SocialOS | Documentation

1 – Introduction

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. The Lua distribution includes a sample host program called
lua, which uses the Lua library to offer a complete, stand-alone Lua
interpreter.

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 the technical
papers available at Lua's web site. For a detailed introduction to
programming in Lua, see Roberto's book, Programming in Lua (Second
Edition)
.

2 – The 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. Non-terminals are shown like non-terminal, keywords are
shown like kword, and other terminal symbols are shown like
`=´. The complete syntax of Lua can be found in §8 at the end
of this manual.

2.1 – Lexical Conventions

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 names in most languages. (The definition of letter
depends on the current locale: any character considered alphabetic by
the current locale can be used in an identifier.) Identifiers are used
to name variables and table fields.

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:

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

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]), and '\''
(apostrophe [single quote]). Moreover, a backslash followed by a real
newline results in a newline in the string. A character in a string can
also be specified by its numerical value using the escape sequence
\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 can contain any
8-bit value, including embedded zeros, which can be specified as '\0'.

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
can run for several lines, do not interpret any escape sequences, and
ignore long brackets of any other level. They can contain anything
except a closing bracket of the proper level.

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 literal strings below denote
the same string:

     a = 'alo\n123"'
     a = "alo\n123\""
     a = '\97lo\10\04923"'
     a = [[alo
     123"]]
     a = [==[
     alo
     123"]==]

A numerical constant can 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

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

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.

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 can contain any 8-bit character,
including embedded zeros ('\0') (see §2.1).

Lua can call (and manipulate) functions written in Lua and functions
written in C (see §2.5.8).

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 §2.8). 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.

The type thread represents independent threads of execution and it is
used to implement coroutines (see §2.11). Do not confuse Lua
threads with operating-system threads. Lua supports coroutines on all
systems, even those that do not support threads.

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 can be used to represent ordinary arrays, symbol tables,
sets, records, graphs, trees, etc. To represent records, Lua uses the
field name as an index. The language supports this representation by
providing a.name as syntactic sugar for a["name"]. There are several
convenient ways to create tables in Lua (see §2.5.7).

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 can contain functions. Thus tables can also carry methods (see
§2.5.9).

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.

2.2.1 – Coercion

Lua provides automatic conversion between string and number values at
run time. Any arithmetic operation applied to a string tries to convert
this string to a number, following the usual conversion rules.
Conversely, whenever a number is used where a string is expected, the
number is converted to a string, in a reasonable format. For complete
control over how numbers are converted to strings, use the format
function from the string library (see
string.format).

2.3 – Variables

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 kind of local
variable):

    var ::= Name

Name denotes identifiers, as defined in §2.1.

Any variable is assumed to be global unless explicitly declared as a
local (see §2.4.7). Local variables are lexically scoped:
local variables can be freely accessed by functions defined inside their
scope (see §2.6).

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

Square brackets are used to index a table:

    var ::= prefixexp `[´ exp `]´

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 §2.8 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.)

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 §2.9). 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. To replace it, you call
setfenv. (You can only manipulate the environment of
C functions through the debug library; (see §5.9).)

An access to a global variable x is equivalent to _env.x, which in
turn is equivalent to

     gettable_event(_env, "x")

where _env is the environment of the running function. (See
§2.8 for a complete description of the gettable_event
function. This function is not defined or callable in Lua. Similarly,
the _env variable is not defined in Lua. We use them here only for
explanatory purposes.)

2.4 – Statements

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

2.4.1 – Chunks

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 §2.5.9). As such, chunks can define
local variables, receive arguments, and return values.

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

Chunks can 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

A block is a list of statements; syntactically, a block is the same as a
chunk:

    block ::= chunk

A block can 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
§2.4.4).

2.4.3 – Assignment

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

    stat ::= varlist `=´ explist
    varlist ::= var {`,´ var}
    explist ::= exp {`,´ exp}

Expressions are discussed in §2.5.

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 that call enter
the list of values, before the adjustment (except when the call is
enclosed in parentheses; see §2.5).

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

     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

     x, y = y, x

exchanges the values of x and y, and

     x, y, z = y, z, x

cyclically permutes the values of x, y, and z.

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
§2.8 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

     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

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 §2.4.5).

The condition expression of a control structure can 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 repeatuntil 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.

