




<p>
<hr><h3><a name="pdf-math.randomseed"><code>math.randomseed (x)</code></a></h3>


<p>
Sets <code>x</code> as the "seed"
for the pseudo-random generator:
equal seeds produce equal sequences of numbers.




<p>
<hr><h3><a name="pdf-math.sin"><code>math.sin (x)</code></a></h3>


<p>
Returns the sine of <code>x</code> (assumed to be in radians).




<p>
<hr><h3><a name="pdf-math.sinh"><code>math.sinh (x)</code></a></h3>


<p>
Returns the hyperbolic sine of <code>x</code>.




<p>
<hr><h3><a name="pdf-math.sqrt"><code>math.sqrt (x)</code></a></h3>


<p>
Returns the square root of <code>x</code>.
(You can also use the expression <code>x^0.5</code> to compute this value.)




<p>
<hr><h3><a name="pdf-math.tan"><code>math.tan (x)</code></a></h3>


<p>
Returns the tangent of <code>x</code> (assumed to be in radians).




<p>
<hr><h3><a name="pdf-math.tanh"><code>math.tanh (x)</code></a></h3>


<p>
Returns the hyperbolic tangent of <code>x</code>.







<h2>5.7 - <a name="5.7">File Input and Output Facilities</a></h2>

<div class='teliva'>
While Teliva supports the standard Lua primitives for managing files,
idiomatic Teliva uses some slightly different primitives.

<p>
<hr><h3><a name="pdf-start_reading"><code>start_reading (fs, filename)</code></a></h3>

<p>
This function opens a file exclusively for reading, and returns a
<tt>channel</tt> (NOT a file as regular Lua) or <b>nil</b> on error.
<tt>recv()</tt> from the channel to read a line at a time from the file.

<p>
(The <tt>fs</tt> parameter is currently unused. It will be used to pass in
fake file systems for tests.)

<p>
<hr><h3><a name="pdf-character_by_character"><code>character_by_character(chanin, chanout)</code></a></h3>

<p>
This function converts a channel that can <tt>recv()</tt> a line at a time
from a file into a channel that can <tt>recv()</tt> a character at a time.
Don't try to mix by-line reads with by-character reads. Once a channel is
passed into <tt>character_by_character</tt>, calling code should stop trying
to use it.


<p>
<hr><h3><a name="pdf-start_writing"><code>start_writing(fs, filename)</code></a></h3>

<p>
This function opens a file exclusively for writing, and returns a
<tt>channel</tt> (NOT a file as regular Lua) or <b>nil</b> on error.
<tt>send()</tt> to the channel will write to the file. <tt>close()</tt> on the
channel will persist the changes and make them externally visible. All writes
are hidden until <tt>close()</tt>.

<p>
(The <tt>fs</tt> parameter is currently unused. It will be used to pass in
fake file systems for tests.)


<hr>
The rest of this section describes Lua's standard primitives for File I/O.
</div>


<p>
Unless otherwise stated,
all File I/O functions return <b>nil</b> on failure
(plus an error message as a second result and
a system-dependent error code as a third result)
and some value different from <b>nil</b> on success.


<p>
<hr><h3><a name="pdf-io.close"><code>io.close (file)</code></a></h3>


<p>
Equivalent to <code>file:close()</code>.



<p>
<hr><h3><a name="pdf-io.open"><code>io.open (filename [, mode])</code></a></h3>


<p>
This function opens a file,
in the mode specified in the string <code>mode</code>.
It returns a new file handle,
or, in case of errors, <b>nil</b> plus an error message.


<p>
The <code>mode</code> string can be any of the following:

<ul>
<li><b>"r":</b> read mode (the default);</li>
<li><b>"w":</b> write mode;</li>
<li><b>"a":</b> append mode;</li>
<li><b>"r+":</b> update mode, all previous data is preserved;</li>
<li><b>"w+":</b> update mode, all previous data is erased;</li>
<li><b>"a+":</b> append update mode, previous data is preserved,
  writing is only allowed at the end of file.</li>
</ul><p>
The <code>mode</code> string can also have a '<code>b</code>' at the end,
which is needed in some systems to open the file in binary mode.
This string is exactly what is used in the
standard&nbsp;C function <code>fopen</code>.




