Extension:Scribunto/Lua reference manual: Difference between revisions

Content deleted Content added

Inline

Revision as of 10:16, 15 August 2012

Introduction

This manual documents Lua as it is used in MediaWiki with the Scribunto extension. The manual is derived from the Lua 5.1 reference manual, which is available under an MIT-style license. This derivative manual may thus be copied under the terms of the same license.

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.

Getting started

On a MediaWiki wiki with Lua enabled, create a page with a title starting with "Module:", for example "Module:Bananas". Into this new page, copy the following text:

local p = {}

function p.hello()
    return "Hello, world!"
end

return p

Save that, then on another (non-module) page, write:

{{#invoke:Bananas|hello}}

Except that you should replace "Bananas" with whatever you called your module. This will call the "hello" function exported from that module. The {{#invoke:Bananas|hello}} will be replaced with the text that the function returned, in this case, "Hello, world!"

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 at the end of this manual.

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.

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 (which are not currently supported in MediaWiki). 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.

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).

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. 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.

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.)

In plain Lua, it is possible to get and set the environment of a function, however this is not possible in MediaWiki.

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.

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.

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).

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.)

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 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.

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.

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.

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.

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.

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.)

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.

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.

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.)

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).

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).

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.

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.

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

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

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.

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. 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 (message [, level])|error]] function. If you need to catch errors in Lua, you can use the pcall function.

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 (e [, base])|tonumber]], etc.) are described in §3.1. 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

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).

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

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

You can change these numbers by calling collectgarbage. With this function you can also control the collector directly (e.g., stop and restart it).

Garbage-Collection Metamethods

Using the C API, you can set garbage-collector metamethods for userdata (see §2.8). 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).

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:

     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 those collected in that cycle. That is, the first finalizer to be called is the one associated with the userdata created last in the program. The userdata itself is freed only in the next garbage-collection cycle.

Weak Tables

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.

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 __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 __mode field. Otherwise, the weak behavior of the tables controlled by this metatable is undefined.

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 (table [, comp])|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.

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!"

`collectgarbage ([opt [, arg]])`

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

"collect": performs a full garbage-collection cycle. This is the default option.
"stop": stops the garbage collector.
"restart": restarts the garbage collector.
"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 as the new value for the pause of the collector (see §2.10). Returns the previous value for pause.
"setstepmul": sets arg as the new value for the step multiplier of the collector (see §2.10). Returns the previous value for step.

`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.

`_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.

`getmetatable (object)`

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)`

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 (table [, index])|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 (table [, index])|next]] for the caveats of modifying the table during its traversal.

`pcall (f, arg1, ···)`

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 (msg)`

This function is only available in code executed from the debug console, available in some MediaWiki installations when editing a module. It is equivalent to mw.log().

`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.

`setmetatable (table, metatable)`

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])`

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).

`_VERSION`

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

`xpcall (f, err)`

This function is similar to 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.

Modules

The package library provides basic facilities for loading and building modules in Lua. It exports the require function directly in the global environment. Everything else is exported in a table package.

`require (modname)`

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, require is guided by the package.loaders array. By changing this array, we can change how require looks for a module.

`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.loaders`

A table used by require to control how to load modules.

Each entry in this table is a searcher function. When looking for a module, require calls each of these searchers in ascending order, with the module name (the argument given to require) as its sole parameter. The function can return another function (the module loader) or a string explaining why it did not find that module (or nil if it has nothing to say). Lua initializes this table with four functions.

The first searcher simply looks for a loader in the package.preload table.

The second searcher calls back to MediaWiki. MediaWiki provides several built-in modules, and allows modules on the local wiki to be loaded by using a "Module:" prefix.

`package.preload`

A table to store loaders for specific modules (see require).

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.

`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.

`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.

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.

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.

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 e^x.

`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 = m2^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`

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

`math.ldexp (m, e)`

Returns m2^e (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 x^y. (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.

The Debug Library

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

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.traceback ([thread,] [message [, level]])`

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).

MediaWiki libraries

The `mw` table

The mw table contains some general utility functions. It is always available.

`mw.clone (value)`

Returns an independent copy of the specified value. Tables in Lua are assigned and passed to functions by reference, which means that if you change a table via one reference, the underlying table will immediately appear to have changed in the other references as well. For example:

a = {x = 1}
b = a
b.x = 2
return a.x      -- returns 2

mw.clone() recursively copies all table elements, and also any metatable attached to the table value or its elements.

Cloning a table that contains a circular reference gives a table copy with an equivalent circular reference. For example:

a = {}
a.a = a
b = mw.clone(a)
return b.a.a.a.a     -- Returns a reference to the table "b"

`mw.log (msg)`

Appends the specified message to the log buffer, with a newline automatically added. In the Scribunto debug console, any log messages added to the buffer in this way during console execution will be displayed on the screen after execution completes.

The Complete Syntax of Lua

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

   chunk ::= {stat [`;´]} [laststat [`;´]]
   
   block ::= chunk
   
   stat ::=  varlist `=´ explist | 
        functioncall | 
        do block end | 
        while exp do block end | 
        repeat block until exp | 
        if exp then block {elseif exp then block} [else block] end | 
        for Name `=´ exp `,´ exp [`,´ exp] do block end | 
        for namelist in explist do block end | 
        function funcname funcbody | 
        local function Name funcbody | 
        local namelist [`=´ explist] 
   
   laststat ::= return [explist] | break
   
   funcname ::= Name {`.´ Name} [`:´ Name]
   
   varlist ::= var {`,´ var}
   
   var ::=  Name | prefixexp `[´ exp `]´ | prefixexp `.´ Name 
   
   namelist ::= Name {`,´ Name}
   
   explist ::= {exp `,´} exp
   
   exp ::=  nil | false | true | Number | String | `...´ | function | 
        prefixexp | tableconstructor | exp binop exp | unop exp 
   
   prefixexp ::= var | functioncall | `(´ exp `)´
   
   functioncall ::=  prefixexp args | prefixexp `:´ Name args 
   
   args ::=  `(´ [explist] `)´ | tableconstructor | String 
   
   function ::= function funcbody
   
   funcbody ::= `(´ [parlist] `)´ block end
   
   parlist ::= namelist [`,´ `...´] | `...´
   
   tableconstructor ::= `{´ [fieldlist] `}´
   
   fieldlist ::= field {fieldsep field} [fieldsep]
   
   field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp
   
   fieldsep ::= `,´ | `;´
   
   binop ::= `+´ | `-´ | `*´ | `/´ | `^´ | `%´ | `..´ | 
        `<´ | `<=´ | `>´ | `>=´ | `==´ | `~=´ | 
        and | or
   
   unop ::= `-´ | not | `#´