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

    stat ::= return [explist]

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

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 v = e1, e2, e3 do block end

is equivalent to the code:

     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
         local v = var
         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 shown
    here are 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 v 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.

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 explist do block end
    namelist ::= Name {`,´ Name}

A for statement like

     for var_1, ···, var_n in explist do block end

is equivalent to the code:

     do
       local f, s, var = explist
       while true do
         local var_1, ···, var_n = f(s, var)
         var = var_1
         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 var_i 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

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 §2.5.8.

2.4.7 – Local Declarations

Local variables can be declared anywhere inside a block. The declaration
can include an initial assignment:

    stat ::= local namelist [`=´ explist]

If present, an initial assignment has the same semantics of a multiple
assignment (see §2.4.3). Otherwise, all variables are
initialized with nil.

A chunk is also a block (see §2.4.1), 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 §2.6.

2.5 – Expressions

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 §2.1; variables are
explained in §2.3; function definitions are explained in
§2.5.9; function calls are explained in §2.5.8;
table constructors are explained in §2.5.7. Vararg
expressions, denoted by three dots ('...'), can only be used when
directly inside a vararg function; they are explained in
§2.5.9.

Binary operators comprise arithmetic operators (see §2.5.1),
relational operators (see §2.5.2), logical operators (see
§2.5.3), and the concatenation operator (see
§2.5.4). Unary operators comprise the unary minus (see
§2.5.1), the unary not (see §2.5.3), and the
unary length operator (see §2.5.5).

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

Here are some examples:

     f()                -- adjusted to 0 results
     g(f(), x)          -- f() is adjusted to 1 result
     g(x, f())          -- g gets x plus all results from f()
     a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)
     a,b = ...          -- a gets the first vararg parameter, b gets
                        -- the second (both a and b can get nil if there
                        -- is no corresponding vararg parameter)

     a,b,c = x, f()     -- f() is adjusted to 2 results
     a,b,c = f()        -- f() is adjusted to 3 results
     return f()         -- returns all results from f()
     return ...         -- returns all received vararg parameters
     return x,y,f()     -- returns x, y, and all results from f()
     {f()}              -- creates a list with all results from f()
     {...}              -- creates a list with all vararg parameters
     {f(), nil}         -- f() is adjusted to 1 result

Any 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

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
§2.2.1), 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

     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

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,
thread, or function), this new object is different from any previously
existing object.

You can change the way that Lua compares tables and userdata by using
the "eq" metamethod (see §2.8).

The conversion rules of §2.2.1 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
§2.8). A comparison a > b is translated to b < a and
a >= b is translated to b <= a.

2.5.3 – Logical Operators

The logical operators in Lua are and, or, and not. Like the
control structures (see §2.4.4), all logical operators
consider both false and nil as false and anything else as true.

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

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
§2.2.1. Otherwise, the "concat" metamethod is called (see
§2.8).

2.5.5 – The Length Operator

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 can be zero. For a regular array, with non-nil values
from 1 to a given n, its length is exactly that n, the index of its
last value. If the array has "holes" (that is, nil values between
other non-nil values), then #t can be any of the indices that directly
precedes a nil value (that is, it may consider any such nil
value as the end of the array).

2.5.6 – Precedence

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 of an
expression. The concatenation ('..') and exponentiation ('^')
operators are right associative. All other binary operators are left
associative.

2.5.7 – Table Constructors

Table constructors are expressions that create tables. Every time a
constructor is evaluated, a new table is created. A constructor can be
used to create an empty table or to create a table and initialize some
of its fields. The general syntax for constructors is

    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,

     a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }

is equivalent to

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

If the last field in the list has the form exp and the expression is a
function call or a vararg expression, then all values returned by this
expression enter the list consecutively (see §2.5.8). To avoid
this, enclose the function call or the vararg expression in parentheses
(see §2.5).

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

2.5.8 – Function Calls

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 §2.8).

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 ::= `(´ [explist] `)´
    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

     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).

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:

     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

The syntax for function definition is

    function ::= function funcbody
    funcbody ::= `(´ [parlist] `)´ 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 () body end

not to

     local f = function () body end

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

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 can refer to different external local variables and
can have different environment tables.

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

    parlist ::= namelist [`,´ `...´] | `...´

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 that last expression is enclosed in
parentheses).