<p>
<hr><h3><a name="pdf-io.tmpfile"><code>io.tmpfile ()</code></a></h3>


<p>
Returns a handle for a temporary file.
This file is opened in update mode
and it is automatically removed when the program ends.




<p>
<hr><h3><a name="pdf-io.type"><code>io.type (obj)</code></a></h3>


<p>
Checks whether <code>obj</code> is a valid file handle.
Returns the string <code>"file"</code> if <code>obj</code> is an open file handle,
<code>"closed file"</code> if <code>obj</code> is a closed file handle,
or <b>nil</b> if <code>obj</code> is not a file handle.




<p>
<hr><h3><a name="pdf-file:close"><code>file:close ()</code></a></h3>


<p>
Closes <code>file</code>.
Note that files are automatically closed when
their handles are garbage collected,
but that takes an unpredictable amount of time to happen.




<p>
<hr><h3><a name="pdf-file:flush"><code>file:flush ()</code></a></h3>


<p>
Saves any written data to <code>file</code>.




<p>
<hr><h3><a name="pdf-file:lines"><code>file:lines ()</code></a></h3>


<p>
Returns an iterator function that,
each time it is called,
returns a new line from the file.
Therefore, the construction

<pre>
     for line in file:lines() do <em>body</em> end
</pre><p>
will iterate over all lines of the file.
(Unlike <a href="#pdf-io.lines"><code>io.lines</code></a>, this function does not close the file
when the loop ends.)




<p>
<hr><h3><a name="pdf-file:read"><code>file:read (&middot;&middot;&middot;)</code></a></h3>


<p>
Reads the file <code>file</code>,
according to the given formats, which specify what to read.
For each format,
the function returns a string (or a number) with the characters read,
or <b>nil</b> if it cannot read data with the specified format.
When called without formats,
it uses a default format that reads the entire next line
(see below).


<p>
The available formats are

<ul>

<li><b>"*n":</b>
reads a number;
this is the only format that returns a number instead of a string.
</li>

<li><b>"*a":</b>
reads the whole file, starting at the current position.
On end of file, it returns the empty string.
</li>

<li><b>"*l":</b>
reads the next line (skipping the end of line),
returning <b>nil</b> on end of file.
This is the default format.
</li>

<li><b><em>number</em>:</b>
reads a string with up to this number of characters,
returning <b>nil</b> on end of file.
If number is zero,
it reads nothing and returns an empty string,
or <b>nil</b> on end of file.
</li>

</ul>



<p>
<hr><h3><a name="pdf-file:seek"><code>file:seek ([whence] [, offset])</code></a></h3>


<p>
Sets and gets the file position,
measured from the beginning of the file,
to the position given by <code>offset</code> plus a base
specified by the string <code>whence</code>, as follows:

<ul>
<li><b>"set":</b> base is position 0 (beginning of the file);</li>
<li><b>"cur":</b> base is current position;</li>
<li><b>"end":</b> base is end of file;</li>
</ul><p>
In case of success, function <code>seek</code> returns the final file position,
measured in bytes from the beginning of the file.
If this function fails, it returns <b>nil</b>,
plus a string describing the error.


<p>
The default value for <code>whence</code> is <code>"cur"</code>,
and for <code>offset</code> is 0.
Therefore, the call <code>file:seek()</code> returns the current
file position, without changing it;
the call <code>file:seek("set")</code> sets the position to the
beginning of the file (and returns 0);
and the call <code>file:seek("end")</code> sets the position to the
end of the file, and returns its size.




