neovim/runtime/doc/luaref.txt

*luaref.txt*           Nvim
                                                          *luaref* *Lua-Reference*

                             LUA REFERENCE MANUAL


                                 Version 0.3.0
                                August 7th, 2022


                    Vimdoc version (c) 2006 by Luis Carvalho
                         <lexcarvalho at gmail dot com>

                    Adapted from "Lua: 5.1 reference manual"
                 R. Ierusalimschy, L. H. de Figueiredo, W. Celes
                      Copyright (c) 2006 Lua.org, PUC-Rio.


                 See |luaref-doc| for information on this manual.
                 See |luaref-copyright| for copyright and licenses.


Type |gO| to see the table of contents.

==============================================================================
1  INTRODUCTION                                                   *luaref-intro*

Lua is an extension programming language designed to support general
procedural programming with data description facilities.  It also offers good
support for object-oriented programming, functional programming, and
data-driven programming.  Lua is intended to be used as a powerful,
light-weight scripting language for any program that needs one.  Lua is
implemented as a library, written in clean C (that is, in the common subset of
ANSI C and C++).

Being an extension language, Lua has no notion of a "main" program: it only
works embedded in a host client, called the embedding program or simply the
host. This host program can invoke functions to execute a piece of Lua code,
can write and read Lua variables, and can register C functions to be called by
Lua code.  Through the use of C functions, Lua can be augmented to cope with a
wide range of different domains, thus creating customized programming
languages sharing a syntactical framework.

Lua is free software, and is provided as usual with no guarantees, as stated
in its license. The implementation described in this manual is available at
Lua's official web site, www.lua.org.

Like any other reference manual, this document is dry in places. For a
discussion of the decisions behind the design of Lua, see references at
|luaref-bibliography|. For a detailed introduction to programming in Lua, see
Roberto's book, Programming in Lua.

Lua means "moon" in Portuguese and is pronounced LOO-ah.

==============================================================================
2  THE LANGUAGE                                                *luaref-language*

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.

The language constructs will be explained using the usual extended BNF
notation, in which `{ a }` means 0 or more `a`'s, and `[ a ]` means an optional `a`.

==============================================================================
2.1  Lexical Conventions                                    *luaref-langLexConv*

                                               *luaref-names* *luaref-identifiers*
Names (also called identifiers) in Lua can be any string of letters, digits,
and underscores, not beginning with a digit. This coincides with the
definition of identifiers in most languages. (The definition of letter depends
on the current locale: any character considered alphabetic by the current
locale can be used in an identifier.) Identifiers are used to name variables
and table fields.

The following keywords are reserved and cannot be used as names:
>
       and       break     do        else      elseif
       end       false     for       function  if
       in        local     nil       not       or
       repeat    return    then      true      until     while
<
Lua is a case-sensitive language: `and` is a reserved word, but `And` and `AND` are
two different, valid names. As a convention, names starting with an underscore
followed by uppercase letters (such as `_VERSION`) are reserved for internal
global variables used by Lua.

The following strings denote other tokens:
>
       +     -     *     /     %     ^     #
       ==    ~=    <=    >=    <     >     =
       (     )     {     }     [     ]
       ;     :     ,     .     ..    ...
<
                                                                *luaref-literal*
Literal strings can be delimited by matching single or double quotes, and can
contain the following C-like escape sequences:

    - `\a`  bell
    - `\b`  backspace
    - `\f`  form feed
    - `\n`  newline
    - `\r`  carriage return
    - `\t`  horizontal tab
    - `\v`  vertical tab
    - `\\`  backslash
    - `\"`  quotation mark (double quote)
    - `\'`  apostrophe (single quote)

Moreover, a backslash followed by a real newline results in a newline in the
string. A character in a string may also be specified by its numerical value
using the escape sequence `\ddd`, where `ddd` 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 may contain
any 8-bit value, including embedded zeros, which can be specified as `\0`.

To put a double (single) quote, a newline, a backslash, or an embedded zero
inside a literal string enclosed by double (single) quotes you must use an
escape sequence. Any other character may be directly inserted into the
literal. (Some control characters may cause problems for the file system, but
Lua has no problem with them.)

Literal strings can also be defined using a long format enclosed by long
brackets. We define an opening long bracket of level n as an opening square
bracket followed by n equal signs followed by another opening square bracket.
So, an opening long bracket of level 0 is written as `[[`, an opening long
bracket of level 1 is written as `[=[`, and so on.
A closing long bracket is defined similarly; for instance, a closing long
bracket of level 4 is written as `]====]`. 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 may run for several lines,
do not interpret any escape sequences, and ignore long brackets of any other
level. They may contain anything except a closing bracket of the proper level.

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 `a` is coded as 97, newline is coded as 10, and `1` is
coded as 49), the five literals below denote the same string:
>lua
       a = 'alo\n123"'
       a = "alo\n123\""
       a = '\97lo\10\04923"'
       a = [[alo
       123"]]
       a = [==[
       alo
       123"]==]
<
                                                            *luaref-numconstant*
A numerical constant may be written with an optional decimal part and an
optional decimal exponent. Lua also accepts integer hexadecimal constants, by
prefixing them with `0x`. Examples of valid numerical constants are
>
     3     3.0     3.1416  314.16e-2   0.31416E1   0xff   0x56
<
                                                                *luaref-comment*
A comment starts with a double hyphen (`--`) anywhere outside a string. If the
text immediately after `--` is not an opening long bracket, the comment is a
short comment, which runs until the end of the line. Otherwise, it is a long
comment, which runs until the corresponding closing long bracket. Long
comments are frequently used to disable code temporarily.

==============================================================================
2.2  Values and Types                                      *luaref-langValTypes*

Lua is a dynamically typed language. 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.

All values in Lua are first-class values. This means that all values can be
stored in variables, passed as arguments to other functions, and returned as
results.

                                                       *luaref-types* *luaref-nil*
                                                      *luaref-true* *luaref-false*
                                                   *luaref-number* *luaref-string*
There are eight basic types in Lua: `nil`, `boolean`, `number`, `string`,
`function`, `userdata`, `thread`, and `table`. Nil is the type of the value
`nil`, whose main property is to be different from any other value; it usually
represents the absence of a useful value. Boolean is the type of the values
`false` and `true`. Both `nil` and `false` make a condition false; any other
value makes it true. Number 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 `luaconf.h`.) String represents arrays of characters. Lua is 8-bit
clean: strings may contain any 8-bit character, including embedded zeros
(`\0`) (see |luaref-literal|).

Lua can call (and manipulate) functions written in Lua and functions written
in C (see |luaref-langFuncCalls|).

                                                           *luaref-userdatatype*
The type userdata is provided to allow arbitrary C data to be stored in Lua
variables. This type corresponds to a block of raw memory and has no
pre-defined operations in Lua, except assignment and identity test. However,
by using metatables, the programmer can define operations for userdata values
(see |luaref-langMetatables|). Userdata values cannot be created or modified
in Lua, only through the C API. This guarantees the integrity of data owned by
the host program.

                                                                 *luaref-thread*
The type `thread` represents independent threads of execution and it is used to
implement coroutines (see |luaref-langCoro|). Do not confuse Lua threads with
operating-system threads. Lua supports coroutines on all systems, even those
that do not support threads.

                                                                  *luaref-table*
The type `table` implements associative arrays, that is, arrays that can be
indexed not only with numbers, but with any value (except `nil`). Tables can
be heterogeneous; that is, they can contain values of all types (except
`nil`). Tables are the sole data structuring mechanism in Lua; they may be
used to represent ordinary arrays, symbol tables, sets, records, graphs,
trees, etc. To represent records, Lua uses the field name as an index. The
language supports this representation by providing `a.name` as syntactic sugar
for `a["name"]`. There are several convenient ways to create tables in Lua
(see |luaref-langTableConst|).

Like indices, the value of a table field can be of any type (except `nil`). In
particular, because functions are first-class values, table fields may contain
functions. Thus tables may also carry methods (see |luaref-langFuncDefs|).

Tables, functions, threads and (full) userdata values are objects: variables
do not actually contain these values, only references to them. Assignment,
parameter passing, and function returns always manipulate references to such
values; these operations do not imply any kind of copy.

The library function `type` returns a string describing the type of a given
value (see |luaref-type()|).

------------------------------------------------------------------------------
2.2.1  Coercion                                            *luaref-langCoercion*

Lua provides automatic conversion between string and number values at run
time. Any arithmetic operation applied to a string tries to convert that
string to a number, following the usual 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 of how numbers are
converted to strings, use the `format` function from the string library (see
|string.format()|).

==============================================================================
2.3  Variables                                            *luaref-langVariables*

Variables are places that store values. There are three kinds of variables in
Lua: global variables, local variables, and table fields.

A single name can denote a global variable or a local variable (or a
function's formal parameter, which is a particular form of local variable):
>
       var ::= Name
<
Name denotes identifiers, as defined in |luaref-langLexConv|.

Any variable is assumed to be global unless explicitly declared as a local
(see |luaref-langLocalDec|). Local variables are lexically scoped: local
variables can be freely accessed by functions defined inside their scope (see
|luaref-langVisibRules|).

Before the first assignment to a variable, its value is `nil`.

Square brackets are used to index a table:
>
       var ::= prefixexp [ exp ]
<
The first expression (`prefixexp`) should result in a table value; the second
expression (`exp`) identifies a specific entry inside that table. The
expression denoting the table to be indexed has a restricted syntax; see
|luaref-langExpressions| for details.

The syntax `var.NAME` is just syntactic sugar for `var["NAME"]` :
>
       var ::= prefixexp . Name
<
All global variables live as fields in ordinary Lua tables, called environment
tables or simply environments (see |luaref-langEnvironments|). 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 `getfenv` (see
|lua_getfenv()|). To replace it, you call `setfenv` (see |luaref-setfenv()|).
(You can only manipulate the environment of C functions through the debug
library; see |luaref-libDebug|.)

An access to a global variable `x` is equivalent to `_env.x`, which in turn is
equivalent to
>lua
       gettable_event(_env, "x")
<
where `_env` is the environment of the running function. (The `_env` variable is
not defined in Lua. We use it here only for explanatory purposes.)

The meaning of accesses to global variables and table fields can be changed
via metatables. An access to an indexed variable `t[i]` is equivalent to a
call `gettable_event(t,i)`. (See |luaref-langMetatables| for a complete
description of the `gettable_event` function. This function is not defined or
callable in Lua. We use it here only for explanatory purposes.)

==============================================================================
2.4  Statements                                               *luaref-langStats*

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

------------------------------------------------------------------------------
2.4.1  Chunks                                   *luaref-chunk* *luaref-langChunks*

The unit of execution of Lua is called a chunk. A chunk is simply a sequence
of statements, which are executed sequentially. Each statement can be
optionally followed by a semicolon:
>
       chunk ::= {stat [ ; ]}
<
There are no empty statements and thus `;;` is not legal.

Lua handles a chunk as the body of an anonymous function with a variable
number of arguments (see |luaref-langFuncDefs|). As such, chunks can define
local variables, receive arguments, and return values.

A chunk may be stored in a file or in a string inside the host program. When a
chunk is executed, first it is pre-compiled into instructions for a virtual
machine, and then the compiled code is executed by an interpreter for the
virtual machine.

Chunks may also be pre-compiled into binary form; see program `luac` for
details. Programs in source and compiled forms are interchangeable; Lua
automatically detects the file type and acts accordingly.

------------------------------------------------------------------------------
2.4.2  Blocks                                   *luaref-block* *luaref-langBlocks*

A block is a list of statements; syntactically, a block is the same as a
chunk:
>
       block ::= chunk
<
                                                          *luaref-do* *luaref-end*
A block may be explicitly delimited to produce a single statement:
>
       stat ::= do block end
<
Explicit blocks are useful to control the scope of variable declarations.
Explicit blocks are also sometimes used to add a `return` or `break` statement
in the middle of another block (see |luaref-langContStructs|).

------------------------------------------------------------------------------
2.4.3  Assignment                                            *luaref-langAssign*

Lua allows multiple assignment. 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:
>
       stat ::= varlist1 = explist1
       varlist1 ::= var { , var }
       explist1 ::= exp { , exp }
<
Expressions are discussed in |luaref-langExpressions|.

Before the assignment, the list of values is adjusted 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 `nil`s as needed. If the list of expressions ends with a function
call, then all values returned by this call enter in the list of values,
before the adjustment (except when the call is enclosed in parentheses; see
|luaref-langExpressions|).

The assignment statement first evaluates all its expressions and only then are
the assignments performed. Thus the code
>lua
       i = 3
       i, a[i] = i+1, 20
<
sets `a[3]` to 20, without affecting `a[4]` because the `i` in `a[i]` is evaluated (to
3) before it is assigned 4. Similarly, the line
>lua
       x, y = y, x
<
exchanges the values of `x` and `y`.

The meaning of assignments to global variables and table fields can be changed
via metatables. An assignment to an indexed variable `t[i] = val` is
equivalent to `settable_event(t,i,val)`. (See |luaref-langMetatables| for a
complete description of the `settable_event` function. This function is not
defined or callable in Lua. We use it here only for explanatory purposes.)

An assignment to a global variable `x = val` is equivalent to the
assignment `_env.x = val`, which in turn is equivalent to
>lua
       settable_event(_env, "x", val)
<
where `_env` is the environment of the running function. (The `_env` variable is
not defined in Lua. We use it here only for explanatory purposes.)

------------------------------------------------------------------------------
2.4.4  Control Structures                               *luaref-langContStructs*

                               *luaref-if* *luaref-then* *luaref-else* *luaref-elseif*
                                       *luaref-while* *luaref-repeat* *luaref-until*
The control structures `if`, `while`, and `repeat` have the usual meaning and
familiar syntax:
>
       stat ::=  while  exp do block end
       stat ::=  repeat  block until exp
       stat ::=  if  exp then block { elseif exp then block }
                 [ else block ] end
<
Lua also has a `for` statement, in two flavors (see |luaref-langForStat|).

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

In the `repeat-until` loop, the inner block does not end at the `until` keyword,
but only after the condition. So, the condition can refer to local variables
declared inside the loop block.

                                                                 *luaref-return*
The `return` statement is used to return values from a function or a chunk
(which is just a function). Functions and chunks may return more than one
value, so the syntax for the `return` statement is

       `stat ::=`  `return`  `[explist1]`

                                                                  *luaref-break*
The `break` statement is used to terminate the execution of a `while`, `repeat`,
or `for` loop, skipping to the next statement after the loop:

       `stat ::=`  `break`

A `break` ends the innermost enclosing loop.

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

------------------------------------------------------------------------------
2.4.5  For Statement                             *luaref-for* *luaref-langForStat*

The `for` statement has two forms: one numeric and one generic.

The numeric `for` loop repeats a block of code while a control variable runs
through an arithmetic progression. It has the following syntax:
>
       stat ::=  for  Name = exp , exp [ , exp ] do block end
<
The `block` is repeated for `name` starting at the value of the first `exp`, until
it passes the second `exp` by steps of the third `exp`. More precisely,
a `for` statement like

       `for var =  e1, e2, e3  do  block  end`

is equivalent to the code: >lua

       do
         local  var, limit, step  = tonumber(e1), tonumber(e2), tonumber(e3)
         if not (  var  and  limit  and  step  ) then error() end
         while (  step  >0 and  var  <=  limit  )
                 or (  step  <=0 and  var  >=  limit  ) do
            block
            var  =  var  +  step
         end
       end
<

Note the following:

 - All three control expressions are evaluated only once, before the loop
   starts. They must all result in numbers.
 - `var`, `limit` and `step` are invisible variables. The names are here for
   explanatory purposes only.
 - If the third expression (the step) is absent, then a step of 1 is used.
 - You can use `break` to exit a `for` loop.
 - The loop variable `var` is local to the loop; you cannot use its value
   after the `for` ends or is broken. If you need this value, assign it to
   another variable before breaking or exiting the loop.

                                                                     *luaref-in*
The generic `for` statement works over functions, called iterators. On each
iteration, the iterator function is called to produce a new value, stopping
when this new value is `nil`. The generic `for` loop has the following syntax:
>
       stat ::=  for  namelist in explist1 do block end
       namelist ::= Name { , Name }
<
A `for` statement like

       `for`  `var1, ..., varn`  `in`  `explist`  `do`  `block`  `end`

is equivalent to the code: >lua

       do
         local  f, s, var  =  explist
         while true do
             local  var1, ..., varn  =  f(s, var)
             var  =  var1
             if  var  == nil then break end
             block
         end
       end
<
Note the following:

 - `explist` is evaluated only once. Its results are an iterator function,
   a `state`, and an initial value for the first iterator variable.
 - `f`, `s`, and `var` are invisible variables. The names are here for
   explanatory purposes only.
 - You can use `break` to exit a `for` loop.
 - The loop variables `var1, ..., varn` are local to the loop; you cannot use
   their values after the `for` ends. If you need these values, then assign
   them to other variables before breaking or exiting the loop.

------------------------------------------------------------------------------
2.4.6  Function Calls as Statements                        *luaref-langFuncStat*

To allow possible side-effects, function calls can be executed as statements:
>
       stat ::= functioncall
<
In this case, all returned values are thrown away. Function calls are
explained in |luaref-langFuncCalls|.

------------------------------------------------------------------------------
2.4.7  Local Declarations                     *luaref-local* *luaref-langLocalDec*

Local variables may be declared anywhere inside a block. The declaration may
include an initial assignment:
>
       stat ::=  local  namelist [ = explist1 ]
       namelist ::= Name { , Name }
<
If present, an initial assignment has the same semantics of a multiple
assignment (see |luaref-langAssign|). Otherwise, all variables are initialized
with `nil`.

A chunk is also a block (see |luaref-langChunks|), 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.

The visibility rules for local variables are explained in
|luaref-langVisibRules|.

==============================================================================
2.5  Expressions                                        *luaref-langExpressions*

The basic expressions in Lua are the following:
>
       exp ::= prefixexp
       exp ::=  nil  |  false  |  true
       exp ::= Number
       exp ::= String
       exp ::= function
       exp ::= tableconstructor
       exp ::= ...
       exp ::= exp binop exp
       exp ::= unop exp
       prefixexp ::= var | functioncall | ( exp )
<
Numbers and literal strings are explained in |luaref-langLexConv|; variables are
explained in |luaref-langVariables|; function definitions are explained in
|luaref-langFuncDefs|; function calls are explained in |luaref-langFuncCalls|;
table constructors are explained in |luaref-langTableConst|. Vararg expressions,
denoted by three dots (`...`), can only be used inside vararg functions;
they are explained in |luaref-langFuncDefs|.

Binary operators comprise arithmetic operators (see |luaref-langArithOp|),
relational operators (see |luaref-langRelOp|), logical operators (see
|luaref-langLogOp|), and the concatenation operator (see |luaref-langConcat|).
Unary operators comprise the unary minus (see |luaref-langArithOp|), the unary
`not` (see |luaref-langLogOp|), and the unary length operator (see
|luaref-langLength|).

Both function calls and vararg expressions may result in multiple values. If
the expression is used as a statement (see |luaref-langFuncStat|)
(only possible for function calls), then its return list is adjusted to zero
elements, thus discarding all returned values. If the 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.

Here are some examples:
>lua
       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 may 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
<
An expression enclosed in parentheses always results in only one value. Thus,
`(f(x,y,z))` is always a single value, even if `f` returns several values.
(The value of `(f(x,y,z))` is the first value returned by `f` or `nil` if `f` does not
return any values.)