As an example, consider the following definitions:

     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
§2.4.4). If control reaches the end of a function without
encountering a return statement, then the function returns with no
results.

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

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:

     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.

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:

     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

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 printing an error message).

Lua code can explicitly generate an error by calling the
error function. If you need to catch errors in Lua, you
can use the pcall function.

2.8 – Metatables

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

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

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

A metatable controls how an object behaves in arithmetic operations,
order comparisons, concatenation, length operation, and indexing. A
metatable also can define a function to be called when a userdata is
garbage collected. For each of these operations Lua associates a
specific key called an event. When Lua performs one of these
operations over a value, it checks whether this value has a metatable
with the corresponding event. If so, the value associated with that key
(the metamethod) controls how Lua will perform the operation.

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 the 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 §5.1. In particular, to retrieve the metamethod of
a given object, we use the expression

     metatable(obj)[event]

This should be read as

     rawget(getmetatable(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": 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.

         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

         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": the - operation. Behavior similar to the "add"
    operation.

  • "mul": the * operation. Behavior similar to the "add"
    operation.
  • "div": the / operation. Behavior similar to the "add"
    operation.
  • "mod": the % operation. Behavior similar to the "add"
    operation, with the operation o1 - floor(o1/o2)*o2 as the
    primitive operation.
  • "pow": the ^ (exponentiation) operation. Behavior similar to
    the "add" operation, with the function pow (from the C math
    library) as the primitive operation.
  • "unm": the unary - operation.

         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": the .. (concatenation) operation.

         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": the # operation.

         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
    

    See §2.5.5 for a description of the length of a table.

  • "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.

         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:

         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": the < operation.

         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": the <= operation.

         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": The indexing access table[key].

         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
         end
    
  • "newindex": The indexing assignment table[key] = value.

         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
             h(table, key,value)           -- call the handler
           else h[key] = value             -- or repeat operation on it
           end
         end
    
  • "call": called when Lua calls a value.

         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

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.

Threads are created sharing the environment of the creating thread.
Userdata and C functions are created sharing the environment of the
creating C function. Non-nested Lua functions (created by
loadfile, loadstring or
load) are created sharing the environment of the creating
thread. Nested Lua functions are created sharing the environment of the
creating Lua function.

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 threads and non-nested Lua
functions created by the thread and can be directly accessed by C code
(see §3.3).

The environment associated with a C function can be directly accessed by
C code (see §3.3). It is used as the default environment for
other C functions and userdata created by the function.

Environments associated with Lua functions are used to resolve all
accesses to global variables within the function (see §2.3).
They are used as the default environment for nested 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.
To manipulate the environment of other objects (userdata, C functions,
other threads) you must use the C API.

2.10 – Garbage Collection

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

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. Both use percentage
points as units (so that a value of 100 means an internal value of 1).

5 – Standard Libraries

The standard Lua 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., table.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, which includes the coroutine sub-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.

To have access to these libraries, the C host program should call the
luaL_openlibs function, which opens all standard
libraries. Alternatively, it can open them 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 library), luaopen_os
(for the Operating System library), 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.

5.1 – Basic Functions

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])

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!"


error (message [, level])

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.


ipairs (t)

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.


next (table [, index])

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 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)

Returns three values: the 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.

See function next for the caveats of modifying the table
during its traversal.


rawequal (v1, v2)

Checks whether v1 is equal to v2, without invoking any metamethod.
Returns a boolean.


rawget (table, index)

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)

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, ···)

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.


tonumber (e [, base])

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 can have a decimal part, as well as an optional
exponent part (see §2.1). In other bases, only unsigned integers
are accepted.


tostring (e)

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.

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


type (v)

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]])

Returns the elements from the given table. This function is equivalent
to

     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 §2.5.5).


5.2 – Coroutine Manipulation

Coroutines are not available to SocialOS scripts.

5.3 – Modules

Modules, other than the built-in modules and the SocialOS api, are not
available to SocialOS scripts.


5.4 – String Manipulation

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).

The string library assumes one-byte character encodings.


string.byte (s [, i [, j]])

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 (···)

Receives zero or more integers. Returns a string with length equal to
the number of arguments, in which each character has the internal
numerical code equal to its corresponding argument.