<p>
<hr><h3><a name="pdf-file:setvbuf"><code>file:setvbuf (mode [, size])</code></a></h3>


<p>
Sets the buffering mode for an output file.
There are three available modes:

<ul>

<li><b>"no":</b>
no buffering; the result of any output operation appears immediately.
</li>

<li><b>"full":</b>
full buffering; output operation is performed only
when the buffer is full (or when you explicitly <code>flush</code> the file
(see <a href="#pdf-io.flush"><code>io.flush</code></a>)).
</li>

<li><b>"line":</b>
line buffering; output is buffered until a newline is output
or there is any input from some special files
(such as a terminal device).
</li>

</ul><p>
For the last two cases, <code>size</code>
specifies the size of the buffer, in bytes.
The default is an appropriate size.




<p>
<hr><h3><a name="pdf-file:write"><code>file:write (&middot;&middot;&middot;)</code></a></h3>


<p>
Writes the value of each of its arguments to
the <code>file</code>.
The arguments must be strings or numbers.
To write other values,
use <a href="#pdf-tostring"><code>tostring</code></a> or <a href="#pdf-string.format"><code>string.format</code></a> before <code>write</code>.







<h2>5.8 - <a name="5.8">Operating System Facilities</a></h2>

<p>
This library is implemented through table <a name="pdf-os"><code>os</code></a>.


<p>
<hr><h3><a name="pdf-os.clock"><code>os.clock ()</code></a></h3>


<p>
Returns an approximation of the amount in seconds of CPU time
used by the program.




<p>
<hr><h3><a name="pdf-os.date"><code>os.date ([format [, time]])</code></a></h3>


<p>
Returns a string or a table containing date and time,
formatted according to the given string <code>format</code>.


<p>
If the <code>time</code> argument is present,
this is the time to be formatted
(see the <a href="#pdf-os.time"><code>os.time</code></a> function for a description of this value).
Otherwise, <code>date</code> formats the current time.


<p>
If <code>format</code> starts with '<code>!</code>',
then the date is formatted in Coordinated Universal Time.
After this optional character,
if <code>format</code> is the string "<code>*t</code>",
then <code>date</code> returns a table with the following fields:
<code>year</code> (four digits), <code>month</code> (1--12), <code>day</code> (1--31),
<code>hour</code> (0--23), <code>min</code> (0--59), <code>sec</code> (0--61),
<code>wday</code> (weekday, Sunday is&nbsp;1),
<code>yday</code> (day of the year),
and <code>isdst</code> (daylight saving flag, a boolean).


<p>
If <code>format</code> is not "<code>*t</code>",
then <code>date</code> returns the date as a string,
formatted according to the same rules as the C&nbsp;function <code>strftime</code>.


<p>
When called without arguments,
<code>date</code> returns a reasonable date and time representation that depends on
the host system and on the current locale
(that is, <code>os.date()</code> is equivalent to <code>os.date("%c")</code>).




<p>
<hr><h3><a name="pdf-os.difftime"><code>os.difftime (t2, t1)</code></a></h3>


<p>
Returns the number of seconds from time <code>t1</code> to time <code>t2</code>.
In POSIX, Windows, and some other systems,
this value is exactly <code>t2</code><em>-</em><code>t1</code>.




<p>
<hr><h3><a name="pdf-os.execute"><code>os.execute ([command])</code></a></h3>


<p>
This function is equivalent to the C&nbsp;function <code>system</code>.
It passes <code>command</code> to be executed by an operating system shell.
It returns a status code, which is system-dependent.
If <code>command</code> is absent, then it returns nonzero if a shell is available
and zero otherwise.




<p>
<hr><h3><a name="pdf-os.exit"><code>os.exit ([code])</code></a></h3>


<p>
Calls the C&nbsp;function <code>exit</code>,
with an optional <code>code</code>,
to terminate the host program.
The default value for <code>code</code> is the success code.




<p>
<hr><h3><a name="pdf-os.getenv"><code>os.getenv (varname)</code></a></h3>


<p>
Returns the value of the process environment variable <code>varname</code>,
or <b>nil</b> if the variable is not defined.