------------------------------------------------------------------------------
2.5.1  Arithmetic Operators                                 *luaref-langArithOp*

Lua supports the usual arithmetic operators: the binary `+` (addition),
`-` (subtraction), `*` (multiplication), `/` (division), `%` (modulo)
and `^` (exponentiation); and unary `-` (negation). If the operands are numbers,
or strings that can be converted to numbers (see |luaref-langCoercion|), then all
operations have the usual meaning. Exponentiation works for any exponent. For
instance, `x^(-0.5)` computes the inverse of the square root of `x`. Modulo is
defined as
>lua
       a % b == a - math.floor(a/b)*b
<
That is, it is the remainder of a division that rounds the quotient towards
minus infinity.

------------------------------------------------------------------------------
2.5.2  Relational Operators                                   *luaref-langRelOp*

The relational operators in Lua are
>
                    ==    ~=    <     >     <=    >=
<
These operators always result in `false` or `true`.

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

You can change the way that Lua compares tables and userdata using the "eq"
metamethod (see |luaref-langMetatables|).

The conversion rules of coercion |luaref-langCoercion| do not apply to
equality comparisons. Thus, `"0"==0` evaluates to `false`, and `t[0]` and
`t["0"]` denote different entries in a table.

The operator `~=` is exactly the negation of equality (`==`).

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 |luaref-langMetatables|).

------------------------------------------------------------------------------
2.5.3  Logical Operators                                      *luaref-langLogOp*

The logical operators in Lua are
>
       and    or    not
<
Like the control structures (see |luaref-langContStructs|), all logical operators
consider both `false` and `nil` as false and anything else as true.

                                               *luaref-not* *luaref-and* *luaref-or*
The negation operator `not` always returns `false` or `true`. The conjunction
operator `and` returns its first argument if this value is `false` or `nil`;
otherwise, `and` returns its second argument. The disjunction
operator `or` returns its first argument if this value is different
from `nil` and `false`; otherwise, `or` returns its second argument.
Both `and` and `or` use short-cut evaluation, that is, the second operand is
evaluated only if necessary. Here are some examples:
>
       10 or 20            --> 10
       10 or error()       --> 10
       nil or "a"          --> "a"
       nil and 10          --> nil
       false and error()   --> false
       false and nil       --> false
       false or nil        --> nil
       10 and 20           --> 20
<
(In this manual, `-->` indicates the result of the preceding expression.)

------------------------------------------------------------------------------
2.5.4  Concatenation                                         *luaref-langConcat*

The string concatenation operator in Lua is denoted by two dots (`..`).
If both operands are strings or numbers, then they are converted to strings
according to the rules mentioned in |luaref-langCoercion|. Otherwise, the
"concat" metamethod is called (see |luaref-langMetatables|).

------------------------------------------------------------------------------
2.5.5  The Length Operator                                   *luaref-langLength*

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

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

------------------------------------------------------------------------------
2.5.6  Precedence                                              *luaref-langPrec*

Operator precedence in Lua follows the table below, from lower to higher
priority:
>
       or
       and
       <     >     <=    >=    ~=    ==
       ..
       +     -
       *     /
       not   #     - (unary)
       ^
<
As usual, you can use parentheses to change the precedences in an expression.
The concatenation (`..`) and exponentiation (`^`) operators are right
associative. All other binary operators are left associative.

------------------------------------------------------------------------------
2.5.7  Table Constructors                                *luaref-langTableConst*

Table constructors are expressions that create tables. Every time a
constructor is evaluated, a new table is created. Constructors can be used to
create empty tables, or to create a table and initialize some of its fields.
The general syntax for constructors is
>
       tableconstructor ::= { [ fieldlist ] }
       fieldlist ::= field { fieldsep field } [ fieldsep ]
       field ::= [ exp ]  = exp | Name = exp | exp
       fieldsep ::=  , |  ;
<
Each field of the form `[exp1] = exp2` adds to the new table an entry with
key `exp1` and value `exp2`. A field of the form `name = exp` is equivalent to
`["name"] = exp`. Finally, fields of the form `exp` are equivalent to
`[i] = exp`, where `i` are consecutive numerical integers, starting with 1.
Fields in the other formats do not affect this counting. For example,
>lua
       a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
<
is equivalent to
>lua
       do
         local t = {}
         t[f(1)] = g
         t[1] = "x"         -- 1st exp
         t[2] = "y"         -- 2nd exp
         t.x = 1            -- temp["x"] = 1
         t[3] = f(x)        -- 3rd exp
         t[30] = 23
         t[4] = 45          -- 4th exp
         a = t
       end
<
If the last field in the list has the form `exp` and the expression is a
function call, then all values returned by the call enter the list
consecutively (see |luaref-langFuncCalls|). To avoid this, enclose the function
call in parentheses (see |luaref-langExpressions|).

The field list may have an optional trailing separator, as a convenience for
machine-generated code.

------------------------------------------------------------------------------
2.5.8  Function Calls                     *luaref-function* *luaref-langFuncCalls*

A function call in Lua has the following syntax:
>
       functioncall ::= prefixexp args
<
In a function call, first `prefixexp` and `args` are evaluated. If the value
of `prefixexp` has type `function`, 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 |luaref-langMetatables|).

The form
>
       functioncall ::= prefixexp : Name args
<
can be used to call "methods". A call `v:name(` `args` `)` is syntactic sugar
for `v.name(v,` `args` `)`, except that `v` is evaluated only once.

Arguments have the following syntax:
>
       args ::=  ( [ explist1 ] )
       args ::= tableconstructor
       args ::= String
<
All argument expressions are evaluated before the call.  A call of the
form `f{` `fields` `}` is syntactic sugar for `f({` `fields` `})`, that is, the
argument list is a single new table. A call of the form `f'` `string` `'`
(or `f"` `string` `"` or `f[[` `string` `]]`) is syntactic sugar for
`f('` `string` `')`, that is, the argument list is a single literal string.

As an exception to the free-format syntax of Lua, you cannot put a line break
before the `(` in a function call. This restriction avoids some ambiguities
in the language. If you write
>lua
       a = f
       (g).x(a)
<
Lua would see that as a single statement, `a = f(g).x(a)`. So, if you want two
statements, you must add a semi-colon between them. If you actually want to
call `f`, you must remove the line break before `(g)`.

                                                               *luaref-tailcall*
A call of the form `return` `functioncall` is called a tail call. Lua
implements proper tail calls (or proper tail recursion): 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 `return` 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:
>lua
       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
<

------------------------------------------------------------------------------
2.5.9  Function Definitions                                *luaref-langFuncDefs*

The syntax for function definition is
>
       function ::= function funcbody
       funcbody ::= ( [ parlist1 ] ) block end
<
The following syntactic sugar simplifies function definitions:
>
       stat ::= function funcname funcbody
       stat ::= local function Name funcbody
       funcname ::= Name { . Name } [ : Name ]
<
The statement

       `function f ()`  `body`  `end`

translates to

       `f = function ()`  `body`  `end`

The statement

       `function t.a.b.c.f ()`  `body`  `end`

translates to

       `t.a.b.c.f = function ()`  `body`  `end`

The statement

       `local function f ()`  `body`  `end`

translates to

       `local f; f = function f ()`  `body`  `end`

not to

       `local f = function f ()`  `body`  `end`

(This only makes a difference when the body of the function contains
references to `f`.)

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

Parameters act as local variables that are initialized with the argument
values:
>
       parlist1 ::= namelist [ , ... ] | ...
<
                                                                 *luaref-vararg*
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 vararg function,
which is indicated by three dots (`...`) 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 vararg expression,
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 the call is enclosed in parentheses).

As an example, consider the following definitions:
>lua
       function f(a, b) end
       function g(a, b, ...) end
       function r() return 1,2,3 end
<
Then, we have the following mapping from arguments to parameters and to the
vararg expression:
>
       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, ... -->  (nothing)
       g(3, 4)          a=3, b=4,   ... -->  (nothing)
       g(3, 4, 5, 8)    a=3, b=4,   ... -->  5  8
       g(5, r())        a=5, b=1,   ... -->  2  3
<
Results are returned using the `return` statement (see |luaref-langContStructs|).
If control reaches the end of a function without encountering
a `return` statement, then the function returns with no results.

                                                            *luaref-colonsyntax*
The colon syntax is used for defining methods, that is, functions that have an
implicit extra parameter `self`. Thus, the statement

       `function t.a.b.c:f (`  `params`  `)`  `body`  `end`

is syntactic sugar for

       `t.a.b.c:f = function (self, (`  `params`  `)`  `body`  `end`

==============================================================================
2.6  Visibility Rules                                    *luaref-langVisibRules*

Lua is a lexically scoped language. The scope of variables begins at the first
statement after their declaration and lasts until the end of the innermost
block that includes the declaration. Consider the following example:
>lua
       x = 10                -- global variable
       do                    -- new block
         local x = x         -- new `x`, with value 10
         print(x)            --> 10
         x = x+1
         do                  -- another block
           local x = x+1     -- another `x`
           print(x)          --> 12
         end
         print(x)            --> 11
       end
       print(x)              --> 10  (the global one)
<
Notice that, in a declaration like `local x = x`, the new `x` being declared is
not in scope yet, and so the second `x` refers to the outside variable.

                                                                *luaref-upvalue*
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 upvalue, or external local variable, inside the inner
function.

Notice that each execution of a local statement defines new local variables.
Consider the following example:
>lua
       a = {}
       local x = 20
       for i=1,10 do
         local y = 0
         a[i] = function () y=y+1; return x+y end
       end
<
The loop creates ten closures (that is, ten instances of the anonymous
function). Each of these closures uses a different `y` variable, while all of
them share the same `x`.

==============================================================================
2.7  Error Handling                                           *luaref-langError*

Because Lua is an embedded extension language, all Lua actions start from
C code in the host program calling a function from the Lua library (see
|lua_pcall()|). Whenever an error occurs during Lua compilation or
execution, control returns to C, which can take appropriate measures (such as
print an error message).

Lua code can explicitly generate an error by calling the `error` function (see
|luaref-error()|). If you need to catch errors in Lua, you can use
the `pcall` function (see |luaref-pcall()|).

==============================================================================
2.8  Metatables                         *luaref-metatable* *luaref-langMetatables*

Every value in Lua may have a metatable. This metatable is an ordinary Lua
table that defines the behavior of the original table and userdata under
certain special operations. You can change several aspects of the behavior of
an object 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 `"__add"` in its metatable. If it finds one, Lua calls that function
to perform the addition.

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

You can query the metatable of any value through the `getmetatable` function
(see |luaref-getmetatable()|).

You can replace the metatable of tables through the `setmetatable` function (see
|luaref-setmetatable()|). You cannot change the metatable of other types from Lua
(except using the debug library); you must use the C API for that.

Tables and userdata have individual metatables (although multiple tables and
userdata can share a same table as their metatable); values of all other types
share one single metatable per type. So, there is one single metatable for all
numbers, and for all strings, etc.

A metatable may control how an object behaves in arithmetic operations, order
comparisons, concatenation, length operation, and indexing. A metatable can
also define a function to be called when a userdata is garbage collected. For
each of those operations Lua associates a specific key called an event. When
Lua performs one of those 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.

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, `__`; for instance, the key for operation "add"
is the string "__add". The semantics of these operations is better explained
by a Lua function describing how the interpreter executes that operation.

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 (`rawget`, `tonumber`, etc.) are
described in |luaref-libBasic|. In particular, to retrieve the metamethod of a
given object, we use the expression
>
       metatable(obj)[event]
<
This should be read as
>lua
       rawget(metatable(obj) or {}, event)
<
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 `nil`).

"add":                                                                 *__add()*
------
the `+` operation.

The function `getbinhandler` 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.
>lua
       function getbinhandler (op1, op2, event)
         return metatable(op1)[event] or metatable(op2)[event]
       end
<
By using this function, the behavior of the `op1 + op2` is
>lua
       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(...)
           end
         end
       end
<
"sub":                                                                 *__sub()*
------
the `-` operation. Behavior similar to the "add" operation.

"mul":                                                                 *__mul()*
------
the `*` operation. Behavior similar to the "add" operation.

"div":                                                                 *__div()*
------
the `/` operation. Behavior similar to the "add" operation.

"mod":                                                                 *__mod()*
------
the `%` operation. Behavior similar to the "add" operation, with the
operation `o1 - floor(o1/o2)*o2` as the primitive operation.

"pow":                                                                 *__pow()*
------
the `^` (exponentiation) operation. Behavior similar to the "add" operation,
with the function `pow` (from the C math library) as the primitive operation.

"unm":                                                                 *__unm()*
------
the unary `-` operation.
>lua
       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(...)
           end
         end
       end
<
"concat":                                                           *__concat()*
---------
the `..` (concatenation) operation.
>lua
       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(...)
           end
         end
       end
<
"len":                                                                 *__len()*
------
the `#` operation.
>lua
       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(...)
           end
         end
       end
<
"eq":                                                                   *__eq()*
-----
the `==` operation.

The function `getcomphandler` 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.
>lua
       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
<
The "eq" event is defined as follows:
>lua
       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
<
`a ~= b` is equivalent to `not (a == b)`.

"lt":                                                                   *__lt()*
-----
the `<` operation.
>lua
       function lt_event (op1, op2)
         if type(op1) == "number" and type(op2) == "number" then
           return op1 < op2   -- numeric comparison
         elseif type(op1) == "string" and type(op2) == "string" then
           return op1 < op2   -- lexicographic comparison
         else
           local h = getcomphandler(op1, op2, "__lt")
           if h then
             return h(op1, op2)
           else
             error(...);
           end
         end
       end
<
`a > b` is equivalent to `b < a`.

"le":                                                                   *__le()*
-----
the `<=` operation.
>lua
       function le_event (op1, op2)
         if type(op1) == "number" and type(op2) == "number" then
           return op1 <= op2   -- numeric comparison
         elseif type(op1) == "string" and type(op2) == "string" then
           return op1 <= 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(...);
             end
           end
         end
       end
<
`a >= b` is equivalent to `b <= a`. Note that, in the absence of a "le"
metamethod, Lua tries the "lt", assuming that `a <= b` is equivalent
to `not (b < a)`.

"index":                                                             *__index()*
--------
The indexing access `table[key]`.
>lua
       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(...);
           end
         end
         if type(h) == "function" then
           return h(table, key)      -- call the handler
         else return h[key]          -- or repeat operation on it
       end
<
"newindex":                                                       *__newindex()*
-----------
The indexing assignment `table[key] = value`.
>lua
       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(...);
           end
         end
         if type(h) == "function" then
           return h(table, key,value)    -- call the handler
         else h[key] = value             -- or repeat operation on it
       end
<
"call":                                                               *__call()*
-------
called when Lua calls a value.
>lua
       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(...)
           end
         end
       end
<

==============================================================================
2.9  Environments                   *luaref-environment* *luaref-langEnvironments*

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

Environments associated with userdata have no meaning for Lua. It is only a
convenience feature for programmers to associate a table to a userdata.

Environments associated with threads are called global environments. They are
used as the default environment for their threads and non-nested functions
created by the thread (through `loadfile` |luaref-loadfile()|, `loadstring`
|luaref-loadstring()| or `load` |luaref-load()|) and can be directly accessed by C
code (see |luaref-apiPseudoIndices|).

Environments associated with C functions can be directly accessed by C code
(see |luaref-apiPseudoIndices|). They are used as the default environment for
other C functions created by the function.

Environments associated with Lua functions are used to resolve all accesses to
global variables within the function (see |luaref-langVariables|). They are
used as the default environment for other Lua functions created by the
function.

You can change the environment of a Lua function or the running thread by
calling `setfenv`. You can get the environment of a Lua function or the
running thread by calling `getfenv` (see |lua_getfenv()|). To manipulate the
environment of other objects (userdata, C functions, other threads) you must
use the C API.

==============================================================================
2.10  Garbage Collection                                         *luaref-langGC*

Lua performs automatic memory management. This means that you do not 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 garbage collector from time to time to collect all dead objects
(that is, these objects that are no longer accessible from Lua). All objects
in Lua are subject to automatic management: tables, userdata, functions,
threads, and strings.

Lua implements an incremental mark-and-sweep collector. It uses two numbers to
control its garbage-collection cycles: the garbage-collector pause and the
garbage-collector step multiplier.

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 1 mean the collector will not wait to start a new cycle. A value
of 2 means that the collector waits for the total memory in use to double
before starting a new cycle.

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 1 make the
collector too slow and may result in the collector never finishing a cycle.
The default, 2, means that the collector runs at "twice" the speed of memory
allocation.

You can change these numbers by calling `lua_gc` (see |lua_gc()|) in C or
`collectgarbage` (see |luaref-collectgarbage()|) in Lua. Both get percentage
points as arguments (so an argument of 100 means a real value of 1). With
these functions you can also control the collector directly (e.g., stop and
restart it).

------------------------------------------------------------------------------
2.10.1  Garbage-Collection Metamethods                       *luaref-langGCMeta*

Using the C API, you can set garbage-collector metamethods for userdata (see
|luaref-langMetatables|). These metamethods are also called finalizers.
Finalizers allow you to coordinate Lua's garbage collection with external
resource management (such as closing files, network or database connections,
or freeing your own memory).

                                                                          *__gc*
Garbage userdata with a field `__gc` in their metatables are not collected
immediately by the garbage collector. Instead, Lua puts them in a list. After
the collection, Lua does the equivalent of the following function for each
userdata in that list:
>lua
       function gc_event (udata)
         local h = metatable(udata).__gc
         if h then
           h(udata)
         end
       end
<
At the end of each garbage-collection cycle, the finalizers for userdata are
called in reverse order of their creation, among these collected in that
cycle. That is, the first finalizer to be called is the one associated with
the userdata created last in the program.

------------------------------------------------------------------------------
2.10.2 - Weak Tables                    *luaref-weaktable* *luaref-langWeaktables*

A weak table is a table whose elements are weak references. 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.

                                                                        *__mode*
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 value of the `__mode` field of its metatable. If the `__mode` field is a
string containing the character `k`, the keys in the table are weak.
If `__mode` contains `v`, the values in the table are weak.

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

==============================================================================
2.11  Coroutines                              *luaref-coroutine* *luaref-langCoro*

Lua supports coroutines, also called collaborative multithreading. 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.

You create a coroutine with a call to `coroutine.create` (see
|coroutine.create()|). Its sole argument is a function that is the main
function of the coroutine. The `create` function only creates a new coroutine
and returns a handle to it (an object of type `thread`); it does not start the
coroutine execution.

When you first call `coroutine.resume` (see |coroutine.resume()|),
passing as its first argument the thread returned by `coroutine.create`, the
coroutine starts its execution, at the first line of its main function. Extra
arguments passed to `coroutine.resume` are passed on to the coroutine main
function. After the coroutine starts running, it runs until it terminates or
`yields`.

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,
`coroutine.resume` returns `true`, plus any values returned by the coroutine
main function. In case of errors, `coroutine.resume` returns `false` plus an
error message.

A coroutine yields by calling `coroutine.yield` (see
|coroutine.yield()|). When a coroutine yields, the corresponding
`coroutine.resume` 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,
`coroutine.resume` also returns `true`, plus any values passed to
`coroutine.yield`. The next time you resume the same coroutine, it continues
its execution from the point where it yielded, with the call to
`coroutine.yield` returning any extra arguments passed to `coroutine.resume`.

Like `coroutine.create`, the `coroutine.wrap` function (see
|coroutine.wrap()|) 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
`coroutine.resume`. `coroutine.wrap` returns all the values returned by
`coroutine.resume`, except the first one (the boolean error code). Unlike
`coroutine.resume`, `coroutine.wrap` does not catch errors; any error is
propagated to the caller.