Note that numerical codes are not necessarily portable across platforms.


string.dump (function)

Returns a string containing a binary representation of the given
function, so that a later loadstring on this string
returns a copy of the function. function must be a Lua function
without upvalues.


string.find (s, pattern [, init [, plain]])

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 can 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, ···)

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

     string.format('%q', 'a string with "quotes" and \n new line')

will produce the string:

     "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,
except as arguments to the q option.


string.gmatch (s, pattern)

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

     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:

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

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


string.gsub (s, pattern, repl [, n])

Returns a copy of s in which all (or the first n, if given)
occurrences of the pattern have been replaced by a replacement string
specified by repl, which can be a string, a table, or a function.
gsub also returns, as its second value, the total number of matches
that occurred.

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).

Here are some examples:

     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)

Receives a string and returns its length. The empty string "" has
length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.


string.lower (s)

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])

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 can be negative.


string.rep (s, n)

Returns a string that is the concatenation of n copies of the string
s.


string.reverse (s)

Returns a string that is the string s reversed.


string.sub (s, i [, j])

Returns the substring of s that starts at i and continues until j;
i and j can 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)

Receives a string and returns a copy of this string with all lowercase
letters changed to uppercase. All other characters are left unchanged.
The definition of what a lowercase letter is depends on the current
locale.

5.4.1 – Patterns

Character Class:

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 can be specified by
    separating the end characters of the range with a '-'. All classes
    %x described above can 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.

Pattern Item:

A pattern item can 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
    occurrence 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.

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.

Captures:

A pattern can 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

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 these functions, when we talk about the "length" of
a table we mean the result of the length operator.


table.concat (table [, sep [, i [, j]]])

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.insert (table, [pos,] value)

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 §2.5.5), so
that a call table.insert(t,x) inserts x at the end of table t.


table.maxn (table)

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])

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, so that a call table.remove(t) removes the last
element of table t.


table.sort (table [, comp])

Sorts table elements in a given order, in-place, from table[1] to
table[n], where n is the length of the table. 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

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


math.abs (x)

Returns the absolute value of x.


math.acos (x)

Returns the arc cosine of x (in radians).


math.asin (x)

Returns the arc sine of x (in radians).


math.atan (x)

Returns the arc tangent of x (in radians).


math.atan2 (y, x)

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


math.ceil (x)

Returns the smallest integer larger than or equal to x.


math.cos (x)

Returns the cosine of x (assumed to be in radians).


math.cosh (x)

Returns the hyperbolic cosine of x.


math.deg (x)

Returns the angle x (given in radians) in degrees.


math.exp (x)

Returns the value ex.


math.floor (x)

Returns the largest integer smaller than or equal to x.


math.fmod (x, y)

Returns the remainder of the division of x by y that rounds the
quotient towards zero.


math.frexp (x)

Returns m and e such that x = m2e, 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

The value HUGE_VAL, a value larger than or equal to any other
numerical value.


math.ldexp (m, e)

Returns m2e (e should be an integer).


math.log (x)

Returns the natural logarithm of x.


math.log10 (x)

Returns the base-10 logarithm of x.


math.max (x, ···)

Returns the maximum value among its arguments.


math.min (x, ···)

Returns the minimum value among its arguments.


math.modf (x)

Returns two numbers, the integral part of x and the fractional part of
x.


math.pi

The value of pi.


math.pow (x, y)

Returns xy. (You can also use the expression x^y to
compute this value.)


math.rad (x)

Returns the angle x (given in degrees) in radians.


math.random ([m [, n]])

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 uniform pseudo-random real
number in the range [0,1). When called with an integer number m,
math.random returns a uniform pseudo-random integer in the range [1,
m]
. When called with two integer numbers m and n, math.random
returns a uniform pseudo-random integer in the range [m, n].


math.randomseed (x)

Sets x as the "seed" for the pseudo-random generator: equal seeds
produce equal sequences of numbers.


math.sin (x)

Returns the sine of x (assumed to be in radians).


math.sinh (x)

Returns the hyperbolic sine of x.


math.sqrt (x)

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


math.tan (x)

Returns the tangent of x (assumed to be in radians).


math.tanh (x)

Returns the hyperbolic tangent of x.


Last update: Tue Nov 13 19:16:29 BRST 2012

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