<p>
<hr><h3><a name="pdf-os.remove"><code>os.remove (filename)</code></a></h3>


<p>
Deletes the file or directory with the given name.
Directories must be empty to be removed.
If this function fails, it returns <b>nil</b>,
plus a string describing the error.




<p>
<hr><h3><a name="pdf-os.rename"><code>os.rename (oldname, newname)</code></a></h3>


<p>
Renames file or directory named <code>oldname</code> to <code>newname</code>.
If this function fails, it returns <b>nil</b>,
plus a string describing the error.




<p>
<hr><h3><a name="pdf-os.setlocale"><code>os.setlocale (locale [, category])</code></a></h3>


<p>
Sets the current locale of the program.
<code>locale</code> is a string specifying a locale;
<code>category</code> is an optional string describing which category to change:
<code>"all"</code>, <code>"collate"</code>, <code>"ctype"</code>,
<code>"monetary"</code>, <code>"numeric"</code>, or <code>"time"</code>;
the default category is <code>"all"</code>.
The function returns the name of the new locale,
or <b>nil</b> if the request cannot be honored.


<p>
If <code>locale</code> is the empty string,
the current locale is set to an implementation-defined native locale.
If <code>locale</code> is the string "<code>C</code>",
the current locale is set to the standard C locale.


<p>
When called with <b>nil</b> as the first argument,
this function only returns the name of the current locale
for the given category.




<p>
<hr><h3><a name="pdf-os.time"><code>os.time ([table])</code></a></h3>


<p>
Returns the current time when called without arguments,
or a time representing the date and time specified by the given table.
This table must have fields <code>year</code>, <code>month</code>, and <code>day</code>,
and may have fields <code>hour</code>, <code>min</code>, <code>sec</code>, and <code>isdst</code>
(for a description of these fields, see the <a href="#pdf-os.date"><code>os.date</code></a> function).


<p>
The returned value is a number, whose meaning depends on your system.
In POSIX, Windows, and some other systems, this number counts the number
of seconds since some given start time (the "epoch").
In other systems, the meaning is not specified,
and the number returned by <code>time</code> can be used only as an argument to
<code>date</code> and <code>difftime</code>.




<p>
<hr><h3><a name="pdf-os.tmpname"><code>os.tmpname ()</code></a></h3>


<p>
Returns a string with a file name that can
be used for a temporary file.
The file must be explicitly opened before its use
and explicitly removed when no longer needed.


<p>
On some systems (POSIX),
this function also creates a file with that name,
to avoid security risks.
(Someone else might create the file with wrong permissions
in the time between getting the name and creating the file.)
You still have to open the file to use it
and to remove it (even if you do not use it).


<p>
When possible,
you may prefer to use <a href="#pdf-io.tmpfile"><code>io.tmpfile</code></a>,
which automatically removes the file when the program ends.







<h2>5.9 - <a name="5.9">The Debug Library</a></h2>

<p>
This library provides
the functionality of the debug interface to Lua programs.
You should exert care when using this library.
The functions provided here should be used exclusively for debugging
and similar tasks, such as profiling.
Please resist the temptation to use them as a
usual programming tool:
they can be very slow.
Moreover, several of these functions
violate some assumptions about Lua code
(e.g., that variables local to a function
cannot be accessed from outside or
that userdata metatables cannot be changed by Lua code)
and therefore can compromise otherwise secure code.
<div class='teliva'>TODO: what do we need to secure here?</div>


<p>
All functions in this library are provided
inside the <a name="pdf-debug"><code>debug</code></a> table.
All functions that operate over a thread
have an optional first argument which is the
thread to operate over.
The default is always the current thread.


<p>
<hr><h3><a name="pdf-debug.debug"><code>debug.debug ()</code></a></h3>


<p>
Enters an interactive mode with the user,
running each string that the user enters.
Using simple commands and other debug facilities,
the user can inspect global and local variables,
change their values, evaluate expressions, and so on.
A line containing only the word <code>cont</code> finishes this function,
so that the caller continues its execution.