As an example, consider the next code:
>lua
       function foo1 (a)
         print("foo", a)
         return coroutine.yield(2*a)
       end

       co = coroutine.create(function (a,b)
             print("co-body", a, b)
             local r = foo1(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"))
<
When you run it, it produces the following output:
>
       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
<

==============================================================================
3  THE APPLICATION PROGRAM INTERFACE                                *luaref-API*

This section describes the C API for Lua, that is, the set of C functions
available to the host program to communicate with Lua. All API functions and
related types and constants are declared in the header file `lua.h`.

Even when we use the term "function", any facility in the API may be provided
as a `macro` instead. All such macros use each of its arguments exactly once
(except for the first argument, which is always a Lua state), and so do not
generate hidden side-effects.

As in most C libraries, the Lua API functions do not check their arguments for
validity or consistency. However, you can change this behavior by compiling
Lua with a proper definition for the macro `luai_apicheck`,in file
`luaconf.h`.

==============================================================================
3.1  The Stack                                    *luaref-stack* *luaref-apiStack*

Lua uses a virtual stack to pass values to and from C. Each element in this
stack represents a Lua value (`nil`, number, string, etc.).

Whenever Lua calls C, the called function gets a new stack, which is
independent of previous stacks and of stacks of C functions that are still
active. This stack initially contains any arguments to the C function and it
is where the C function pushes its results to be returned to the caller (see
|lua_CFunction()|).

                                                             *luaref-stackindex*
For convenience, most query operations in the API do not follow a strict stack
discipline. Instead, they can refer to any element in the stack by using an
index: a positive index represents an absolute stack position (starting at 1);
a negative index represents an offset from the top of the stack. More
specifically, if the stack has `n` elements, then index 1 represents the first
element (that is, the element that was pushed onto the stack first) and index
`n` represents the last element; index `-1` also represents the last element
(that is, the element at the top) and index `-n` represents the first element.
We say that an index is valid if it lies between 1 and the stack top (that is,
if `1 <= abs(index) <= top`).

==============================================================================
3.2  Stack Size                                            *luaref-apiStackSize*

When you interact with Lua API, you are responsible for ensuring consistency.
In particular, you are responsible for controlling stack overflow. You can
use the function `lua_checkstack` to grow the stack size (see
|lua_checkstack()|).

Whenever Lua calls C, it ensures that at least `LUA_MINSTACK` stack positions
are available. `LUA_MINSTACK` is defined as 20, so that usually you do not
have to worry about stack space unless your code has loops pushing elements
onto the stack.

Most query functions accept as indices any value inside the available stack
space, that is, indices up to the maximum stack size you have set through
`lua_checkstack`. Such indices are called acceptable indices. More formally,
we define an acceptable index as follows:
>lua
    (index < 0 && abs(index) <= top) || (index > 0 && index <= stackspace)
<
Note that 0 is never an acceptable index.

==============================================================================
3.3  Pseudo-Indices                 *luaref-pseudoindex* *luaref-apiPseudoIndices*

Unless otherwise noted, any function that accepts valid indices can also be
called with pseudo-indices, which represent some Lua values that are
accessible to the C code but which are not in the stack. Pseudo-indices are
used to access the thread environment, the function environment, the registry,
and the upvalues of a C function (see |luaref-apiCClosures|).

The thread environment (where global variables live) is always at pseudo-index
`LUA_GLOBALSINDEX`. The environment of the running C function is always at
pseudo-index `LUA_ENVIRONINDEX`.

To access and change the value of global variables, you can use regular table
operations over an environment table. For instance, to access the value of a
global variable, do
>c
       lua_getfield(L, LUA_GLOBALSINDEX, varname);
<

==============================================================================
3.4  C Closures                            *luaref-cclosure* *luaref-apiCClosures*

When a C function is created, it is possible to associate some values with it,
thus creating a C closure; these values are called upvalues and are accessible
to the function whenever it is called (see |lua_pushcclosure()|).

Whenever a C function is called, its upvalues are located at specific
pseudo-indices. These pseudo-indices are produced by the macro
`lua_upvalueindex`. The first value associated with a function is at position
`lua_upvalueindex(1)`, and so on. Any access to `lua_upvalueindex(` `n` `)`,
where `n` is greater than the number of upvalues of the current function,
produces an acceptable (but invalid) index.

==============================================================================
3.5  Registry                               *luaref-registry* *luaref-apiRegistry*

Lua provides a registry, a pre-defined table that can be used by any C code to
store whatever Lua value it needs to store. This table is always located at
pseudo-index `LUA_REGISTRYINDEX`. Any C library can store data into this
table, but it should take care to choose keys different from those used by
other libraries, to avoid collisions. Typically, you should use as key a
string containing your library name or a light userdata with the address of a
C object in your code.

The integer keys in the registry are used by the reference mechanism,
implemented by the auxiliary library, and therefore should not be used for
other purposes.

==============================================================================
3.6  Error Handling in C                                       *luaref-apiError*

Internally, Lua uses the C `longjmp` facility to handle errors. (You can also
choose to use exceptions if you use C++; see file `luaconf.h`.) When Lua faces
any error (such as memory allocation errors, type errors, syntax errors, and
runtime errors) it raises an error; that is, it does a long jump. A protected
environment uses `setjmp` to set a recover point; any error jumps to the most
recent active recover point.

Almost any function in the API may raise an error, for instance due to a
memory allocation error. The following functions run in protected mode (that
is, they create a protected environment to run), so they never raise an error:
`lua_newstate`, `lua_close`, `lua_load`, `lua_pcall`, and `lua_cpcall` (see
|lua_newstate()|, |lua_close()|, |lua_load()|,
|lua_pcall()|, and |lua_cpcall()|).

Inside a C function you can raise an error by calling `lua_error`  (see
|lua_error()|).

==============================================================================
3.7  Functions and Types                                   *luaref-apiFunctions*

Here we list all functions and types from the C API in alphabetical order.

lua_Alloc                                                          *lua_Alloc()*
>c
    typedef void * (*lua_Alloc) (void *ud,
                                 void *ptr,
                                 size_t osize,
                                 size_t nsize);
<
        The type of the memory-allocation function used by Lua states. The
        allocator function must provide a functionality similar to `realloc`,
        but not exactly the same. Its arguments are `ud`, an opaque pointer
        passed to `lua_newstate` (see |lua_newstate()|); `ptr`, a pointer
        to the block being allocated/reallocated/freed; `osize`, the original
        size of the block; `nsize`, the new size of the block. `ptr` is `NULL`
        if and only if `osize` is zero. When `nsize` is zero, the allocator
        must return `NULL`; if `osize` is not zero, it should free the block
        pointed to by `ptr`. When `nsize` is not zero, the allocator returns
        `NULL` if and only if it cannot fill the request. When `nsize` is not
        zero and `osize` is zero, the allocator should behave like `malloc`.
        When `nsize` and `osize` are not zero, the allocator behaves like
        `realloc`. Lua assumes that the allocator never fails when `osize >=
        nsize`.

        Here is a simple implementation for the allocator function. It is used
        in the auxiliary library by `luaL_newstate` (see
        |luaL_newstate()|).
>c
            static void *l_alloc (void *ud, void *ptr, size_t osize,
                                                       size_t nsize) {
              (void)ud;  (void)osize;  /* not used */
              if (nsize == 0) {
                free(ptr);
                return NULL;
              }
              else
                return realloc(ptr, nsize);
            }
<
        This code assumes that `free(NULL)` has no effect and that
        `realloc(NULL, size)` is equivalent to `malloc(size)`. ANSI C ensures both
        behaviors.

lua_atpanic                                                      *lua_atpanic()*
>c
    lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);
<
        Sets a new panic function and returns the old one.

        If an error happens outside any protected environment, Lua calls a
        `panic` `function` and then calls `exit(EXIT_FAILURE)`, thus exiting
        the host application. Your panic function may avoid this exit by never
        returning (e.g., doing a long jump).

        The panic function can access the error message at the top of the
        stack.

lua_call                                                            *lua_call()*
>c
    void lua_call (lua_State *L, int nargs, int nresults);
<
        Calls a function.

        To call a function you must use the following protocol: first, the
        function to be called is pushed onto the stack; then, the arguments to
        the function are pushed in direct order; that is, the first argument
        is pushed first. Finally you call `lua_call`; `nargs` is the number of
        arguments that you pushed onto the stack. All arguments and the
        function value are popped from the stack when the function is called.
        The function results are pushed onto the stack when the function
        returns. The number of results is adjusted to `nresults`, unless
        `nresults` is `LUA_MULTRET`. In this case, `all` results from the
        function are pushed. Lua takes care that the returned values fit into
        the stack space. The function results are pushed onto the stack in
        direct order (the first result is pushed first), so that after the
        call the last result is on the top of the stack.

        Any error inside the called function is propagated upwards (with a
        `longjmp`).

        The following example shows how the host program may do the equivalent
        to this Lua code:
>lua
            a = f("how", t.x, 14)
<
        Here it is in C:
>c
            lua_getfield(L, LUA_GLOBALSINDEX, "f"); // function to be called
            lua_pushstring(L, "how");                        // 1st argument
            lua_getfield(L, LUA_GLOBALSINDEX, "t");   // table to be indexed
            lua_getfield(L, -1, "x");        // push result of t.x (2nd arg)
            lua_remove(L, -2);                  // remove 't' from the stack
            lua_pushinteger(L, 14);                          // 3rd argument
            lua_call(L, 3, 1);     // call 'f' with 3 arguments and 1 result
            lua_setfield(L, LUA_GLOBALSINDEX, "a");        // set global 'a'
<
        Note that the code above is "balanced": at its end, the stack is back
        to its original configuration. This is considered good programming
        practice.

lua_CFunction                             *luaref-cfunction* *lua_CFunction()*
>c
    typedef int (*lua_CFunction) (lua_State *L);
<
        Type for C functions.

        In order to communicate properly with Lua, a C function must use the
        following protocol, which defines the way parameters and results are
        passed: a C function receives its arguments from Lua in its stack in
        direct order (the first argument is pushed first). So, when the
        function starts, `lua_gettop(L)` (see |lua_gettop()|) returns the
        number of arguments received by the function. The first argument (if
        any) is at index 1 and its last argument is at index `lua_gettop(L)`.
        To return values to Lua, a C function just pushes them onto the stack,
        in direct order (the first result is pushed first), and returns the
        number of results. Any other value in the stack below the results will
        be properly discarded by Lua. Like a Lua function, a C function called
        by Lua can also return many results.

                                                       *luaref-cfunctionexample*
        As an example, the following function receives a variable number of
        numerical arguments and returns their average and sum:
>c
            static int foo (lua_State *L) {
              int n = lua_gettop(L);    /* number of arguments */
              lua_Number sum = 0;
              int i;
              for (i = 1; i &lt;= n; i++) {
                if (!lua_isnumber(L, i)) {
                  lua_pushstring(L, "incorrect argument");
                  lua_error(L);
                }
                sum += lua_tonumber(L, i);
              }
              lua_pushnumber(L, sum/n); /* first result */
              lua_pushnumber(L, sum);   /* second result */
              return 2;                 /* number of results */
            }
<

lua_checkstack                                                *lua_checkstack()*
>c
    int lua_checkstack (lua_State *L, int extra);
<
        Ensures that there are at least `extra` free stack slots in the stack.
        It returns false if it cannot grow the stack to that size. This
        function never shrinks the stack; if the stack is already larger than
        the new size, it is left unchanged.

lua_close                                                          *lua_close()*
>c
    void lua_close (lua_State *L);
<
        Destroys all objects in the given Lua state (calling the corresponding
        garbage-collection metamethods, if any) and frees all dynamic memory
        used by this state. On several platforms, you may not need to call
        this function, because all resources are naturally released when the
        host program ends. On the other hand, long-running programs, such as a
        daemon or a web server, might need to release states as soon as they
        are not needed, to avoid growing too large.

lua_concat                                                        *lua_concat()*
>c
    void lua_concat (lua_State *L, int n);
<
        Concatenates the `n` values at the top of the stack, pops them, and
        leaves the result at the top. If `n` is 1, the result is the single
        string on the stack (that is, the function does nothing); if `n` is 0,
        the result is the empty string. Concatenation is done following the
        usual semantics of Lua (see |luaref-langConcat|).

lua_cpcall                                                        *lua_cpcall()*
>c
    int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);
<
        Calls the C function `func` in protected mode. `func` starts with only
        one element in its stack, a light userdata containing `ud`. In case of
        errors, `lua_cpcall` returns the same error codes as `lua_pcall` (see
        |lua_pcall()|), plus the error object on the top of the stack;
        otherwise, it returns zero, and does not change the stack. All values
        returned by `func` are discarded.

lua_createtable                                              *lua_createtable()*
>c
    void lua_createtable (lua_State *L, int narr, int nrec);
<
        Creates a new empty table and pushes it onto the stack. The new table
        has space pre-allocated for `narr` array elements and `nrec` non-array
        elements. This pre-allocation is useful when you know exactly how many
        elements the table will have. Otherwise you can use the function
        `lua_newtable`  (see |lua_newtable()|).

lua_dump                                                            *lua_dump()*
>c
    int lua_dump (lua_State *L, lua_Writer writer, void *data);
<
        Dumps a function as a binary chunk. Receives a Lua function on the top
        of the stack and produces a binary chunk that, if loaded again,
        results in a function equivalent to the one dumped. As it produces
        parts of the chunk, `lua_dump` calls function `writer` (see
        |lua_Writer()|) with the given `data` to write them.

        The value returned is the error code returned by the last call to the
        writer; 0 means no errors.

        This function does not pop the Lua function from the stack.

lua_equal                                                          *lua_equal()*
>c
    int lua_equal (lua_State *L, int index1, int index2);
<
        Returns 1 if the two values in acceptable indices `index1` and
        `index2` are equal, following the semantics of the Lua `==` operator
        (that is, may call metamethods). Otherwise returns 0. Also returns 0
        if any of the indices is non valid.

lua_error                                                          *lua_error()*
>c
    int lua_error (lua_State *L);
<
        Generates a Lua error. The error message (which can actually be a Lua
        value of any type) must be on the stack top. This function does a long
        jump, and therefore never returns (see |luaL_error()|).

lua_gc                                                                *lua_gc()*
>c
    int lua_gc (lua_State *L, int what, int data);
<
        Controls the garbage collector.

        This function performs several tasks, according to the value of the
        parameter `what`:

        - `LUA_GCSTOP`      stops the garbage collector.
        - `LUA_GCRESTART`   restarts the garbage collector.
        - `LUA_GCCOLLECT`   performs a full garbage-collection cycle.
        - `LUA_GCCOUNT`     returns the current amount of memory (in Kbytes) in
                          use by Lua.
        - `LUA_GCCOUNTB`    returns the remainder of dividing the current
                          amount of bytes of memory in use by Lua by 1024.
        - `LUA_GCSTEP`      performs an incremental step of garbage collection.
                          The step "size" is controlled by `data` (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 `data`. The
                          function returns 1 if the step finished a
                          garbage-collection cycle.
        - `LUA_GCSETPAUSE`  sets `data` /100 as the new value for the
                          `pause` of the collector (see |luaref-langGC|).
                          The function returns the previous value of the
                          pause.
        - `LUA_GCSETSTEPMUL`sets `data` /100 as the new value for the
                          `step` `multiplier`  of the collector (see
                          |luaref-langGC|). The function returns the
                          previous value of the step multiplier.

lua_getallocf                                                  *lua_getallocf()*
>c
    lua_Alloc lua_getallocf (lua_State *L, void **ud);
<
        Returns the memory-allocation function of a given state. If `ud` is
        not `NULL`, Lua stores in `*ud` the opaque pointer passed to
        `lua_newstate` (see |lua_newstate()|).

lua_getfenv                                                      *lua_getfenv()*
>c
    void lua_getfenv (lua_State *L, int index);
<
        Pushes onto the stack the environment table of the value at the given
        index.

lua_getfield                                                    *lua_getfield()*
>c
    void lua_getfield (lua_State *L, int index, const char *k);
<
        Pushes onto the stack the value `t[k]`, where `t` is the value at the
        given valid index `index`. As in Lua, this function may trigger a
        metamethod for the "index" event (see |luaref-langMetatables|).

lua_getglobal                                                  *lua_getglobal()*
>c
    void lua_getglobal (lua_State *L, const char *name);
<
        Pushes onto the stack the value of the global `name`. It is defined as
        a macro:
>c
            #define lua_getglobal(L,s)  lua_getfield(L, LUA_GLOBALSINDEX, s)
<

lua_getmetatable                                            *lua_getmetatable()*
>c
    int lua_getmetatable (lua_State *L, int index);
<
        Pushes onto the stack the metatable of the value at the given
        acceptable index. If the index is not valid, or if the value does not
        have a metatable, the function returns 0 and pushes nothing on the
        stack.

lua_gettable                                                    *lua_gettable()*
>c
    void lua_gettable (lua_State *L, int index);
<
        Pushes onto the stack the value `t[k]`, where `t` is the value at the
        given valid index `index` and `k` is the value at the top of the
        stack.

        This function pops the key from the stack (putting the resulting value
        in its place). As in Lua, this function may trigger a metamethod for
        the "index" event (see |luaref-langMetatables|).

lua_gettop                                                        *lua_gettop()*
>c
    int lua_gettop (lua_State *L);
<
        Returns the index of the top element in the stack. Because indices
        start at 1, this result is equal to the number of elements in the
        stack (and so
        0 means an empty stack).

lua_insert                                                        *lua_insert()*
>c
    void lua_insert (lua_State *L, int index);
<
        Moves the top element into the given valid index, shifting up the
        elements above this index to open space. Cannot be called with a
        pseudo-index, because a pseudo-index is not an actual stack position.

lua_Integer                                                      *lua_Integer()*
>c
    typedef ptrdiff_t lua_Integer;
<
        The type used by the Lua API to represent integral values.

        By default it is a `ptrdiff_t`, which is usually the largest integral
        type the machine handles "comfortably".

lua_isboolean                                                  *lua_isboolean()*
>c
    int lua_isboolean (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index has type boolean,
        and 0 otherwise.

lua_iscfunction                                              *lua_iscfunction()*
>c
    int lua_iscfunction (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a C function,
        and 0 otherwise.

lua_isfunction                                                *lua_isfunction()*
>c
    int lua_isfunction (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a function
        (either C or Lua), and 0 otherwise.

lua_islightuserdata                                      *lua_islightuserdata()*
>c
    int lua_islightuserdata (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a light
        userdata, and 0 otherwise.

lua_isnil                                                          *lua_isnil()*
>c
    int lua_isnil (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is `nil`, and 0
        otherwise.

lua_isnumber                                                    *lua_isnumber()*
>c
    int lua_isnumber (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a number or a
        string convertible to a number, and 0 otherwise.

lua_isstring                                                    *lua_isstring()*
>c
    int lua_isstring (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a string or a
        number (which is always convertible to a string), and 0 otherwise.

lua_istable                                                      *lua_istable()*
>c
    int lua_istable (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a table, and
        0 otherwise.

lua_isthread                                                    *lua_isthread()*
>c
    int lua_isthread (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a thread, and
        0 otherwise.

lua_isuserdata                                                *lua_isuserdata()*
>c
    int lua_isuserdata (lua_State *L, int index);
<
        Returns 1 if the value at the given acceptable index is a userdata
        (either full or light), and 0 otherwise.

lua_lessthan                                                    *lua_lessthan()*
>c
    int lua_lessthan (lua_State *L, int index1, int index2);
<
        Returns 1 if the value at acceptable index `index1` is smaller than
        the value at acceptable index `index2`, following the semantics of the
        Lua `<` operator (that is, may call metamethods). Otherwise returns 0.
        Also returns 0 if any of the indices is non valid.

lua_load                                                            *lua_load()*
>c
    int lua_load (lua_State *L,
                  lua_Reader reader,
                  void *data,
                  const char *chunkname);
<
        Loads a Lua chunk. If there are no errors, `lua_load` pushes the
        compiled chunk as a Lua function on top of the stack. Otherwise, it
        pushes an error message. The return values of `lua_load` are:

         - `0`: no errors;
         - `LUA_ERRSYNTAX` : syntax error during pre-compilation;
         - `LUA_ERRMEM` : memory allocation error.

        This function only loads a chunk; it does not run it.

        `lua_load` automatically detects whether the chunk is text or binary,
        and loads it accordingly (see program `luac`).

        The `lua_load` function uses a user-supplied `reader` function to read
        the chunk (see |lua_Reader()|). The `data` argument is an opaque
        value passed to the reader function.

        The `chunkname` argument gives a name to the chunk, which is used for
        error messages and in debug information (see |luaref-apiDebug|).

lua_newstate                                                    *lua_newstate()*
>c
    lua_State *lua_newstate (lua_Alloc f, void *ud);
<
        Creates a new, independent state. Returns `NULL` if cannot create the
        state (due to lack of memory). The argument `f` is the allocator
        function; Lua does all memory allocation for this state through this
        function. The second argument, `ud`, is an opaque pointer that Lua
        simply passes to the allocator in every call.

lua_newtable                                                    *lua_newtable()*
>c
    void lua_newtable (lua_State *L);
<
        Creates a new empty table and pushes it onto the stack. It is
        equivalent to `lua_createtable(L, 0, 0)` (see
        |lua_createtable()|).

lua_newthread                                                  *lua_newthread()*
>c
    lua_State *lua_newthread (lua_State *L);
<
        Creates a new thread, pushes it on the stack, and returns a pointer to
        a `lua_State`  (see |lua_State()|) that represents this new
        thread. The new state returned by this function shares with the
        original state all global objects (such as tables), but has an
        independent execution stack.

        There is no explicit function to close or to destroy a thread. Threads
        are subject to garbage collection, like any Lua object.

lua_newuserdata                                              *lua_newuserdata()*
>c
    void *lua_newuserdata (lua_State *L, size_t size);
<
        This function allocates a new block of memory with the given size,
        pushes onto the stack a new full userdata with the block address, and
        returns this address.
                                                               *luaref-userdata*
        Userdata represents C values in Lua. A full userdata represents a
        block of memory. It is an object (like a table): you must create it,
        it can have its own metatable, and you can detect when it is being
        collected. A full userdata is only equal to itself (under raw
        equality).

        When Lua collects a full userdata with a `gc` metamethod, Lua calls
        the metamethod and marks the userdata as finalized. When this userdata
        is collected again then Lua frees its corresponding memory.

lua_next                                                            *lua_next()*
>c
    int lua_next (lua_State *L, int index);
<
        Pops a key from the stack, and pushes a key-value pair from the table
        at the given index (the "next" pair after the given key). If there are
        no more elements in the table, then `lua_next` returns 0 (and pushes
        nothing).

                                                         *luaref-tabletraversal*
        A typical traversal looks like this:
>c
               /* table is in the stack at index 't' */
               lua_pushnil(L);  /* first key */
               while (lua_next(L, t) != 0) {
                 /* uses 'key' (at index -2) and 'value' (at index -1) */
                 printf("%s - %s\n",
                        lua_typename(L, lua_type(L, -2)),
                        lua_typename(L, lua_type(L, -1)));
                 /* removes 'value'; keeps 'key' for next iteration */
                 lua_pop(L, 1);
               }
<
        While traversing a table, do not call `lua_tolstring` (see
        |lua_tolstring()|) directly on a key, unless you know that the
        key is actually a string. Recall that `lua_tolstring` `changes` the
        value at the given index; this confuses the next call to `lua_next`.

lua_Number                                                        *lua_Number()*
>c
    typedef double lua_Number;
<
        The type of numbers in Lua. By default, it is double, but that can be
        changed in `luaconf.h`.

        Through the configuration file you can change Lua to operate with
        another type for numbers (e.g., float or long).

lua_objlen                                                        *lua_objlen()*
>c
    size_t lua_objlen (lua_State *L, int index);
<
        Returns the "length" of the value at the given acceptable index: for
        strings, this is the string length; for tables, this is the result of
        the length operator (`#`); for userdata, this is the size of the
        block of memory allocated for the userdata; for other values, it is 0.

lua_pcall                                                          *lua_pcall()*
>c
    lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);
<
        Calls a function in protected mode.

        Both `nargs` and `nresults` have the same meaning as in `lua_call`
        (see |lua_call()|). If there are no errors during the call,
        `lua_pcall` behaves exactly like `lua_call`. However, if there is any
        error, `lua_pcall` catches it, pushes a single value on the stack (the
        error message), and returns an error code. Like `lua_call`,
        `lua_pcall` always removes the function and its arguments from the
        stack.

        If `errfunc` is 0, then the error message returned on the stack is
        exactly the original error message. Otherwise, `errfunc` is the stack
        index of an `error` `handler function`. (In the current
        implementation, this index cannot be a pseudo-index.) In case of
        runtime errors, this function will be called with the error message
        and its return value will be the message returned on the stack by
        `lua_pcall`.

        Typically, the error handler function is used to add more debug
        information to the error message, such as a stack traceback. Such
        information cannot be gathered after the return of `lua_pcall`, since
        by then the stack has unwound.

        The `lua_pcall` function returns 0 in case of success or one of the
        following error codes (defined in `lua.h`):

        - `LUA_ERRRUN`  a runtime error.
        - `LUA_ERRMEM`  memory allocation error. For such errors, Lua does
                      not call the error handler function.
        - `LUA_ERRERR`  error while running the error handler function.

lua_pop                                                              *lua_pop()*
>c
    void lua_pop (lua_State *L, int n);
<
        Pops `n` elements from the stack.

lua_pushboolean                                              *lua_pushboolean()*
>c
    void lua_pushboolean (lua_State *L, int b);
<
        Pushes a boolean value with value `b` onto the stack.

lua_pushcclosure                                            *lua_pushcclosure()*
>c
    void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);
<
        Pushes a new C closure onto the stack.

        When a C function is created, it is possible to associate some values
        with it, thus creating a C closure (see |luaref-apiCClosures|); these
        values are then accessible to the function whenever it is called. To
        associate values with a C function, first these values should be
        pushed onto the stack (when there are multiple values, the first value
        is pushed first). Then `lua_pushcclosure` is called to create and push
        the C function onto the stack, with the argument `n` telling how many
        values should be associated with the function. `lua_pushcclosure` also
        pops these values from the stack.

lua_pushcfunction                                          *lua_pushcfunction()*
>c
    void lua_pushcfunction (lua_State *L, lua_CFunction f);
<
        Pushes a C function onto the stack. This function receives a pointer
        to a C function and pushes onto the stack a Lua value of type
        `function` that, when called, invokes the corresponding C function.

        Any function to be registered in Lua must follow the correct protocol
        to receive its parameters and return its results (see
        |lua_CFunction()|).

        `lua_pushcfunction` is defined as a macro:
>c
            #define lua_pushcfunction(L,f)  lua_pushcclosure(L,f,0)
<

lua_pushfstring                                              *lua_pushfstring()*
>c
    const char *lua_pushfstring (lua_State *L, const char *fmt, ...);
<
        Pushes onto the stack a formatted string and returns a pointer to this
        string. It is similar to the C function `sprintf`, but has some
        important differences:

         - You do not have to allocate space for the result: the result is a
           Lua string and Lua takes care of memory allocation (and
           deallocation, through garbage collection).
         - The conversion specifiers are quite restricted. There are no flags,
           widths, or precisions. The conversion specifiers can only be `%%`
           (inserts a `%` in the string), `%s` (inserts a zero-terminated
           string, with no size restrictions), `%f` (inserts a
           `lua_Number`), `%p` (inserts a pointer as a hexadecimal numeral),
           `%d` (inserts an `int`), and `%c` (inserts an `int` as a
           character).

lua_pushinteger                                              *lua_pushinteger()*
>c
    void lua_pushinteger (lua_State *L, lua_Integer n);
<
        Pushes a number with value `n` onto the stack.

lua_pushlightuserdata                                  *lua_pushlightuserdata()*
>c
    void lua_pushlightuserdata (lua_State *L, void *p);
<
        Pushes a light userdata onto the stack.
                                                          *luaref-lightuserdata*
        Userdata represents C values in Lua. A light userdata represents a
        pointer. It is a value (like a number): you do not create it, it has
        no individual metatable, and it is not collected (as it was never
        created). A light userdata is equal to "any" light userdata with the
        same C address.

lua_pushlstring                                              *lua_pushlstring()*
>c
    void lua_pushlstring (lua_State *L, const char *s, size_t len);
<
        Pushes the string pointed to by `s` with size `len` onto the stack.
        Lua makes (or reuses) an internal copy of the given string, so the
        memory at `s` can be freed or reused immediately after the function
        returns. The string can contain embedded zeros.

lua_pushnil                                                      *lua_pushnil()*
>c
    void lua_pushnil (lua_State *L);
<
        Pushes a nil value onto the stack.

lua_pushnumber                                                *lua_pushnumber()*
>c
    void lua_pushnumber (lua_State *L, lua_Number n);
<
        Pushes a number with value `n` onto the stack.

lua_pushstring                                                *lua_pushstring()*
>c
    void lua_pushstring (lua_State *L, const char *s);
<
        Pushes the zero-terminated string pointed to by `s` onto the stack.
        Lua makes (or reuses) an internal copy of the given string, so the
        memory at `s` can be freed or reused immediately after the function
        returns. The string cannot contain embedded zeros; it is assumed to
        end at the first zero.

lua_pushthread                                                *lua_pushthread()*
>c
    int lua_pushthread (lua_State *L);
<
        Pushes the thread represented by `L` onto the stack. Returns 1 if this
        thread is the main thread of its state.

lua_pushvalue                                                  *lua_pushvalue()*
>c
    void lua_pushvalue (lua_State *L, int index);
<
        Pushes a copy of the element at the given valid index onto the stack.

lua_pushvfstring                                            *lua_pushvfstring()*
>c
    const char *lua_pushvfstring (lua_State *L,
                                  const char *fmt,
                                  va_list argp);
<
        Equivalent to `lua_pushfstring` (see |lua_pushfstring()|), except
        that it receives a `va_list` instead of a variable number of
        arguments.

lua_rawequal                                                    *lua_rawequal()*
>c
    int lua_rawequal (lua_State *L, int index1, int index2);
<
        Returns 1 if the two values in acceptable indices `index1` and
        `index2` are primitively equal (that is, without calling metamethods).
        Otherwise returns 0. Also returns 0 if any of the indices are non
        valid.

lua_rawget                                                        *lua_rawget()*
>c
    void lua_rawget (lua_State *L, int index);
<
        Similar to `lua_gettable` (see |lua_gettable()|), but does a raw
        access (i.e., without metamethods).

lua_rawgeti                                                      *lua_rawgeti()*
>c
    void lua_rawgeti (lua_State *L, int index, int n);
<
        Pushes onto the stack the value `t[n]`, where `t` is the value at the
        given valid index `index`. The access is raw; that is, it does not
        invoke metamethods.

lua_rawset                                                        *lua_rawset()*
>c
    void lua_rawset (lua_State *L, int index);
<
        Similar to `lua_settable` (see |lua_settable()|), but does a raw
        assignment (i.e., without metamethods).

lua_rawseti                                                      *lua_rawseti()*
>c
    void lua_rawseti (lua_State *L, int index, int n);
<
        Does the equivalent of `t[n] = v`, where `t` is the value at the given
        valid index `index` and `v` is the value at the top of the stack.

        This function pops the value from the stack. The assignment is raw;
        that is, it does not invoke metamethods.

lua_Reader                                                        *lua_Reader()*
>c
    typedef const char * (*lua_Reader) (lua_State *L,
                                        void *data,
                                        size_t *size);
<
        The reader function used by `lua_load` (see |lua_load()|). Every
        time it needs another piece of the chunk, `lua_load` calls the reader,
        passing along its `data` parameter. The reader must return a pointer
        to a block of memory with a new piece of the chunk and set `size` to
        the block size. The block must exist until the reader function is
        called again. To signal the end of the chunk, the reader must return
        `NULL`. The reader function may return pieces of any size greater than
        zero.

lua_register                                                    *lua_register()*
>c
    void lua_register (lua_State *L,
                       const char *name,
                       lua_CFunction f);
<
        Sets the C function `f` as the new value of global `name`. It is
        defined as a macro:
>c
            #define lua_register(L,n,f) \
                   (lua_pushcfunction(L, f), lua_setglobal(L, n))
<

lua_remove                                                        *lua_remove()*
>c
    void lua_remove (lua_State *L, int index);
<
        Removes the element at the given valid index, shifting down the
        elements above this index to fill the gap. Cannot be called with a
        pseudo-index, because a pseudo-index is not an actual stack position.

lua_replace                                                      *lua_replace()*
>c
    void lua_replace (lua_State *L, int index);
<
        Moves the top element into the given position (and pops it), without
        shifting any element (therefore replacing the value at the given
        position).

lua_resume                                                        *lua_resume()*
>c
    int lua_resume (lua_State *L, int narg);
<
        Starts and resumes a coroutine in a given thread.

        To start a coroutine, you first create a new thread (see
        |lua_newthread()|); then you push onto its stack the main
        function plus any arguments; then you call `lua_resume` (see
        |lua_resume()|) with `narg` being the number of arguments. This
        call returns when the coroutine suspends or finishes its execution.
        When it returns, the stack contains all values passed to `lua_yield`
        (see |lua_yield()|), or all values returned by the body function.
        `lua_resume` returns `LUA_YIELD` if the coroutine yields, 0 if the
        coroutine finishes its execution without errors, or an error code in
        case of errors (see |lua_pcall()|). In case of errors, the stack
        is not unwound, so you can use the debug API over it. The error
        message is on the top of the stack. To restart a coroutine, you put on
        its stack only the values to be passed as results from `lua_yield`,
        and then call `lua_resume`.

lua_setallocf                                                  *lua_setallocf()*
>c
    void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);
<
        Changes the allocator function of a given state to `f` with user data
        `ud`.

lua_setfenv                                                      *lua_setfenv()*
>c
    int lua_setfenv (lua_State *L, int index);
<
        Pops a table from the stack and sets it as the new environment for the
        value at the given index. If the value at the given index is neither a
        function nor a thread nor a userdata, `lua_setfenv` returns 0.
        Otherwise it returns 1.

lua_setfield                                                    *lua_setfield()*
>c
    void lua_setfield (lua_State *L, int index, const char *k);
<
        Does the equivalent to `t[k] = v`, where `t` is the value at the given
        valid index `index` and `v` is the value at the top of the stack.

        This function pops the value from the stack. As in Lua, this function
        may trigger a metamethod for the "newindex" event (see
        |luaref-langMetatables|).

lua_setglobal                                                  *lua_setglobal()*
>c
    void lua_setglobal (lua_State *L, const char *name);
<
        Pops a value from the stack and sets it as the new value of global
        `name`. It is defined as a macro:
>c
            #define lua_setglobal(L,s)   lua_setfield(L, LUA_GLOBALSINDEX, s)
<

lua_setmetatable                                            *lua_setmetatable()*
>c
    int lua_setmetatable (lua_State *L, int index);
<
        Pops a table from the stack and sets it as the new metatable for the
        value at the given acceptable index.

lua_settable                                                    *lua_settable()*
>c
    void lua_settable (lua_State *L, int index);
<
        Does the equivalent to `t[k] = v`, where `t` is the value at the given
        valid index `index`, `v` is the value at the top of the stack, and `k`
        is the value just below the top.

        This function pops both the key and the value from the stack. As in
        Lua, this function may trigger a metamethod for the "newindex" event
        (see |luaref-langMetatables|).

lua_settop                                                        *lua_settop()*
>c
    void lua_settop (lua_State *L, int index);
<
        Accepts any acceptable index, or 0, and sets the stack top to this
        index. If the new top is larger than the old one, then the new
        elements are filled with `nil`. If `index` is 0, then all stack
        elements are removed.

lua_State                                                          *lua_State()*
>c
    typedef struct lua_State lua_State;
<
        Opaque structure that keeps the whole state of a Lua interpreter. The
        Lua library is fully reentrant: it has no global variables. All
        information about a state is kept in this structure.

        A pointer to this state must be passed as the first argument to every
        function in the library, except to `lua_newstate` (see
        |lua_newstate()|), which creates a Lua state from scratch.

lua_status                                                        *lua_status()*
>c
    int lua_status (lua_State *L);
<
        Returns the status of the thread `L`.

        The status can be 0 for a normal thread, an error code if the thread
        finished its execution with an error, or `LUA_YIELD` if the thread is
        suspended.

lua_toboolean                                                  *lua_toboolean()*
>c
    int lua_toboolean (lua_State *L, int index);
<
        Converts the Lua value at the given acceptable index to a C boolean
        value (0 or 1). Like all tests in Lua, `lua_toboolean` returns 1 for
        any Lua value different from `false` and `nil`; otherwise it returns
        0. It also returns 0 when called with a non-valid index. (If you want
           to accept only actual boolean values, use `lua_isboolean`
           |lua_isboolean()| to test the value's type.)

lua_tocfunction                                              *lua_tocfunction()*
>c
    lua_CFunction lua_tocfunction (lua_State *L, int index);
<
        Converts a value at the given acceptable index to a C function. That
        value must be a C function; otherwise it returns `NULL`.

lua_tointeger                                                  *lua_tointeger()*
>c
    lua_Integer lua_tointeger (lua_State *L, int idx);
<
        Converts the Lua value at the given acceptable index to the signed
        integral type `lua_Integer` (see |lua_Integer()|). The Lua value
        must be a number or a string convertible to a number (see
        |luaref-langCoercion|); otherwise, `lua_tointeger` returns 0.

        If the number is not an integer, it is truncated in some non-specified
        way.

lua_tolstring                                                  *lua_tolstring()*
>c
    const char *lua_tolstring (lua_State *L, int index, size_t *len);
<
        Converts the Lua value at the given acceptable index to a C string. If
        `len` is not `NULL`, it also sets `*len` with the string length. The
        Lua value must be a string or a number; otherwise, the function
        returns `NULL`. If the value is a number, then `lua_tolstring`  also
        `changes the actual value in the stack to a` `string`. (This change
        confuses `lua_next` |lua_next()| when `lua_tolstring` is applied
        to keys during a table traversal.)

        `lua_tolstring` returns a fully aligned pointer to a string inside the
        Lua state. This string always has a zero (`\0`) after its last
        character (as in C), but may contain other zeros in its body. Because
        Lua has garbage collection, there is no guarantee that the pointer
        returned by `lua_tolstring` will be valid after the corresponding
        value is removed from the stack.

lua_tonumber                                                    *lua_tonumber()*
>c
    lua_Number lua_tonumber (lua_State *L, int index);
<
        Converts the Lua value at the given acceptable index to the C type
        `lua_Number` (see |lua_Number()|). The Lua value must be a number
        or a string convertible to a number (see |luaref-langCoercion|);
        otherwise, `lua_tonumber` returns 0.

lua_topointer                                                  *lua_topointer()*
>c
    const void *lua_topointer (lua_State *L, int index);
<
        Converts the value at the given acceptable index to a generic C
        pointer (`void*`). The value may be a userdata, a table, a thread, or
        a function; otherwise, `lua_topointer` returns `NULL`. Different
        objects will give different pointers. There is no way to convert the
        pointer back to its original value.