<p>
Note that commands for <code>debug.debug</code> are not lexically nested
within any function, and so have no direct access to local variables.




<p>
<hr><h3><a name="pdf-debug.getfenv"><code>debug.getfenv (o)</code></a></h3>
Returns the environment of object <code>o</code>.




<p>
<hr><h3><a name="pdf-debug.gethook"><code>debug.gethook ([thread])</code></a></h3>


<p>
Returns the current hook settings of the thread, as three values:
the current hook function, the current hook mask,
and the current hook count
(as set by the <a href="#pdf-debug.sethook"><code>debug.sethook</code></a> function).




<p>
<hr><h3><a name="pdf-debug.getinfo"><code>debug.getinfo ([thread,] function [, what])</code></a></h3>


<p>
Returns a table with information about a function.
You can give the function directly,
or you can give a number as the value of <code>function</code>,
which means the function running at level <code>function</code> of the call stack
of the given thread:
level&nbsp;0 is the current function (<code>getinfo</code> itself);
level&nbsp;1 is the function that called <code>getinfo</code>;
and so on.
If <code>function</code> is a number larger than the number of active functions,
then <code>getinfo</code> returns <b>nil</b>.


<p>
The returned table can contain all the fields returned by <a href="#lua_getinfo"><code>lua_getinfo</code></a>,
with the string <code>what</code> describing which fields to fill in.
The default for <code>what</code> is to get all information available,
except the table of valid lines.
If present,
the option '<code>f</code>'
adds a field named <code>func</code> with the function itself.
If present,
the option '<code>L</code>'
adds a field named <code>activelines</code> with the table of
valid lines.


<p>
For instance, the expression <code>debug.getinfo(1,"n").name</code> returns
a table with a name for the current function,
if a reasonable name can be found,
and the expression <code>debug.getinfo(print)</code>
returns a table with all available information
about the <a href="#pdf-print"><code>print</code></a> function.




<p>
<hr><h3><a name="pdf-debug.getlocal"><code>debug.getlocal ([thread,] level, local)</code></a></h3>


<p>
This function returns the name and the value of the local variable
with index <code>local</code> of the function at level <code>level</code> of the stack.
(The first parameter or local variable has index&nbsp;1, and so on,
until the last active local variable.)
The function returns <b>nil</b> if there is no local
variable with the given index,
and raises an error when called with a <code>level</code> out of range.
(You can call <a href="#pdf-debug.getinfo"><code>debug.getinfo</code></a> to check whether the level is valid.)


<p>
Variable names starting with '<code>(</code>' (open parentheses)
represent internal variables
(loop control variables, temporaries, and C&nbsp;function locals).




<p>
<hr><h3><a name="pdf-debug.getmetatable"><code>debug.getmetatable (object)</code></a></h3>


<p>
Returns the metatable of the given <code>object</code>
or <b>nil</b> if it does not have a metatable.




<p>
<hr><h3><a name="pdf-debug.getregistry"><code>debug.getregistry ()</code></a></h3>


<p>
Returns the registry table (see <a href="#3.5">&sect;3.5</a>).




<p>
<hr><h3><a name="pdf-debug.getupvalue"><code>debug.getupvalue (func, up)</code></a></h3>


<p>
This function returns the name and the value of the upvalue
with index <code>up</code> of the function <code>func</code>.
The function returns <b>nil</b> if there is no upvalue with the given index.




<p>
<hr><h3><a name="pdf-debug.setfenv"><code>debug.setfenv (object, table)</code></a></h3>


<p>
Sets the environment of the given <code>object</code> to the given <code>table</code>.
Returns <code>object</code>.