        Typically this function is used only for debug information.

lua_tostring                                                    *lua_tostring()*
>c
    const char *lua_tostring (lua_State *L, int index);
<
        Equivalent to `lua_tolstring` (see |lua_tolstring()|) with `len`
        equal to `NULL`.

lua_tothread                                                    *lua_tothread()*
>c
    lua_State *lua_tothread (lua_State *L, int index);
<
        Converts the value at the given acceptable index to a Lua thread
        (represented as `lua_State*` |lua_State()|). This value must be a
        thread; otherwise, the function returns `NULL`.

lua_touserdata                                                *lua_touserdata()*
>c
    void *lua_touserdata (lua_State *L, int index);
<
        If the value at the given acceptable index is a full userdata, returns
        its block address. If the value is a light userdata, returns its
        pointer. Otherwise, it returns `NULL`.

lua_type                                                            *lua_type()*
>c
    int lua_type (lua_State *L, int index);
<
        Returns the type of the value in the given acceptable index, or
        `LUA_TNONE` for a non-valid index (that is, an index to an "empty"
        stack position). The types returned by `lua_type` are coded by the
        following constants defined in `lua.h` : `LUA_TNIL`, `LUA_TNUMBER`,
        `LUA_TBOOLEAN`, `LUA_TSTRING`, `LUA_TTABLE`, `LUA_TFUNCTION`,
        `LUA_TUSERDATA`, `LUA_TTHREAD`, and `LUA_TLIGHTUSERDATA`.

lua_typename                                                    *lua_typename()*
>c
    const char *lua_typename  (lua_State *L, int tp);
<
        Returns the name of the type encoded by the value `tp`, which must be
        one the values returned by `lua_type`.

lua_Writer                                                        *lua_Writer()*
>c
    typedef int (*lua_Writer) (lua_State *L,
                               const void* p,
                               size_t sz,
                               void* ud);
<
        The writer function used by `lua_dump` (see |lua_dump()|). Every
        time it produces another piece of chunk, `lua_dump` calls the writer,
        passing along the buffer to be written (`p`), its size (`sz`), and the
        `data` parameter supplied to `lua_dump`.

        The writer returns an error code: 0 means no errors; any other value
        means an error and stops `lua_dump` from calling the writer again.

lua_xmove                                                          *lua_xmove()*
>c
    void lua_xmove (lua_State *from, lua_State *to, int n);
<
        Exchange values between different threads of the `same` global state.

        This function pops `n` values from the stack `from`, and pushes them
        onto the stack `to`.

lua_yield                                                          *lua_yield()*
>c
    int lua_yield (lua_State *L, int nresults);
<
        Yields a coroutine.

        This function should only be called as the return expression of a C
        function, as follows:
>c
               return lua_yield (L, nresults);
<
        When a C function calls `lua_yield` in that way, the running coroutine
        suspends its execution, and the call to `lua_resume` (see
        |lua_resume()|) that started this coroutine returns. The
        parameter `nresults` is the number of values from the stack that are
        passed as results to `lua_resume`.

                                                           *luaref-stackexample*
        As an example of stack manipulation, if the stack starts as
        `10 20 30 40 50*` (from bottom to top; the `*` marks the top), then
>
               lua_pushvalue(L, 3)    --> 10 20 30 40 50 30*
               lua_pushvalue(L, -1)   --> 10 20 30 40 50 30 30*
               lua_remove(L, -3)      --> 10 20 30 40 30 30*
               lua_remove(L,  6)      --> 10 20 30 40 30*
               lua_insert(L,  1)      --> 30 10 20 30 40*
               lua_insert(L, -1)      --> 30 10 20 30 40*  (no effect)
               lua_replace(L, 2)      --> 30 40 20 30*
               lua_settop(L, -3)      --> 30 40*
               lua_settop(L,  6)      --> 30 40 nil nil nil nil*
<

==============================================================================
3.8  The Debug Interface                                       *luaref-apiDebug*

Lua has no built-in debugging facilities. Instead, it offers a special
interface by means of functions and hooks. This interface allows the
construction of different kinds of debuggers, profilers, and other tools that
need "inside information" from the interpreter.

lua_Debug                                                          *lua_Debug()*

>c
    typedef struct lua_Debug {
        int event;
        const char *name;           /* (n) */
        const char *namewhat;       /* (n) */
        const char *what;           /* (S) */
        const char *source;         /* (S) */
        int currentline;            /* (l) */
        int nups;                   /* (u) number of upvalues */
        int linedefined;            /* (S) */
        int lastlinedefined;        /* (S) */
        char short_src[LUA_IDSIZE]; /* (S) */
        /* private part */
        other fields
    } lua_Debug;
<

A structure used to carry different pieces of information about an active
function. `lua_getstack` (see |lua_getstack()|) fills only the private part
of this structure, for later use. To fill the other fields of `lua_Debug` with
useful information, call `lua_getinfo` (see |lua_getinfo()|).

The fields of `lua_Debug`  have the following meaning:

- `source`             If the function was defined in a string, then `source` is
                     that string. If the function was defined in a file, then
                     `source` starts with a `@` followed by the file name.
- `short_src`          a "printable" version of `source`, to be used in error messages.
- `linedefined`        the line number where the definition of the function starts.
- `lastlinedefined`    the line number where the definition of the function ends.
- `what`               the string `"Lua"` if the function is a Lua function,
                     `"C"` if it is a C function, `"main"` if it is the main
                     part of a chunk, and `"tail"` if it was a function that
                     did a tail call. In the latter case, Lua has no other
                     information about the function.
- `currentline`        the current line where the given function is executing.
                     When no line information is available, `currentline` is
                     set to -1.
- `name`               a reasonable name for the given function. Because
                     functions in Lua are first-class values, they do not have
                     a fixed name: some functions may be the value of multiple
                     global variables, while others may be stored only in a
                     table field. The `lua_getinfo` function checks how the
                     function was called to find a suitable name. If it cannot
                     find a name, then `name` is set to `NULL`.
- `namewhat`           explains the `name` field. The value of `namewhat` can be
                     `"global"`, `"local"`, `"method"`, `"field"`,
                     `"upvalue"`, or `""` (the empty string), according to how
                     the function was called. (Lua uses the empty string when
                     no other option seems to apply.) `nups`  the number of
                     upvalues of the function.

lua_gethook                                                      *lua_gethook()*
>c
    lua_Hook lua_gethook (lua_State *L);
<
        Returns the current hook function.

lua_gethookcount                                            *lua_gethookcount()*
>c
    int lua_gethookcount (lua_State *L);
<
        Returns the current hook count.

lua_gethookmask                                              *lua_gethookmask()*
>c
    int lua_gethookmask (lua_State *L);
<
        Returns the current hook mask.

lua_getinfo                                                      *lua_getinfo()*
>c
    int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);
<
        Returns information about a specific function or function invocation.

        To get information about a function invocation, the parameter `ar`
        must be a valid activation record that was filled by a previous call
        to `lua_getstack` (see |lua_getstack()|) or given as argument to
        a hook (see |lua_Hook()|).

        To get information about a function you push it onto the stack and
        start the `what` string with the character `>`. (In that case,
        `lua_getinfo` pops the function in the top of the stack.) For
        instance, to know in which line a function `f` was defined, you can
        write the following code:
>c
               lua_Debug ar;
               lua_getfield(L, LUA_GLOBALSINDEX, "f");  /* get global 'f' */
               lua_getinfo(L, ">S", &ar);
               printf("%d\n", ar.linedefined);
<
        Each character in the string `what` selects some fields of the
        structure `ar` to be filled or a value to be pushed on the stack:

        `'n'`  fills in the field `name` and `namewhat`
        `'S'`  fills in the fields `source`, `short_src`, `linedefined`,
             `lastlinedefined`, and `what`
        `'l'`  fills in the field `currentline`
        `'u'`  fills in the field `nups`
        `'f'`  pushes onto the stack the function that is running at the
             given level
        `'L'`  pushes onto the stack a table whose indices are the numbers
             of the lines that are valid on the function. (A `valid line` is a
             line with some associated code, that is, a line where you can put
             a break point. Non-valid lines include empty lines and comments.)

        This function returns 0 on error (for instance, an invalid option in
        `what`).

lua_getlocal                                                    *lua_getlocal()*
>c
    const char *lua_getlocal (lua_State *L, lua_Debug *ar, int n);
<
        Gets information about a local variable of a given activation record.
        The parameter `ar` must be a valid activation record that was filled
        by a previous call to `lua_getstack` (see |lua_getstack()|) or
        given as argument to a hook (see |lua_Hook()|). The index `n`
        selects which local variable to inspect (1 is the first parameter or
        active local variable, and so on, until the last active local
        variable). `lua_getlocal` pushes the variable's value onto the stack
        and returns its name.

        Variable names starting with `(` (open parentheses) represent
        internal variables (loop control variables, temporaries, and C
        function locals).

        Returns `NULL` (and pushes nothing) when the index is greater than the
        number of active local variables.

lua_getstack                                                    *lua_getstack()*
>c
    int lua_getstack (lua_State *L, int level, lua_Debug *ar);
<
        Gets information about the interpreter runtime stack.

        This function fills parts of a `lua_Debug` (see |lua_Debug()|)
        structure with an identification of the `activation record` of the
        function executing at a given level. Level 0 is the current running
        function, whereas level `n+1` is the function that has called level
        `n`. When there are no errors, `lua_getstack` returns 1; when called
        with a level greater than the stack depth, it returns 0.

lua_getupvalue                                                *lua_getupvalue()*
>c
    const char *lua_getupvalue (lua_State *L, int funcindex, int n);
<
        Gets information about a closure's upvalue. (For Lua functions,
        upvalues are the external local variables that the function uses, and
        that are consequently included in its closure.) `lua_getupvalue` gets
        the index `n` of an upvalue, pushes the upvalue's value onto the
        stack, and returns its name. `funcindex` points to the closure in the
        stack. (Upvalues have no particular order, as they are active through
        the whole function. So, they are numbered in an arbitrary order.)

        Returns `NULL` (and pushes nothing) when the index is greater than the
        number of upvalues. For C functions, this function uses the empty
        string `""` as a name for all upvalues.

lua_Hook                                                            *lua_Hook()*
>c
    typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);
<
        Type for debugging hook functions.

        Whenever a hook is called, its `ar` argument has its field `event` set
        to the specific event that triggered the hook. Lua identifies these
        events with the following constants: `LUA_HOOKCALL`, `LUA_HOOKRET`,
        `LUA_HOOKTAILRET`, `LUA_HOOKLINE`, and `LUA_HOOKCOUNT`. Moreover, for
        line events, the field `currentline` is also set. To get the value of
        any other field in `ar`, the hook must call `lua_getinfo` (see
        |lua_getinfo()|). For return events, `event` may be
        `LUA_HOOKRET`, the normal value, or `LUA_HOOKTAILRET`. In the latter
        case, Lua is simulating a return from a function that did a tail call;
        in this case, it is useless to call `lua_getinfo`.

        While Lua is running a hook, it disables other calls to hooks.
        Therefore, if a hook calls back Lua to execute a function or a chunk,
        this execution occurs without any calls to hooks.


lua_sethook                                                      *lua_sethook()*
>c
    int lua_sethook (lua_State *L, lua_Hook f, int mask, int count);
<
        Sets the debugging hook function.

        Argument `f` is the hook function. `mask` specifies on which events
        the hook will be called: it is formed by a bitwise `or` of the
        constants `LUA_MASKCALL`, `LUA_MASKRET`, `LUA_MASKLINE`, and
        `LUA_MASKCOUNT`. The `count` argument is only meaningful when the mask
        includes `LUA_MASKCOUNT`. For each event, the hook is called as
        explained below:

         - `The call hook`: is called when the interpreter calls a function.
           The hook is called just after Lua enters the new function, before
           the function gets its arguments.
         - `The return hook`: is called when the interpreter returns from a
           function. The hook is called just before Lua leaves the function.
           You have no access to the values to be returned by the function.
         - `The line hook`: is called when the interpreter is about to start
           the execution of a new line of code, or when it jumps back in the
           code (even to the same line). (This event only happens while Lua is
           executing a Lua function.)
         - `The count hook`: is called after the interpreter executes every
           `count` instructions. (This event only happens while Lua is
           executing a Lua function.)

        A hook is disabled by setting `mask` to zero.

lua_setlocal                                                    *lua_setlocal()*
>c
    const char *lua_setlocal (lua_State *L, lua_Debug *ar, int n);
<
        Sets the value of a local variable of a given activation record.
        Parameters `ar` and `n` are as in `lua_getlocal` (see
        |lua_getlocal()|). `lua_setlocal` assigns the value at the top of
        the stack to the variable and returns its name. It also pops the value
        from the stack.

        Returns `NULL` (and pops nothing) when the index is greater than the
        number of active local variables.

lua_setupvalue                                                *lua_setupvalue()*
>c
    const char *lua_setupvalue (lua_State *L, int funcindex, int n);
<
        Sets the value of a closure's upvalue. It assigns the value at the top
        of the stack to the upvalue and returns its name. It also pops the
        value from the stack. Parameters `funcindex` and `n` are as in the
        `lua_getupvalue` (see |lua_getupvalue()|).

        Returns `NULL` (and pops nothing) when the index is greater than the
        number of upvalues.

                                                           *luaref-debugexample*
        As an example, the following function lists the names of all local
        variables and upvalues for a function at a given level of the stack:
>c
               int listvars (lua_State *L, int level) {
                 lua_Debug ar;
                 int i;
                 const char *name;
                 if (lua_getstack(L, level, &ar) == 0)
                   return 0;  /* failure: no such level in the stack */
                 i = 1;
                 while ((name = lua_getlocal(L, &ar, i++)) != NULL) {
                   printf("local %d %s\n", i-1, name);
                   lua_pop(L, 1);  /* remove variable value */
                 }
                 lua_getinfo(L, "f", &ar);  /* retrieves function */
                 i = 1;
                 while ((name = lua_getupvalue(L, -1, i++)) != NULL) {
                   printf("upvalue %d %s\n", i-1, name);
                   lua_pop(L, 1);  /* remove upvalue value */
                 }
                 return 1;
               }
<

==============================================================================
4  THE AUXILIARY LIBRARY                                            *luaref-aux*

The auxiliary library provides several convenient functions to interface C
with Lua. While the basic API provides the primitive functions for all
interactions between C and Lua, the auxiliary library provides higher-level
functions for some common tasks.

All functions from the auxiliary library are defined in header file `lauxlib.h`
and have a prefix `luaL_`.

All functions in the auxiliary library are built on top of the basic API, and
so they provide nothing that cannot be done with this API.

Several functions in the auxiliary library are used to check C function
arguments. Their names are always `luaL_check*` or `luaL_opt*`. All of these
functions raise an error if the check is not satisfied. Because the error
message is formatted for arguments (e.g., "bad argument #1"), you should not
use these functions for other stack values.

==============================================================================
4.1  Functions and Types                                   *luaref-auxFunctions*

Here we list all functions and types from the auxiliary library in
alphabetical order.

luaL_addchar                                                    *luaL_addchar()*
>c
    void luaL_addchar (luaL_Buffer *B, char c);
<
        Adds the character `c` to the buffer `B` (see |luaL_Buffer()|).

luaL_addlstring                                              *luaL_addlstring()*
>c
    void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);
<
        Adds the string pointed to by `s` with length `l` to the buffer `B`
        (see |luaL_Buffer()|). The string may contain embedded zeros.

luaL_addsize                                                    *luaL_addsize()*
>c
    void luaL_addsize (luaL_Buffer *B, size_t n);
<
        Adds to the buffer `B` (see |luaL_Buffer()|) a string of length
        `n` previously copied to the buffer area (see
        |luaL_prepbuffer()|).

luaL_addstring                                                *luaL_addstring()*
>c
    void luaL_addstring (luaL_Buffer *B, const char *s);
<
        Adds the zero-terminated string pointed to by `s` to the buffer `B`
        (see |luaL_Buffer()|). The string may not contain embedded zeros.

luaL_addvalue                                                  *luaL_addvalue()*
>c
    void luaL_addvalue (luaL_Buffer *B);
<
        Adds the value at the top of the stack to the buffer `B` (see
        |luaL_Buffer()|). Pops the value.

        This is the only function on string buffers that can (and must) be
        called with an extra element on the stack, which is the value to be
        added to the buffer.

luaL_argcheck                                                  *luaL_argcheck()*
>c
    void luaL_argcheck (lua_State *L,
                        int cond,
                        int narg,
                        const char *extramsg);
<
        Checks whether `cond` is true. If not, raises an error with the
        following message, where `func` is retrieved from the call stack:
>
               bad argument #<narg> to <func> (<extramsg>)
<

luaL_argerror                                                  *luaL_argerror()*
>c
    int luaL_argerror (lua_State *L, int narg, const char *extramsg);
<
        Raises an error with the following message, where `func` is retrieved
        from the call stack:
>
               bad argument #<narg> to <func> (<extramsg>)
<
        This function never returns, but it is an idiom to use it in C
        functions as `return luaL_argerror(` `args` `)`.

luaL_Buffer                                                      *luaL_Buffer()*
>c
    typedef struct luaL_Buffer luaL_Buffer;
<
        Type for a `string buffer`.

        A string buffer allows C code to build Lua strings piecemeal. Its
        pattern of use is as follows:

         - First you declare a variable `b` of type `luaL_Buffer`.
         - Then you initialize it with a call `luaL_buffinit(L, &b)` (see
           |luaL_buffinit()|).
         - Then you add string pieces to the buffer calling any of the
           `luaL_add*` functions.
         - You finish by calling `luaL_pushresult(&b)` (see
           |luaL_pushresult()|). This call leaves the final string on the
           top of the stack.

        During its normal operation, a string buffer uses a variable number of
        stack slots. So, while using a buffer, you cannot assume that you know
        where the top of the stack is. You can use the stack between
        successive calls to buffer operations as long as that use is balanced;
        that is, when you call a buffer operation, the stack is at the same
        level it was immediately after the previous buffer operation. (The
        only exception to this rule is `luaL_addvalue`
        |luaL_addvalue()|.) After calling `luaL_pushresult` the stack is
        back to its level when the buffer was initialized, plus the final
        string on its top.

luaL_buffinit                                                  *luaL_buffinit()*
>c
    void luaL_buffinit (lua_State *L, luaL_Buffer *B);
<
        Initializes a buffer `B`. This function does not allocate any space;
        the buffer must be declared as a variable (see |luaL_Buffer()|).

luaL_callmeta                                                  *luaL_callmeta()*
>c
    int luaL_callmeta (lua_State *L, int obj, const char *e);
<
        Calls a metamethod.

        If the object at index `obj` has a metatable and this metatable has a
        field `e`, this function calls this field and passes the object as its
        only argument. In this case this function returns 1 and pushes onto
        the stack the value returned by the call. If there is no metatable or
        no metamethod, this function returns
        0 (without pushing any value on the stack).

luaL_checkany                                                  *luaL_checkany()*
>c
    void luaL_checkany (lua_State *L, int narg);
<
        Checks whether the function has an argument of any type (including
        `nil`) at position `narg`.

luaL_checkint                                                  *luaL_checkint()*
>c
    int luaL_checkint (lua_State *L, int narg);
<
        Checks whether the function argument `narg` is a number and returns
        this number cast to an `int`.

luaL_checkinteger                                          *luaL_checkinteger()*
>c
    lua_Integer luaL_checkinteger (lua_State *L, int narg);
<
        Checks whether the function argument `narg` is a number and returns
        this number cast to a `lua_Integer` (see |lua_Integer()|).

luaL_checklong                                                *luaL_checklong()*
>c
    long luaL_checklong (lua_State *L, int narg);
<
        Checks whether the function argument `narg` is a number and returns
        this number cast to a `long`.

luaL_checklstring                                          *luaL_checklstring()*
>c
    const char *luaL_checklstring (lua_State *L, int narg, size_t *l);
<
        Checks whether the function argument `narg` is a string and returns
        this string; if `l` is not `NULL` fills `*l` with the string's length.

luaL_checknumber                                            *luaL_checknumber()*
>c
    lua_Number luaL_checknumber (lua_State *L, int narg);
<
        Checks whether the function argument `narg` is a number and returns
        this number (see |lua_Number()|).

luaL_checkoption                                            *luaL_checkoption()*
>c
    int luaL_checkoption (lua_State *L,
                          int narg,
                          const char *def,
                          const char *const lst[]);
<
        Checks whether the function argument `narg` is a string and searches
        for this string in the array `lst` (which must be NULL-terminated).
        Returns the index in the array where the string was found. Raises an
        error if the argument is not a string or if the string cannot be
        found.

        If `def` is not `NULL`, the function uses `def` as a default value
        when there is no argument `narg` or if this argument is `nil`.

        This is a useful function for mapping strings to C enums. (The usual
        convention in Lua libraries is to use strings instead of numbers to
        select options.)

luaL_checkstack                                              *luaL_checkstack()*
>c
    void luaL_checkstack (lua_State *L, int sz, const char *msg);
<
        Grows the stack size to `top + sz` elements, raising an error if the
        stack cannot grow to that size. `msg` is an additional text to go into
        the error message.

luaL_checkstring                                            *luaL_checkstring()*
>c
    const char *luaL_checkstring (lua_State *L, int narg);
<
        Checks whether the function argument `narg` is a string and returns
        this string.

luaL_checktype                                                *luaL_checktype()*
>c
    void luaL_checktype (lua_State *L, int narg, int t);
<
        Checks whether the function argument `narg` has type `t` (see
        |lua_type()|).

luaL_checkudata                                              *luaL_checkudata()*
>c
    void *luaL_checkudata (lua_State *L, int narg, const char *tname);
<
        Checks whether the function argument `narg` is a userdata of the type
        `tname` (see |luaL_newmetatable()|).

luaL_dofile                                                      *luaL_dofile()*
>c
    int luaL_dofile (lua_State *L, const char *filename);
<
        Loads and runs the given file. It is defined as the following macro:
>c
               (luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))
<
        It returns 0 if there are no errors or 1 in case of errors.

luaL_dostring                                                  *luaL_dostring()*
>c
    int luaL_dostring (lua_State *L, const char *str);
<
        Loads and runs the given string. It is defined as the following macro:
>c
               (luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))
<
        It returns 0 if there are no errors or 1 in case of errors.

luaL_error                                                        *luaL_error()*
>c
    int luaL_error (lua_State *L, const char *fmt, ...);
<
        Raises an error. The error message format is given by `fmt` plus any
        extra arguments, following the same rules of `lua_pushfstring` (see
        |lua_pushfstring()|). It also adds at the beginning of the
        message the file name and the line number where the error occurred, if
        this information is available.

        This function never returns, but it is an idiom to use it in C
        functions as `return luaL_error(` `args` `)`.

luaL_getmetafield                                          *luaL_getmetafield()*
>c
    int luaL_getmetafield (lua_State *L, int obj, const char *e);
<
        Pushes onto the stack the field `e` from the metatable of the object
        at index `obj`. If the object does not have a metatable, or if the
        metatable does not have this field, returns 0 and pushes nothing.

luaL_getmetatable                                          *luaL_getmetatable()*
>c
    void luaL_getmetatable (lua_State *L, const char *tname);
<
        Pushes onto the stack the metatable associated with name `tname` in
        the registry (see |luaL_newmetatable()|).

luaL_gsub                                                          *luaL_gsub()*
>c
    const char *luaL_gsub (lua_State *L,
                           const char *s,
                           const char *p,
                           const char *r);
<
        Creates a copy of string `s` by replacing any occurrence of the string
        `p` with the string `r`. Pushes the resulting string on the stack and
        returns it.

luaL_loadbuffer                                              *luaL_loadbuffer()*
>c
    int luaL_loadbuffer (lua_State *L,
                         const char *buff,
                         size_t sz,
                         const char *name);
<
        Loads a buffer as a Lua chunk. This function uses `lua_load` (see
        |lua_load()|) to load the chunk in the buffer pointed to by
        `buff` with size `sz`.

        This function returns the same results as `lua_load`. `name` is the
        chunk name, used for debug information and error messages.

luaL_loadfile                                                  *luaL_loadfile()*
>c
    int luaL_loadfile (lua_State *L, const char *filename);
<
        Loads a file as a Lua chunk. This function uses `lua_load` (see
        |lua_load()|) to load the chunk in the file named `filename`. If
        `filename` is `NULL`, then it loads from the standard input. The first
        line in the file is ignored if it starts with a `#`.

        This function returns the same results as `lua_load`, but it has an
        extra error code `LUA_ERRFILE` if it cannot open/read the file.

        As `lua_load`, this function only loads the chunk; it does not run it.

luaL_loadstring                                              *luaL_loadstring()*
>c
    int luaL_loadstring (lua_State *L, const char *s);
<
        Loads a string as a Lua chunk. This function uses `lua_load` (see
        |lua_load()|) to load the chunk in the zero-terminated string
        `s`.

        This function returns the same results as `lua_load`.

        Also as `lua_load`, this function only loads the chunk; it does not
        run it.

luaL_newmetatable                                          *luaL_newmetatable()*
>c
    int luaL_newmetatable (lua_State *L, const char *tname);
<
        If the registry already has the key `tname`, returns 0. Otherwise,
        creates a new table to be used as a metatable for userdata, adds it to
        the registry with key `tname`, and returns 1.

        In both cases pushes onto the stack the final value associated with
        `tname` in the registry.

luaL_newstate                                                  *luaL_newstate()*
>c
    lua_State *luaL_newstate (void);
<
        Creates a new Lua state. It calls `lua_newstate` (see
        |lua_newstate()|) with an allocator based on the standard C
        `realloc` function and then sets a panic function (see
        |lua_atpanic()|) that prints an error message to the standard
        error output in case of fatal errors.

        Returns the new state, or `NULL` if there is a memory allocation
        error.

luaL_openlibs                                                  *luaL_openlibs()*
>c
    void luaL_openlibs (lua_State *L);
<
        Opens all standard Lua libraries into the given state. See also
        |luaref-openlibs| for details on how to open individual libraries.

luaL_optint                                                      *luaL_optint()*
>c
    int luaL_optint (lua_State *L, int narg, int d);
<
        If the function argument `narg` is a number, returns this number cast
        to an `int`. If this argument is absent or is `nil`, returns `d`.
        Otherwise, raises an error.

luaL_optinteger                                              *luaL_optinteger()*
>c
    lua_Integer luaL_optinteger (lua_State *L,
                                 int narg,
                                 lua_Integer d);
<
        If the function argument `narg` is a number, returns this number cast
        to a `lua_Integer` (see |lua_Integer()|). If this argument is
        absent or is `nil`, returns `d`. Otherwise, raises an error.

luaL_optlong                                                    *luaL_optlong()*
>c
    long luaL_optlong (lua_State *L, int narg, long d);
<
        If the function argument `narg` is a number, returns this number cast
        to a `long`. If this argument is absent or is `nil`, returns `d`.
        Otherwise, raises an error.

luaL_optlstring                                              *luaL_optlstring()*
>c
    const char *luaL_optlstring (lua_State *L,
                                 int narg,
                                 const char *d,
                                 size_t *l);
<
        If the function argument `narg` is a string, returns this string. If
        this argument is absent or is `nil`, returns `d`. Otherwise, raises an
        error.

        If `l` is not `NULL`, fills the position `*l` with the results' length.

luaL_optnumber                                                *luaL_optnumber()*
>c
    lua_Number luaL_optnumber (lua_State *L, int narg, lua_Number d);
<
        If the function argument `narg` is a number, returns this number. If
        this argument is absent or is `nil`, returns `d`. Otherwise, raises an
        error.

luaL_optstring                                                *luaL_optstring()*
>c
    const char *luaL_optstring (lua_State *L,
                                int narg,
                                const char *d);
<
        If the function argument `narg` is a string, returns this string. If
        this argument is absent or is `nil`, returns `d`. Otherwise, raises an
        error.

luaL_prepbuffer                                              *luaL_prepbuffer()*
>c
    char *luaL_prepbuffer (luaL_Buffer *B);
<
        Returns an address to a space of size `LUAL_BUFFERSIZE` where you can
        copy a string to be added to buffer `B` (see |luaL_Buffer()|).
        After copying the string into this space you must call `luaL_addsize`
        (see |luaL_addsize()|) with the size of the string to actually
        add it to the buffer.

luaL_pushresult                                              *luaL_pushresult()*
>c
    void luaL_pushresult (luaL_Buffer *B);
<
        Finishes the use of buffer `B` leaving the final string on the top of
        the stack.

luaL_ref                                                            *luaL_ref()*
>c
    int luaL_ref (lua_State *L, int t);
<
        Creates and returns a `reference`, in the table at index `t`, for the
        object at the top of the stack (and pops the object).

        A reference is a unique integer key. As long as you do not manually
        add integer keys into table `t`, `luaL_ref` ensures the uniqueness of
        the key it returns. You can retrieve an object referred by reference
        `r` by calling `lua_rawgeti(L, t, r)` (see |lua_rawgeti()|).
        Function `luaL_unref` (see |luaL_unref()|) frees a reference and
        its associated object.

        If the object at the top of the stack is `nil`, `luaL_ref` returns the
        constant `LUA_REFNIL`. The constant `LUA_NOREF` is guaranteed to be
        different from any reference returned by `luaL_ref`.

luaL_Reg                                                            *luaL_Reg()*
>c
    typedef struct luaL_Reg {
        const char *name;
        lua_CFunction func;
    } luaL_Reg;
<
        Type for arrays of functions to be registered by `luaL_register`  (see
        |luaL_register()|). `name` is the function name and `func` is a
        pointer to the function. Any array of `luaL_Reg` must end with a
        sentinel entry in which both `name` and `func` are `NULL`.

luaL_register                                                  *luaL_register()*
>c
    void luaL_register (lua_State *L,
                        const char *libname,
                        const luaL_Reg *l);
<
        Opens a library.

        When called with `libname` equal to `NULL`, it simply registers all
        functions in the list `l` (see |luaL_Reg()|) into the table on
        the top of the stack.

        When called with a non-null `libname`, `luaL_register` creates a new
        table `t`, sets it as the value of the global variable `libname`, sets
        it as the value of `package.loaded[libname]`, and registers on it all
        functions in the list `l`. If there is a table in
        `package.loaded[libname]` or in variable `libname`, reuses this table
        instead of creating a new one.

        In any case the function leaves the table on the top of the stack.

luaL_typename                                                  *luaL_typename()*
>c
    const char *luaL_typename (lua_State *L, int idx);
<
        Returns the name of the type of the value at index `idx`.

luaL_typerror                                                  *luaL_typerror()*
>c
    int luaL_typerror (lua_State *L, int narg, const char *tname);
<
        Generates an error with a message like the following:

          `location`  `: bad argument`  `narg`  `to`  `'func'`  `(`  `tname`
          `expected, got`  `rt`  `)`

        where `location` is produced by `luaL_where`  (see
        |luaL_where()|), `func` is the name of the current function, and
        `rt` is the type name of the actual argument.

luaL_unref                                                        *luaL_unref()*
>c
    void luaL_unref (lua_State *L, int t, int ref);
<
        Releases reference `ref` from the table at index `t` (see
        |luaL_ref()|). The entry is removed from the table, so that the
        referred object can be collected. The reference `ref` is also freed to
        be used again.

        If `ref` is `LUA_NOREF` or `LUA_REFNIL`, `luaL_unref` does nothing.

luaL_where                                                        *luaL_where()*
>c
    void luaL_where (lua_State *L, int lvl);
<
        Pushes onto the stack a string identifying the current position of the
        control at level `lvl` in the call stack. Typically this string has
        the following format:

            `chunkname:currentline:`

        Level 0 is the running function, level 1 is the function that called
        the running function, etc.

        This function is used to build a prefix for error messages.

==============================================================================
5  STANDARD LIBRARIES                                               *luaref-Lib*

The standard libraries provide useful functions that are implemented directly
through the C API. Some of these functions provide essential services to the
language (e.g., `type` and `getmetatable`); 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., `sort`).

All libraries are implemented through the official C API and are provided as
separate C modules. Currently, Lua has the following standard libraries:

- basic library;
- package library;
- string manipulation;
- table manipulation;
- mathematical functions (sin, log, etc.);
- input and output;
- operating system facilities;
- debug facilities.

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.

                                                               *luaref-openlibs*
To have access to these libraries, the C host program should call the
`luaL_openlibs` function, which opens all standard libraries (see
|luaL_openlibs()|). Alternatively, the host program can open the libraries
individually by calling `luaopen_base` (for the basic library),
`luaopen_package` (for the package library), `luaopen_string` (for the string
library), `luaopen_table` (for the table library), `luaopen_math` (for the
mathematical library), `luaopen_io` (for the I/O and the Operating System
libraries), and `luaopen_debug` (for the debug library). These functions are
declared in `lualib.h` and should not be called directly: you must call them
like any other Lua C function, e.g., by using `lua_call` (see |lua_call()|).

==============================================================================
5.1  Basic Functions                                           *luaref-libBasic*

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.

assert({v} [, {message}])                                      *luaref-assert()*
    Issues an error when the value of its argument `v` is false (i.e., `nil` or
    `false`); otherwise, returns all its arguments. `message` is an error message;
    when absent, it defaults to "assertion failed!"

collectgarbage({opt} [, {arg}])                        *luaref-collectgarbage()*
        This function is a generic interface to the garbage collector. It
        performs different functions according to its first argument, {opt}:

        `"stop"`       stops the garbage collector.
        `"restart"`    restarts the garbage collector.
        `"collect"`    performs a full garbage-collection cycle.
        `"count"`      returns the total memory in use by Lua (in Kbytes).
        `"step"`       performs a garbage-collection step. The step "size" is
                     controlled by {arg} (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 {arg}. Returns
                     `true` if the step finished a collection cycle.
        `"setpause"`   sets {arg} /100 as the new value for the `pause` of
                     the collector (see |luaref-langGC|).
        `"setstepmul"` sets {arg} /100 as the new value for the `step
                     multiplier` of the collector (see |luaref-langGC|).

dofile({filename})                                             *luaref-dofile()*
        Opens the named file and executes its contents as a Lua chunk. When
        called without arguments, `dofile` executes the contents of the
        standard input (`stdin`). Returns all values returned by the chunk. In
        case of errors, `dofile` propagates the error to its caller (that is,
        `dofile` does not run in protected mode).

error({message} [, {level}])                                    *luaref-error()*
        Terminates the last protected function called and returns `message` as
        the error message. Function {error} never returns.

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

_G                                                                 *luaref-_G()*
        A global variable (not a function) that holds the global environment
        (that is, `_G._G = _G`). Lua itself does not use this variable;
        changing its value does not affect any environment, nor vice-versa.
        (Use `setfenv` to change environments.)

getfenv({f})                                                  *luaref-getfenv()*
        Returns the current environment in use by the function. {f} can be a
        Lua function or a number that specifies the function at that stack
        level: Level 1 is the function calling `getfenv`. If the given
        function is not a Lua function, or if {f} is 0, `getfenv` returns the
        global environment. The default for {f} is 1.

getmetatable({object})                                   *luaref-getmetatable()*
        If {object} does not have a metatable, returns `nil`. Otherwise, if
        the object's metatable has a `"__metatable"` field, returns the
        associated value. Otherwise, returns the metatable of the given
        object.

ipairs({t})                                                    *luaref-ipairs()*
        Returns three values: an iterator function, the table {t}, and 0, so
        that the construction

               `for i,v in ipairs(t) do`  `body`  `end`

        will iterate over the pairs (`1,t[1]`), (`2,t[2]`), ..., up to the
        first integer key absent from the table.

load({func} [, {chunkname}])                                     *luaref-load()*
        Loads a chunk using function {func} to get its pieces. Each call to
        {func} must return a string that concatenates with previous results. A
        return of `nil` (or no value) signals the end of the chunk.

        If there are no errors, returns the compiled chunk as a function;
        otherwise, returns `nil` plus the error message. The environment of
        the returned function is the global environment.

        {chunkname} is used as the chunk name for error messages and debug
        information.

loadfile([{filename}])                                       *luaref-loadfile()*
        Similar to `load` (see |luaref-load()|), but gets the chunk from file
        {filename} or from the standard input, if no file name is given.

loadstring({string} [, {chunkname}])                       *luaref-loadstring()*
        Similar to `load` (see |luaref-load()|), but gets the chunk from the
        given {string}.

        To load and run a given string, use the idiom
>lua
               assert(loadstring(s))()
<

next({table} [, {index}])                                        *luaref-next()*
        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. `next`
        returns the next index of the table and its associated value. When
        called with `nil` as its second argument, `next` returns an initial
        index and its associated value. When called with the last index, or
        with `nil` in an empty table, `next` returns `nil`. If the second
        argument is absent, then it is interpreted as `nil`. In particular,
        you can use `next(t)` to check whether a table is empty.

        The order in which the indices are enumerated is not specified, `even
        for` `numeric indices`. (To traverse a table in numeric order, use a
        numerical `for` or the `ipairs` |luaref-ipairs()| function.)

        The behavior of `next` is `undefined` 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.

pairs({t})                                                      *luaref-pairs()*
        Returns three values: the `next` |luaref-next()| function, the table
        {t}, and `nil`, so that the construction

               `for k,v in pairs(t) do`  `body`  `end`

        will iterate over all key-value pairs of table {t}.

pcall({f}, {arg1}, {...})                                       *luaref-pcall()*
        Calls function {f} with the given arguments in `protected mode`. This
        means that any error inside {f} is not propagated; instead, `pcall`
        catches the error and returns a status code. Its first result is the
        status code (a boolean), which is `true` if the call succeeds without
        errors. In such case, `pcall` also returns all results from the call,
        after this first result. In case of any error, `pcall` returns `false`
        plus the error message.

print({...})                                                    *luaref-print()*
        Receives any number of arguments, and prints their values to `stdout`,
        using the `tostring` |luaref-tostring()| function to convert them to
        strings. `print` is not intended for formatted output, but only as a
        quick way to show a value, typically for debugging. For formatted
        output, use `string.format` (see |string.format()|).

rawequal({v1}, {v2})                                         *luaref-rawequal()*
        Checks whether {v1} is equal to {v2}, without invoking any metamethod.
        Returns a boolean.

rawget({table}, {index})                                       *luaref-rawget()*
        Gets the real value of `table[index]`, without invoking any
        metamethod. {table} must be a table; {index} may be any value.

rawset({table}, {index}, {value})                              *luaref-rawset()*
        Sets the real value of `table[index]` to {value}, without invoking any
        metamethod. {table} must be a table, {index} any value different from
        `nil`, and {value} any Lua value.

        This function returns {table}.

select({index}, {...})                                         *luaref-select()*
        If {index} is a number, returns all arguments after argument number
        {index}. Otherwise, {index} must be the string `"#"`, and `select`
        returns the total number of extra arguments it received.

setfenv({f}, {table})                                         *luaref-setfenv()*
        Sets the environment to be used by the given function. {f} can be a
        Lua function or a number that specifies the function at that stack
        level: Level 1 is the function calling `setfenv`. `setfenv` returns
        the given function.