<p>
<hr><h3><a name="pdf-debug.sethook"><code>debug.sethook ([thread,] hook, mask [, count])</code></a></h3>


<p>
Sets the given function as a hook.
The string <code>mask</code> and the number <code>count</code> describe
when the hook will be called.
The string mask may have the following characters,
with the given meaning:

<ul>
<li><b><code>"c"</code>:</b> the hook is called every time Lua calls a function;</li>
<li><b><code>"r"</code>:</b> the hook is called every time Lua returns from a function;</li>
<li><b><code>"l"</code>:</b> the hook is called every time Lua enters a new line of code.</li>
</ul><p>
With a <code>count</code> different from zero,
the hook is called after every <code>count</code> instructions.


<p>
When called without arguments,
<a href="#pdf-debug.sethook"><code>debug.sethook</code></a> turns off the hook.


<p>
When the hook is called, its first parameter is a string
describing the event that has triggered its call:
<code>"call"</code>, <code>"return"</code> (or <code>"tail return"</code>,
when simulating a return from a tail call),
<code>"line"</code>, and <code>"count"</code>.
For line events,
the hook also gets the new line number as its second parameter.
Inside a hook,
you can call <code>getinfo</code> with level&nbsp;2 to get more information about
the running function
(level&nbsp;0 is the <code>getinfo</code> function,
and level&nbsp;1 is the hook function),
unless the event is <code>"tail return"</code>.
In this case, Lua is only simulating the return,
and a call to <code>getinfo</code> will return invalid data.




<p>
<hr><h3><a name="pdf-debug.setlocal"><code>debug.setlocal ([thread,] level, local, value)</code></a></h3>


<p>
This function assigns the value <code>value</code> to the local variable
with index <code>local</code> of the function at level <code>level</code> of the stack.
The function returns <b>nil</b> if there is no local
variable with the given index,
and raises an error when called with a <code>level</code> out of range.
(You can call <code>getinfo</code> to check whether the level is valid.)
Otherwise, it returns the name of the local variable.




<p>
<hr><h3><a name="pdf-debug.setmetatable"><code>debug.setmetatable (object, table)</code></a></h3>


<p>
Sets the metatable for the given <code>object</code> to the given <code>table</code>
(which can be <b>nil</b>).




<p>
<hr><h3><a name="pdf-debug.setupvalue"><code>debug.setupvalue (func, up, value)</code></a></h3>


<p>
This function assigns the value <code>value</code> to the upvalue
with index <code>up</code> of the function <code>func</code>.
The function returns <b>nil</b> if there is no upvalue
with the given index.
Otherwise, it returns the name of the upvalue.




<p>
<hr><h3><a name="pdf-debug.traceback"><code>debug.traceback ([thread,] [message [, level]])</code></a></h3>


<p>
Returns a string with a traceback of the call stack.
An optional <code>message</code> string is appended
at the beginning of the traceback.
An optional <code>level</code> number tells at which level
to start the traceback
(default is 1, the function calling <code>traceback</code>).


<div class='teliva'>
<h2>5.10 - <a name="5.10">Curses Window Facilities</a></h2>

Teliva includes curses facilities identical to Lua's <a href='http://lcurses.github.io/lcurses/'>lcurses</a>
library. As there, the top-level module is called <tt>curses</tt>. All apps
start with the terminal window initialized using <tt>curses.initscr()</tt>.
Look at the sample apps for example usage.

</div>

<div class='teliva'>
<h2>5.11 - <a name="5.11">Networking Facilities</a></h2>

Teliva includes the following well-known Lua libraries for networking:

<li><a href='https://w3.impa.br/~diego/software/luasocket/reference.html'>LuaSocket</a>,
consisting of modules <tt>socket</tt>, <tt>http</tt>, <tt>url</tt>,
<tt>headers</tt>, <tt>mime</tt> and <tt>ltn12</tt>.

<li><a href='https://github.com/brunoos/luasec/wiki'>LuaSec</a>,
consisting of modules <tt>https</tt> and <tt>ssl</tt>.

</div>

<div class='teliva'>
<h2>5.12 - <a name="5.12">JSON Facilities</a></h2>

Teliva includes the well-known
<a href='https://github.com/rxi/json.lua'>json.lua</a> library (module
<tt>json</tt>). It also includes a variant in module <tt>jsonf</tt> that can
read or write JSON from channels opened by
<a href='#pdf-start_reading'>start_reading</a>
and
<a href='#pdf-start_writing'>start_writing</a>.