        As a special case, when {f} is 0 `setfenv` changes the environment of
        the running thread. In this case, `setfenv` returns no values.

setmetatable({table}, {metatable})                       *luaref-setmetatable()*
        Sets the metatable for the given table. (You cannot change the
        metatable of other types from Lua, only from C.) If {metatable} is
        `nil`, removes the metatable of the given table. If the original
        metatable has a `"__metatable"` field, raises an error.

        This function returns {table}.

tonumber({e} [, {base}])                                     *luaref-tonumber()*
        Tries to convert its argument to a number. If the argument is already
        a number or a string convertible to a number, then `tonumber` returns
        this number; otherwise, it returns `nil`.

        An optional argument specifies the base to interpret the numeral. The
        base may be any integer between 2 and 36, inclusive. In bases above
        10, the letter `A` (in either upper or lower case) represents 10, `B`
        represents 11, and so forth, with `Z'` representing 35. In base 10
        (the default), the number may have a decimal part, as well as an
        optional exponent part (see |luaref-langLexConv|). In other bases,
        only unsigned integers are accepted.

tostring({e})                                                *luaref-tostring()*
        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 `string.format` (see |string.format()|).

                                                                    *__tostring*
        If the metatable of {e} has a `"__tostring"` field, `tostring` calls
        the corresponding value with {e} as argument, and uses the result of
        the call as its result.

type({v})                                                        *luaref-type()*
        Returns the type of its only argument, coded as a string. The possible
        results of this function are `"nil"` (a string, not the value `nil`),
        `"number"`, `"string"`, `"boolean`, `"table"`, `"function"`,
        `"thread"`, and `"userdata"`.

unpack({list} [, {i} [, {j}]])                                 *luaref-unpack()*
        Returns the elements from the given table. This function is equivalent
        to
>lua
          return list[i], list[i+1], ..., list[j]
<
        except that the above code can be written only for a fixed number of
        elements. By default, {i} is 1 and {j} is the length of the list, as
        defined by the length operator(see |luaref-langLength|).

_VERSION                                                     *luaref-_VERSION()*
        A global variable (not a function) that holds a string containing the
        current interpreter version. The current contents of this string is
        `"Lua 5.1"` .

xpcall({f}, {err})                                             *luaref-xpcall()*
        This function is similar to `pcall` (see |luaref-pcall()|), except that
        you can set a new error handler.

        `xpcall` calls function {f} in protected mode, using {err} as the
        error handler. Any error inside {f} is not propagated; instead,
        `xpcall` catches the error, calls the {err} function with the original
        error object, and returns a status code. Its first result is the
        status code (a boolean), which is true if the call succeeds without
        errors. In this case, `xpcall` also returns all results from the call,
        after this first result. In case of any error, `xpcall` returns
        `false` plus the result from {err}.

==============================================================================
5.2  Coroutine Manipulation                                     *luaref-libCoro*

The operations related to coroutines comprise a sub-library of the basic
library and come inside the table `coroutine`. See |luaref-langCoro| for a
general description of coroutines.

coroutine.create({f})                                       *coroutine.create()*
        Creates a new coroutine, with body {f}. {f} must be a Lua function.
        Returns this new coroutine, an object with type `"thread"`.

coroutine.resume({co} [, {val1}, {...}])                    *coroutine.resume()*
        Starts or continues the execution of coroutine {co}. The first time
        you resume a coroutine, it starts running its body. The values {val1},
        {...} are passed as arguments to the body function. If the coroutine has
        yielded, `resume` restarts it; the values {val1}, {...} are passed as
        the results from the yield.

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

coroutine.running()                                        *coroutine.running()*
        Returns the running coroutine, or `nil` when called by the main
        thread.

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

coroutine.wrap({f})                                           *coroutine.wrap()*
        Creates a new coroutine, with body {f}. {f} 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
        `resume`. Returns the same values returned by `resume`, except the
        first boolean. In case of error, propagates the error.

coroutine.yield({...})                                       *coroutine.yield()*
        Suspends the execution of the calling coroutine. The coroutine cannot
        be running a C function, a metamethod, or an iterator. Any arguments
        to `yield` are passed as extra results to `resume`.

==============================================================================
5.3 - Modules                                                 *luaref-libModule*

The package library provides basic facilities for loading and building modules
in Lua. It exports two of its functions directly in the global environment:
`require` and `module` (see |luaref-require()| and |luaref-module()|). Everything else is
exported in a table `package`.

module({name} [, {...}])                                      *luaref-module()*
        Creates a module. If there is a table in `package.loaded[name]`, this
        table is the module. Otherwise, if there is a global table `t` with
        the given name, this table is the module. Otherwise creates a new
        table `t` and sets it as the value of the global {name} and the value
        of `package.loaded[name]`. This function also initializes `t._NAME`
        with the given name, `t._M` with the module (`t` itself), and
        `t._PACKAGE` with the package name (the full module name minus last
        component; see below). Finally, `module` sets `t` as the new
        environment of the current function and the new value of
        `package.loaded[name]`, so that `require` (see |luaref-require()|)
        returns `t`.

        If {name} is a compound name (that is, one with components separated
        by dots), `module` creates (or reuses, if they already exist) tables
        for each component. For instance, if {name} is `a.b.c`, then `module`
        stores the module table in field `c` of field `b` of global `a`.

        This function may receive optional `options` after the module name,
        where each option is a function to be applied over the module.

require({modname})                                            *luaref-require()*
        Loads the given module. The function starts by looking into the
        `package.loaded` table to determine whether {modname} is already
        loaded. If it is, then `require` returns the value stored at
        `package.loaded[modname]`. Otherwise, it tries to find a `loader` for
        the module.

        To find a loader, first `require` queries `package.preload[modname]`.
        If it has a value, this value (which should be a function) is the
        loader. Otherwise `require` searches for a Lua loader using the path
        stored in `package.path`. If that also fails, it searches for a C
        loader using the path stored in `package.cpath`. If that also fails,
        it tries an `all-in-one` loader (see below).

        When loading a C library, `require` first uses a dynamic link facility
        to link the application with the library. Then it tries to find a C
        function inside this library to be used as the loader. The name of
        this C function is the string `"luaopen_"` concatenated with a copy of
        the module name where each dot is replaced by an underscore. Moreover,
        if the module name has a hyphen, its prefix up to (and including) the
        first hyphen is removed. For instance, if the module name is
        `a.v1-b.c`, the function name will be `luaopen_b_c`.

        If `require` finds neither a Lua library nor a C library for a module,
        it calls the `all-in-one loader`. This loader searches the C path for
        a library for the root name of the given module. For instance, when
        requiring `a.b.c`, it will search for a C library for `a`. If found,
        it looks into it for an open function for the submodule; in our
        example, that would be `luaopen_a_b_c`. With this facility, a package
        can pack several C submodules into one single library, with each
        submodule keeping its original open function.

        Once a loader is found, `require` calls the loader with a single
        argument, {modname}. If the loader returns any value, `require`
        assigns the returned value to `package.loaded[modname]`. If the loader
        returns no value and has not assigned any value to
        `package.loaded[modname]`, then `require` assigns `true` to this
        entry. In any case, `require` returns the final value of
        `package.loaded[modname]`.

        If there is any error loading or running the module, or if it cannot
        find any loader for the module, then `require` signals an error.

package.cpath                                                  *package.cpath*
        The path used by `require` to search for a C loader.

        Lua initializes the C path `package.cpath` in the same way it
        initializes the Lua path `package.path`, using the environment
        variable `LUA_CPATH` (plus another default path defined in
        `luaconf.h`).

package.loaded                                                *package.loaded()*
        A table used by `require` to control which modules are already loaded.
        When you require a module `modname` and `package.loaded[modname]` is
        not false, `require` simply returns the value stored there.

package.loadlib({libname}, {funcname})                     *package.loadlib()*
        Dynamically links the host program with the C library {libname}.
        Inside this library, looks for a function {funcname} and returns this
        function as a C function. (So, {funcname} must follow the protocol
        (see |lua_CFunction()|)).

        This is a low-level function. It completely bypasses the package and
        module system. Unlike `require`, it does not perform any path
        searching and does not automatically adds extensions. {libname} must
        be the complete file name of the C library, including if necessary a
        path and extension. {funcname} must be the exact name exported by the
        C library (which may depend on the C compiler and linker used).

        This function is not supported by ANSI C. As such, it is only
        available on some platforms (Windows, Linux, Mac OS X, Solaris, BSD,
        plus other Unix systems that support the `dlfcn` standard).

package.path                                                    *package.path*
        The path used by `require` to search for a Lua loader.

        At start-up, Lua initializes this variable with the value of the
        environment variable `LUA_PATH` or with a default path defined in
        `luaconf.h`, if the environment variable is not defined. Any `";;"` in
        the value of the environment variable is replaced by the default path.

        A path is a sequence of `templates` separated by semicolons. For each
        template, `require` will change each interrogation mark in the
        template by `filename`, which is `modname` with each dot replaced by a
        "directory separator" (such as `"/"`  in Unix); then it will try to
        load the resulting file name. So, for instance, if the Lua path is
>
               "./?.lua;./?.lc;/usr/local/?/init.lua"
<
        the search for a Lua loader for module `foo` will try to load the
        files `./foo.lua`, `./foo.lc`, and `/usr/local/foo/init.lua`, in that
        order.

package.preload                                              *package.preload()*
        A table to store loaders for specific modules (see |luaref-require()|).

package.seeall({module})                                      *package.seeall()*
        Sets a metatable for {module} with its `__index` field referring to
        the global environment, so that this module inherits values from the
        global environment. To be used as an option to function {module}.

==============================================================================
5.4 - String Manipulation                                     *luaref-libString*

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 1 (not at 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.

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

string.byte({s} [, {i} [, {j}]])                                 *string.byte()*
        Returns the internal numerical codes of the characters `s[i]`,
        `s[i+1]`,..., `s[j]`. The default value for {i} is 1; the default
        value for {j} is {i}.

        Note that numerical codes are not necessarily portable across
        platforms.

string.char({...})                                               *string.char()*
        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 correspondent argument.

        Note that numerical codes are not necessarily portable across
        platforms.

string.dump({function})                                          *string.dump()*
        Returns a string containing a binary representation of the given
        function, so that a later |luaref-loadstring()| on this string returns a
        copy of the function. {function} must be a Lua function without
        upvalues.

string.find({s}, {pattern} [, {init} [, {plain}]])               *string.find()*
        Looks for the first match of {pattern} in the string {s}. If it finds
        a match, then {find} returns the indices of {s} where this occurrence
        starts and ends; otherwise, it returns `nil`. A third, optional
        numerical argument {init} specifies where to start the search; its
        default value is 1 and may be negative. A value of {true} as a fourth,
        optional argument {plain} turns off the pattern matching facilities,
        so the function does a plain "find substring" operation, with no
        characters in {pattern} being considered "magic". Note that if {plain}
        is given, then {init} must be given as well.

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

string.format({formatstring}, {...})                           *string.format()*
        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 `printf`
        family of standard C functions. The only differences are that the
        options/modifiers `*`, `l`, `L`, `n`, `p`, and `h` are not supported
        and that there is an extra option, `q`. The `q` 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
>lua
               string.format('%q', 'a string with "quotes" and \n new line')
<
        will produce the string:
>lua
               "a string with \"quotes\" and \
                new line"
<
        The options `c`, `d`, `E`, `e`, `f`, `g`, `G`, `i`, `o`, `u`, `X`, and
        `x` all expect a number as argument, whereas `q` and `s` expect a
        string.

        This function does not accept string values containing embedded zeros.

string.gmatch({s}, {pattern})                                  *string.gmatch()*
        Returns an iterator function that, each time it is called, returns the
        next captures from {pattern} over string {s}.

        If {pattern} specifies no captures, then the whole match is produced
        in each call.

        As an example, the following loop
>lua
               s = "hello world from Lua"
               for w in string.gmatch(s, "%a+") do
                 print(w)
               end
<
        will iterate over all the words from string {s}, printing one per
        line. The next example collects all pairs `key=value` from the given
        string into a table:
>lua
               t = {}
               s = "from=world, to=Lua"
               for k, v in string.gmatch(s, "(%w+)=(%w+)") do
                 t[k] = v
               end
<

string.gsub({s}, {pattern}, {repl} [, {n}])                    *string.gsub()*
        Returns a copy of {s} in which all occurrences of the {pattern} have
        been replaced by a replacement string specified by {repl}, which may
        be a string, a table, or a function. `gsub` also returns, as its
        second value, the total number of substitutions made.

        If {repl} is a string, then its value is used for replacement. The
        character `%` works as an escape character: any sequence in {repl} of
        the form `%n`, with {n} between 1 and 9, stands for the value of the
        {n} -th captured substring (see below). The sequence `%0` stands for
        the whole match. The sequence `%%` stands for a single `%`.

        If {repl} 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.

        If {repl} 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.

        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 `false` or `nil`, then there is no replacement
        (that is, the original match is kept in the string).

        The optional last parameter {n} limits the maximum number of
        substitutions to occur. For instance, when {n} is 1 only the first
        occurrence of `pattern` is replaced.

        Here are some examples:
>lua
           x = string.gsub("hello world", "(%w+)", "%1 %1")
           --> x="hello hello world world"

           x = string.gsub("hello world", "%w+", "%0 %0", 1)
           --> x="hello hello world"

           x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
           --> x="world hello Lua from"

           x = string.gsub("home =  `HOME, user = ` USER", "%$(%w+)", os.getenv)
           --> x="home = /home/roberto, user = roberto"

           x = string.gsub("4+5 =  `return 4+5` ", "% `(.-)%` ", function (s)
                 return loadstring(s)()
               end)
           --> x="4+5 = 9"

           local t = {name="lua", version="5.1"}
           x = string.gsub(" `name%-` version.tar.gz", "%$(%w+)", t)
           --> x="lua-5.1.tar.gz"
<

string.len({s})                                                   *string.len()*
        Receives a string and returns its length. The empty string `""` has
        length 0. Embedded zeros are counted, so `"a\000b\000c"` has length 5.

string.lower({s})                                               *string.lower()*
        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.

string.match({s}, {pattern} [, {init}])                         *string.match()*
        Looks for the first `match` of {pattern} in the string {s}. If it
        finds one, then `match` returns the captures from the pattern;
        otherwise it returns `nil`. If {pattern} specifies no captures, then
        the whole match is returned. A third, optional numerical argument
        {init} specifies where to start the search; its default value is 1 and
        may be negative.

string.rep({s}, {n})                                              *string.rep()*
        Returns a string that is the concatenation of {n} copies of the string
        {s}.

string.reverse({s})                                           *string.reverse()*
        Returns a string that is the string {s} reversed.

string.sub({s}, {i} [, {j}])                                      *string.sub()*
        Returns the substring of {s} that starts at {i} and continues until
        {j}; {i} and {j} may be negative. If {j} is absent, then it is assumed
        to be equal to `-1` (which is the same as the string length). In
        particular, the call `string.sub(s,1,j)` returns a prefix of {s} with
        length {j}, and `string.sub(s,-i)` returns a suffix of {s} with length
        {i}.

string.upper({s})                                               *string.upper()*
        Receives a string and returns a copy of that 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.

------------------------------------------------------------------------------
5.4.1  Patterns                            *luaref-patterns* *luaref-libStringPat*

A character class is used to represent a set of characters. The following
combinations are allowed in describing a character class:

    - `x`   (where `x` is not one of the magic characters `^$()%.[]*+-?`)
          represents the character `x` itself.
    - `.`   (a dot) represents all characters.
    - `%a`  represents all letters.
    - `%c`  represents all control characters.
    - `%d`  represents all digits.
    - `%l`  represents all lowercase letters.
    - `%p`  represents all punctuation characters.
    - `%s`  represents all space characters.
    - `%u`  represents all uppercase letters.
    - `%w`  represents all alphanumeric characters.
    - `%x`  represents all hexadecimal digits.
    - `%z`  represents the character with representation `0`.
    - `%x`  (where `x` is any non-alphanumeric character) represents the
          character `x`. This is the standard way to escape the magic
          characters. Any punctuation character (even the non-magic) can be
          preceded by a `%` when used to represent itself in a pattern.

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

         The interaction between ranges and classes is not defined. Therefore,
         patterns like `[%a-z]` or `[a-%%]` have no meaning.

    - `[^set]`  represents the complement of `set`, where `set` is interpreted
         as above.

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

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

                                                            *luaref-patternitem*
Pattern Item:~
-------------
A pattern item may be

   - a single character class, which matches any single character in the
      class;
   - a single character class followed by `*`, which matches 0 or more
      repetitions of characters in the class. These repetition items will
      always match the longest possible sequence;
   - a single character class followed by `+`, which matches 1 or more
      repetitions of characters in the class. These repetition items will
      always match the longest possible sequence;
   - a single character class followed by `-`, which also matches 0 or
      more repetitions of characters in the class. Unlike `*`, these
      repetition items will always match the shortest possible sequence;
   - a single character class followed by `?`, which matches 0 or 1
      occurrences of a character in the class;
   - `%n`, for `n` between 1 and 9; such item matches a substring equal to the
      `n` -th captured string (see below);
   - `%bxy`, where `x` and `y` are two distinct characters; such item matches
      strings that start with `x`, end with `y`, and where the `x` and `y`
      are balanced. This means that, if one reads the string from left to
      right, counting `+1` for an `x` and `-1` for a `y`, the ending `y` is the first
      `y` where the count reaches 0. For instance, the item `%b()` matches
      expressions with balanced parentheses.

                                                                *luaref-pattern*
Pattern:~
--------
A pattern is a sequence of pattern items. A `^` at the beginning of a pattern
anchors the match at the beginning of the subject string. A `$` at the end of
a pattern anchors the match at the end of the subject string. At other
positions, `^` and `$` have no special meaning and represent themselves.

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

As a special case, the empty capture `()` captures the current string position
(a number). For instance, if we apply the pattern `"()aa()"` on the
string `"flaaap"`, there will be two captures: 3 and 5.

A pattern cannot contain embedded zeros.  Use `%z` instead.

==============================================================================
5.5  Table Manipulation                                        *luaref-libTable*

This library provides generic functions for table manipulation. It provides
all its functions inside the table `table`.

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

table.concat({table} [, {sep} [, {i} [, {j}]]])                 *table.concat()*
        Given an array where all elements are strings or numbers, returns
        `table[i]..sep..table[i+1] ... sep..table[j]`. The default value for
        {sep} is the empty string, the default for {i} is 1, and the default
        for {j} is the length of the table. If {i} is greater than {j},
        returns the empty string.

table.foreach({table}, {f})                                  *table.foreach()*
        Executes the given {f} over all elements of {table}. For each element,
        {f} is called with the index and respective value as arguments. If {f}
        returns a non-`nil` value, then the loop is broken, and this value is
        returned as the final value of `table.foreach`.

        See |luaref-next()| for extra information about table traversals.

table.foreachi({table}, {f})                                *table.foreachi()*
        Executes the given {f} over the numerical indices of {table}. For each
        index, {f} is called with the index and respective value as arguments.
        Indices are visited in sequential order, from 1 to `n`, where `n` is
        the length of the table. If {f} returns a non-`nil` value, then the
        loop is broken and this value is returned as the result of
        `table.foreachi`.

table.insert({table}, [{pos},] {value})                         *table.insert()*
        Inserts element {value} at position {pos} in {table}, shifting up
        other elements to open space, if necessary. The default value for
        {pos} is `n+1`, where `n` is the length of the table (see
        |luaref-langLength|), so that a call `table.insert(t,x)` inserts `x`
        at the end of table `t`.

table.maxn({table})                                               *table.maxn()*
        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.)

table.remove({table} [, {pos}])                               *table.remove()*
        Removes from {table} the element at position {pos}, shifting down
        other elements to close the space, if necessary. Returns the value of
        the removed element. The default value for {pos} is `n`, where `n` is
        the length of the table (see |luaref-langLength|), so that a call
        `table.remove(t)` removes the last element of table `t`.

table.sort({table} [, {comp}])                                  *table.sort()*
        Sorts table elements in a given order, `in-place`, from `table[1]` to
        `table[n]`, where `n` is the length of the table (see
        |luaref-langLength|). If {comp} 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 `not comp(a[i+1],a[i])` will be true
        after the sort). If {comp} is not given, then the standard Lua
        operator `<` is used instead.

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

==============================================================================
5.6  Mathematical Functions                                     *luaref-libMath*

This library is an interface to most of the functions of the standard C math
library. It provides all its functions inside the table `math`.

math.abs({x})                                                       *math.abs()*
        Returns the absolute value of {x}.

math.acos({x})                                                     *math.acos()*
        Returns the arc cosine of {x} (in radians).

math.asin({x})                                                     *math.asin()*
        Returns the arc sine of {x} (in radians).

math.atan({x})                                                     *math.atan()*
        Returns the arc tangent of {x} (in radians).

math.atan2({x}, {y})                                              *math.atan2()*
        Returns the arc tangent of `x/y` (in radians), but uses the signs of
        both parameters to find the quadrant of the result. (It also handles
        correctly the case of {y} being zero.)

math.ceil({x})                                                     *math.ceil()*
        Returns the smallest integer larger than or equal to {x}.

math.cos({x})                                                       *math.cos()*
        Returns the cosine of {x} (assumed to be in radians).

math.cosh({x})                                                     *math.cosh()*
        Returns the hyperbolic cosine of {x}.

math.deg({x})                                                       *math.deg()*
        Returns the angle {x} (given in radians) in degrees.

math.exp({x})                                                       *math.exp()*
        Returns the value `e^x`.

math.floor({x})                                                   *math.floor()*
        Returns the largest integer smaller than or equal to {x}.

math.fmod({x}, {y})                                                *math.fmod()*
        Returns the remainder of the division of {x} by {y}.

math.frexp({x})                                                   *math.frexp()*
        Returns `m` and `e` such that `x = m * 2^e`, `e` is an integer and the
        absolute value of `m` is in the range `[0.5, 1)` (or zero when {x} is
        zero).

math.huge                                                          *math.huge()*
        The value `HUGE_VAL`, a value larger than or equal to any other
        numerical value.

math.ldexp({m}, {e})                                              *math.ldexp()*
        Returns `m * 2^e` (`e` should be an integer).

math.log({x})                                                       *math.log()*
        Returns the natural logarithm of {x}.

math.log10({x})                                                   *math.log10()*
        Returns the base-10 logarithm of {x}.

math.max({x}, {...})                                                *math.max()*
        Returns the maximum value among its arguments.

math.min({x}, {...})                                                *math.min()*
        Returns the minimum value among its arguments.

math.modf({x})                                                     *math.modf()*
        Returns two numbers, the integral part of {x} and the fractional part
        of {x}.

math.pi                                                              *math.pi()*
        The value of `pi`.

math.pow({x}, {y})                                                  *math.pow()*
        Returns `x^y`. (You can also use the expression `x^y` to compute this
        value.)

math.rad({x})                                                       *math.rad()*
        Returns the angle {x} (given in degrees) in radians.

math.random([{m} [, {n}]])                                       *math.random()*
        This function is an interface to the simple pseudo-random generator
        function `rand` provided by ANSI C. (No guarantees can be given for
        its statistical properties.)

        When called without arguments, returns a pseudo-random real number in
        the range `[0,1)`. When called with a number {m}, `math.random`
        returns a pseudo-random integer in the range `[1, m]`. When called
        with two numbers {m} and {n}, `math.random` returns a pseudo-random
        integer in the range `[m, n]`.

math.randomseed({x})                                         *math.randomseed()*
        Sets {x} as the "seed" for the pseudo-random generator: equal seeds
        produce equal sequences of numbers.

math.sin({x})                                                       *math.sin()*
        Returns the sine of {x} (assumed to be in radians).

math.sinh({x})                                                     *math.sinh()*
        Returns the hyperbolic sine of {x}.

math.sqrt({x})                                                     *math.sqrt()*
        Returns the square root of {x}. (You can also use the expression
        `x^0.5` to compute this value.)

math.tan({x})                                                       *math.tan()*
        Returns the tangent of {x} (assumed to be in radians).

math.tanh({x})                                                     *math.tanh()*
        Returns the hyperbolic tangent of {x}.

==============================================================================
5.6  Input and Output Facilities                                  *luaref-libIO*

The I/O library provides two different styles for file manipulation. The first
one uses implicit file descriptors; that is, there are operations to set a
default input file and a default output file, and all input/output operations
are over these default files. The second style uses explicit file
descriptors.

When using implicit file descriptors, all operations are supplied by
table `io`. When using explicit file descriptors, the operation `io.open` returns
a file descriptor and then all operations are supplied as methods of the file
descriptor.

The table `io` also provides three predefined file descriptors with their usual
meanings from C: `io.stdin`, `io.stdout`, and `io.stderr`.

Unless otherwise stated, all I/O functions return `nil` on failure (plus an
error message as a second result) and some value different from `nil` on
success.

io.close([{file}])                                                  *io.close()*
        Equivalent to `file:close`. Without a {file}, closes the default
        output file.

io.flush()                                                          *io.flush()*
        Equivalent to `file:flush` over the default output file.

io.input([{file}])                                                  *io.input()*
        When called with a file name, it opens the named file (in text mode),
        and sets its handle as the default input file. When called with a file
        handle, it simply sets this file handle as the default input file.
        When called without parameters, it returns the current default input
        file.

        In case of errors this function raises the error, instead of returning
        an error code.

io.lines([{filename}])                                              *io.lines()*
        Opens the given file name in read mode and returns an iterator
        function that, each time it is called, returns a new line from the
        file. Therefore, the construction

        `for line in io.lines(filename) do`  `body`  `end`

        will iterate over all lines of the file. When the iterator function
        detects the end of file, it returns `nil` (to finish the loop) and
        automatically closes the file.

        The call `io.lines()` (without a file name) is equivalent to
        `io.input():lines()`; that is, it iterates over the lines of the
        default input file. In this case it does not close the file when the
        loop ends.

io.open({filename} [, {mode}])                                       *io.open()*
        This function opens a file, in the mode specified in the string
        {mode}. It returns a new file handle, or, in case of errors, `nil`
        plus an error message.

        The {mode} string can be any of the following:

         - `"r"`   read mode (the default);
         - `"w"`   write mode;
         - `"a"`   append mode;
         - `"r+"`  update mode, all previous data is preserved;
         - `"w+"`  update mode, all previous data is erased;
         - `"a+"`  append update mode, previous data is preserved, writing is
                 only allowed at the end of file.

        The {mode} string may also have a `b` 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 C function `fopen`.

io.output([{file}])                                                *io.output()*
        Similar to `io.input`, but operates over the default output file.

io.popen({prog} [, {mode}])                                         *io.popen()*
        Starts program {prog} in a separated process and returns a file handle
        that you can use to read data from this program (if {mode} is `"r"`,
        the default) or to write data to this program (if {mode} is `"w"`).

        This function is system dependent and is not available on all
        platforms.

io.read({...})                                                       *io.read()*
        Equivalent to `io.input():read`.

io.tmpfile()                                                      *io.tmpfile()*
        Returns a handle for a temporary file. This file is opened in update
        mode and it is automatically removed when the program ends.

io.type({obj})                                                       *io.type()*
        Checks whether {obj} is a valid file handle. Returns the string
        `"file"` if {obj} is an open file handle, `"closed file"` if {obj} is
        a closed file handle, or `nil` if {obj} is not a file handle.

io.write({...})                                                     *io.write()*
        Equivalent to `io.output():write`.

file:close()                                               *luaref-file:close()*
        Closes `file`. Note that files are automatically closed when their
        handles are garbage collected, but that takes an unpredictable amount
        of time to happen.

file:flush()                                               *luaref-file:flush()*
        Saves any written data to `file`.

file:lines()                                               *luaref-file:lines()*
        Returns an iterator function that, each time it is called, returns a
        new line from the file. Therefore, the construction

               `for line in file:lines() do`  `body`  `end`

        will iterate over all lines of the file. (Unlike `io.lines`, this
        function does not close the file when the loop ends.)

file:read({...})                                            *luaref-file:read()*
        Reads the file `file`, 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 `nil` 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).

        The available formats are

         `"*n"`    reads a number; this is the only format that returns a
                 number instead of a string.
         `"*a"`    reads the whole file, starting at the current position. On
                 end of file, it returns the empty string.
         `"*l"`    reads the next line (skipping the end of line), returning
                 `nil` on end of file. This is the default format.
         `number`  reads a string with up to that number of characters,
                 returning `nil` on end of file. If number is zero, it reads
                 nothing and returns an empty string, or `nil` on end of file.

file:seek([{whence}] [, {offset}])                          *luaref-file:seek()*
        Sets and gets the file position, measured from the beginning of the
        file, to the position given by {offset} plus a base specified by the
        string {whence}, as follows:

         - `"set"`: base is position 0 (beginning of the file);
         - `"cur"`: base is current position;
         - `"end"`: base is end of file;

        In case of success, function `seek` returns the final file position,
        measured in bytes from the beginning of the file. If this function
        fails, it returns `nil`, plus a string describing the error.

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

file:setvbuf({mode} [, {size}])                          *luaref-file:setvbuf()*
        Sets the buffering mode for an output file. There are three available
        modes:

         `"no"`    no buffering; the result of any output operation appears
                 immediately.
         `"full"`  full buffering; output operation is performed only when
                 the buffer is full (or when you explicitly `flush` the file
                 (see |io.flush()|).
         `"line"`  line buffering; output is buffered until a newline is
                 output or there is any input from some special files (such as
                 a terminal device).

        For the last two cases, {size} specifies the size of the buffer, in
        bytes. The default is an appropriate size.

file:write({...})                                          *luaref-file:write()*
        Writes the value of each of its arguments to `file`. The arguments
        must be strings or numbers. To write other values, use `tostring`
        |luaref-tostring()| or `string.format` |string.format()| before
        `write`.

==============================================================================
5.8  Operating System Facilities                                  *luaref-libOS*

This library is implemented through table `os`.

os.clock()                                                          *os.clock()*
        Returns an approximation of the amount in seconds of CPU time used by
        the program.

os.date([{format} [, {time}]])                                       *os.date()*
        Returns a string or a table containing date and time, formatted
        according to the given string {format}.

        If the {time} argument is present, this is the time to be formatted
        (see the `os.time` function |os.time()| for a description of this
        value). Otherwise, `date` formats the current time.

        If {format} starts with `!`, then the date is formatted in
        Coordinated Universal Time. After this optional character, if {format}
        is the string `"*t"`, then `date` returns a table with the following
        fields: `year` (four digits), `month` (1-12), `day` (1-31), `hour`
        (0-23), `min` (0-59), `sec` (0-61), `wday` (weekday, Sunday is 1),
        `yday` (day of the year), and `isdst` (daylight saving flag, a
        boolean).

        If {format} is not `"*t"`, then `date` returns the date as a string,
        formatted according to the same rules as the C function `strftime`.

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

os.difftime({t2}, {t1})                                          *os.difftime()*
        Returns the number of seconds from time {t1} to time {t2}. In POSIX,
        Windows, and some other systems, this value is exactly `t2 - t1` .

os.execute([{command}])                                           *os.execute()*
        This function is equivalent to the C function `system`. It passes
        {command} to be executed by an operating system shell. It returns a
        status code, which is system-dependent. If {command} is absent, then
        it returns nonzero if a shell is available and zero otherwise.

os.exit([{code}])                                                    *os.exit()*
        Calls the C function `exit`, with an optional {code}, to terminate the
        host program. The default value for {code} is the success code.

os.getenv({varname})                                               *os.getenv()*
        Returns the value of the process environment variable {varname}, or
        `nil` if the variable is not defined.

os.remove({filename})                                              *os.remove()*
        Deletes the file with the given name. Directories must be empty to be
        removed. If this function fails, it returns `nil`, plus a string
        describing the error.

os.rename({oldname}, {newname})                                    *os.rename()*
        Renames file named {oldname} to {newname}. If this function fails, it
        returns `nil`, plus a string describing the error.

os.setlocale({locale} [, {category}])                           *os.setlocale()*
        Sets the current locale of the program. {locale} is a string
        specifying a locale; {category} is an optional string describing which
        category to change: `"all"`, `"collate"`, `"ctype"`, `"monetary"`,
        `"numeric"`, or `"time"`; the default category is `"all"`. The
        function returns the name of the new locale, or `nil` if the request
        cannot be honored.

os.time([{table}])                                                   *os.time()*
        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 `year`, `month`, and `day`, and may have fields
        `hour`, `min`, `sec`, and `isdst` (for a description of these fields,
        see the `os.date` function |os.date()|).

        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
        `time` can be used only as an argument to `date` and `difftime`.

os.tmpname()                                                      *os.tmpname()*
        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.

==============================================================================
5.9  The Debug Library                                         *luaref-libDebug*

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 its 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.

All functions in this library are provided inside the `debug` 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.

debug.debug()                                                    *debug.debug()*
        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
        `cont` finishes this function, so that the caller continues its
        execution.

        Note that commands for `debug.debug` are not lexically nested within
        any function, and so have no direct access to local variables.

debug.getfenv(o)                                               *debug.getfenv()*
        Returns the environment of object {o}.

debug.gethook([{thread}])                                      *debug.gethook()*
        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 `debug.sethook` function).

debug.getinfo([{thread},] {function} [, {what}])               *debug.getinfo()*
        Returns a table with information about a function. You can give the
        function directly, or you can give a number as the value of
        {function}, which means the function running at level {function} of
        the call stack of the given thread: level 0 is the current function
        (`getinfo` itself); level 1 is the function that called `getinfo`; and
        so on. If {function} is a number larger than the number of active
        functions, then `getinfo` returns `nil`.

        The returned table may contain all the fields returned by
        `lua_getinfo` (see |lua_getinfo()|), with the string {what}
        describing which fields to fill in. The default for {what} is to get
        all information available, except the table of valid lines. If
        present, the option `f` adds a field named `func` with the function
        itself. If present, the option `L` adds a field named `activelines`
        with the table of valid lines.

        For instance, the expression `debug.getinfo(1,"n").name` returns the
        name of the current function, if a reasonable name can be found, and
        `debug.getinfo(print)` returns a table with all available information
        about the `print` function.

debug.getlocal([{thread},] {level}, {local})                  *debug.getlocal()*
        This function returns the name and the value of the local variable
        with index {local} of the function at level {level} of the stack. (The
        first parameter or local variable has index 1, and so on, until the
        last active local variable.) The function returns `nil` if there is no
        local variable with the given index, and raises an error when called
        with a {level} out of range. (You can call `debug.getinfo`
        |debug.getinfo()| to check whether the level is valid.)

        Variable names starting with `(` (open parentheses) represent
        internal variables (loop control variables, temporaries, and C
        function locals).

debug.getmetatable({object})                              *debug.getmetatable()*
        Returns the metatable of the given {object} or `nil` if it does not
        have a metatable.

debug.getregistry()                                        *debug.getregistry()*
        Returns the registry table (see |luaref-apiRegistry|).

debug.getupvalue({func}, {up})                              *debug.getupvalue()*
        This function returns the name and the value of the upvalue with index
        {up} of the function {func}. The function returns `nil` if there is no
        upvalue with the given index.

debug.setfenv({object}, {table})                               *debug.setfenv()*
        Sets the environment of the given {object} to the given {table}.
        Returns {object}.

debug.sethook([{thread},] {hook}, {mask} [, {count}])          *debug.sethook()*
        Sets the given function as a hook. The string {mask} and the number
        {count} describe when the hook will be called. The string mask may
        have the following characters, with the given meaning:

         - `"c"` : The hook is called every time Lua calls a function;
         - `"r"` : The hook is called every time Lua returns from a function;
         - `"l"` : The hook is called every time Lua enters a new line of
           code.

        With a {count} different from zero, the hook is called after every
        {count} instructions.

        When called without arguments, the `debug.sethook` turns off the hook.

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

debug.setlocal([{thread},] {level}, {local}, {value})         *debug.setlocal()*
        This function assigns the value {value} to the local variable with
        index {local} of the function at level {level} of the stack. The
        function returns `nil` if there is no local variable with the given
        index, and raises an error when called with a {level} out of range.
        (You can call `getinfo` to check whether the level is valid.)
        Otherwise, it returns the name of the local variable.

debug.setmetatable({object}, {table})                     *debug.setmetatable()*
        Sets the metatable for the given {object} to the given {table} (which
        can be `nil`).

debug.setupvalue({func}, {up}, {value})                     *debug.setupvalue()*
        This function assigns the value {value} to the upvalue with index {up}
        of the function {func}. The function returns `nil` if there is no
        upvalue with the given index. Otherwise, it returns the name of the
        upvalue.

debug.traceback([{thread},] [{message}] [,{level}])          *debug.traceback()*
        Returns a string with a traceback of the call stack. An optional
        {message} string is appended at the beginning of the traceback. An
        optional {level} number tells at which level to start the traceback
        (default is 1, the function calling `traceback`).

==============================================================================
A  BIBLIOGRAPHY                                            *luaref-bibliography*

This help file is a minor adaptation from this main reference:

 - R. Ierusalimschy, L. H. de Figueiredo, and W. Celes.,
   "Lua: 5.1 reference manual", https://www.lua.org/manual/5.1/manual.html

Lua is discussed in these references:

 - R. Ierusalimschy, L. H. de Figueiredo, and W. Celes.,
   "Lua --- an extensible extension language".
   "Software: Practice & Experience" 26, 6 (1996) 635-652.

 - L. H. de Figueiredo, R. Ierusalimschy, and W. Celes.,
   "The design and implementation of a language for extending applications".
   "Proc. of XXI Brazilian Seminar on Software and Hardware" (1994) 273-283.

 - L. H. de Figueiredo, R. Ierusalimschy, and W. Celes.,
   "Lua: an extensible embedded language".
   "Dr. Dobb's Journal" 21, 12 (Dec 1996) 26-33.

 - R. Ierusalimschy, L. H. de Figueiredo, and W. Celes.,
   "The evolution of an extension language: a history of Lua".
   "Proc. of V Brazilian Symposium on Programming Languages" (2001) B-14-B-28.

==============================================================================
B  COPYRIGHT AND LICENSES                                     *luaref-copyright*

This help file has the same copyright and license as Lua 5.1 and the Lua 5.1
 manual:

Copyright (c) 1994-2006 Lua.org, PUC-Rio.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.

==============================================================================
C  LUAREF DOC                 *luarefvim* *luarefvimdoc* *luaref-help* *luaref-doc*

This is a Vim help file containing a reference for Lua 5.1, and it is -- with
a few exceptions and adaptations -- a copy of the Lua 5.1 Reference Manual
(see |luaref-bibliography|). For usage information, refer to
|luaref-doc|. For copyright information, see |luaref-copyright|.

The main ideas and concepts on how to implement this reference were taken from
Christian Habermann's CRefVim project
(https://www.vim.org/scripts/script.php?script_id=614).

Adapted for bundled Nvim documentation; the original plugin can be found at
https://www.vim.org/scripts/script.php?script_id=1291

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