</div>

<div class='teliva'>
<h2>5.13 - <a name="5.13">Tasks and Channels</a></h2>

Teliva includes the well-known
<a href='https://github.com/majek/lua-channels#readme'>lua-channels</a>
library in module <tt>task</tt>. It also transparently starts up
<tt>task.scheduler</tt> for all apps.

</div>



<h1>8 - <a name="8">The Complete Syntax of Lua</a></h1>

<p>
Here is the complete syntax of Lua in extended BNF.
(It does not describe operator precedences.)




<pre>

	chunk ::= {stat [`<b>;</b>&acute;]} [laststat [`<b>;</b>&acute;]]

	block ::= chunk

	stat ::=  varlist `<b>=</b>&acute; explist | 
		 functioncall | 
		 <b>do</b> block <b>end</b> | 
		 <b>while</b> exp <b>do</b> block <b>end</b> | 
		 <b>repeat</b> block <b>until</b> exp | 
		 <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b> | 
		 <b>for</b> Name `<b>=</b>&acute; exp `<b>,</b>&acute; exp [`<b>,</b>&acute; exp] <b>do</b> block <b>end</b> | 
		 <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b> | 
		 <b>function</b> funcname funcbody | 
		 <b>local</b> <b>function</b> Name funcbody | 
		 <b>local</b> namelist [`<b>=</b>&acute; explist] 

	laststat ::= <b>return</b> [explist] | <b>break</b>

	funcname ::= Name {`<b>.</b>&acute; Name} [`<b>:</b>&acute; Name]

	varlist ::= var {`<b>,</b>&acute; var}

	var ::=  Name | prefixexp `<b>[</b>&acute; exp `<b>]</b>&acute; | prefixexp `<b>.</b>&acute; Name 

	namelist ::= Name {`<b>,</b>&acute; Name}

	explist ::= {exp `<b>,</b>&acute;} exp

	exp ::=  <b>nil</b> | <b>false</b> | <b>true</b> | Number | String | `<b>...</b>&acute; | function | 
		 prefixexp | tableconstructor | exp binop exp | unop exp 

	prefixexp ::= var | functioncall | `<b>(</b>&acute; exp `<b>)</b>&acute;

	functioncall ::=  prefixexp args | prefixexp `<b>:</b>&acute; Name args 

	args ::=  `<b>(</b>&acute; [explist] `<b>)</b>&acute; | tableconstructor | String 

	function ::= <b>function</b> funcbody

	funcbody ::= `<b>(</b>&acute; [parlist] `<b>)</b>&acute; block <b>end</b>

	parlist ::= namelist [`<b>,</b>&acute; `<b>...</b>&acute;] | `<b>...</b>&acute;

	tableconstructor ::= `<b>{</b>&acute; [fieldlist] `<b>}</b>&acute;

	fieldlist ::= field {fieldsep field} [fieldsep]

	field ::= `<b>[</b>&acute; exp `<b>]</b>&acute; `<b>=</b>&acute; exp | Name `<b>=</b>&acute; exp | exp

	fieldsep ::= `<b>,</b>&acute; | `<b>;</b>&acute;

	binop ::= `<b>+</b>&acute; | `<b>-</b>&acute; | `<b>*</b>&acute; | `<b>/</b>&acute; | `<b>^</b>&acute; | `<b>%</b>&acute; | `<b>..</b>&acute; | 
		 `<b>&lt;</b>&acute; | `<b>&lt;=</b>&acute; | `<b>&gt;</b>&acute; | `<b>&gt;=</b>&acute; | `<b>==</b>&acute; | `<b>~=</b>&acute; | 
		 <b>and</b> | <b>or</b>

	unop ::= `<b>-</b>&acute; | <b>not</b> | `<b>#</b>&acute;

</pre>

<p>







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