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LUA Reference Manual 5.0


LUA Reference Manual 입니다. 영문

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LUA Reference Manual 입니다. 영문 Lua: 5.0 reference manual

Lua Lua 5.0 Reference Manual

by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes

Copyright © 2003 Tecgraf, PUC-Rio. All rights reserved.


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 configuration 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 stand-alone embedding program, lua, that uses the Lua library to offer a complete Lua interpreter.

Lua is free software, and is provided as usual with no guarantees, as stated in its copyright notice. The implementation described in this manual is available at Lua's official web site, 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 papers below, which are available at Lua's web site.

  • R. Ierusalimschy, L. H. de Figueiredo, and W. Celes. Lua---an extensible extension language. Software: Practice & Experience 26 #6 (1996) 635-652.
  • L. H. de Figueiredo, R. Ierusalimschy, and W. Celes. The design and implementation of a language for extending applications. Proceedings of XXI Brazilian Seminar on Software and Hardware (1994) 273-283.
  • L. H. de Figueiredo, R. Ierusalimschy, and W. Celes. Lua: an extensible embedded language. Dr. Dobb's Journal 21 #12 (Dec 1996) 26-33.
  • R. Ierusalimschy, L. H. de Figueiredo, and W. Celes. The evolution of an extension language: a history of Lua, Proceedings of V Brazilian Symposium on Programming Languages (2001) B-14-B-28.

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

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, in which {a} means 0 or more a's, and [a] means an optional a. Non-terminals are shown in italics, keywords are shown in bold, and other terminal symbols are shown in typewriter font, enclosed in single quotes.

2.1 - Lexical Conventions

Identifiers in Lua can be any string of letters, digits, and underscores, not beginning with a digit. This coincides with the definition of identifiers in most languages. (The definition of letter depends on the current locale: any character considered alphabetic by the current locale can be used in an identifier.)

The following keywords are reserved and cannot be used as identifiers:

       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 identifiers. As a convention, identifiers starting with an underscore followed by uppercase letters (such as _VERSION) are reserved for internal 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
  • \' --- apostrophe
  • \[ --- left square bracket
  • \] --- right square bracket
Moreover, a `\newline´ (that is, a backslash followed by a real newline) results in a newline in the string. A character in a string may also be specified by its numerical value using the escape sequence `\ddd´, where ddd is a sequence of up to three decimal digits. Strings in Lua may contain any 8-bit value, including embedded zeros, which can be specified as `\0´.

Literal strings can also be delimited by matching double square brackets [[ · · · ]]. Literals in this bracketed form may run for several lines, may contain nested [[ · · · ]] pairs, and do not interpret any escape sequences. For convenience, when the opening `[[´ 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 four literals below denote the same string:

      (1)   "alo\n123\""
      (2)   '\97lo\10\04923"'
      (3)   [[alo
            123"]]
      (4)   [[
            alo
            123"]]

Numerical constants may be written with an optional decimal part and an optional decimal exponent. Examples of valid numerical constants are

       3     3.0     3.1416  314.16e-2   0.31416E1

Comments start anywhere outside a string with a double hyphen (--). If the text immediately after -- is different from [[, 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 ]]. Long comments may run for several lines and may contain nested [[ · · · ]] pairs.

For convenience, the first line of a chunk is skipped if it starts with #. This facility allows the use of Lua as a script interpreter in Unix systems (see 6).

2.2 - Values and Types

Lua is a dynamically typed language. That means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.

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; usually it represents the absence of a useful value. Boolean is the type of the values false and true. In Lua, 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.) String represents arrays of characters. Lua is 8-bit clean: Strings may contain any 8-bit character, including embedded zeros ('\0') (see 2.1).

Functions are first-class values in Lua. That means that functions can be stored in variables, passed as arguments to other functions, and returned as results. Lua can call (and manipulate) functions written in Lua and functions written in C (see 2.5.7).

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.

The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any value (except nil). Moreover, tables can be heterogeneous, that is, they can contain values of all types (except nil). Tables are the sole data structuring mechanism in Lua; they may be used to represent ordinary arrays, symbol tables, sets, records, graphs, trees, etc. To represent records, Lua uses the field name as an index. The language supports this representation by providing a.name as syntactic sugar for a["name"]. There are several convenient ways to create tables in Lua (see 2.5.6).

Like indices, the value of a table field can be of any type (except nil). In particular, because functions are first class values, table fields may contain functions. Thus tables may also carry methods (see 2.5.8).

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

The library function type returns a string describing the type of a given value (see 5.1).

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 that string to a number, following the usual rules. Conversely, whenever a number is used where a string is expected, the number is converted to a string, in a reasonable format. For complete control of how numbers are converted to strings, use the format function from the string library (see 5.3).

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 formal parameter of a function, which is a particular form of local variable):

	var ::= Name
Variables are assumed to be global unless explicitly declared 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 first expression (prefixexp)should result in a table value; the second expression (exp) identifies a specific entry inside that table. The expression denoting the table to be indexed has a restricted syntax; see __EXPRESSION__s">2.5 for details.

The syntax var.NAME is just syntactic sugar for var["NAME"]:

	var ::= prefixexp `.´ Name

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.)

All global variables live as fields in ordinary Lua tables, called environment tables or simply environments. Functions written in C and exported to Lua (C functions) all share a common global environment. Each function written in Lua (a Lua function) has its own reference to an environment, so that all global variables in that function will refer to that environment table. When a function is created, it inherits the environment from the function that created it. To change or get the environment table of a Lua function, you call setfenv or getfenv (see 5.1).

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. (The _env variable is not defined in Lua. We use it 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 assignment, control structures, procedure calls, table constructors, 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 [`;´]}

Lua handles a chunk as the body of an anonymous function (see 2.5.8). As such, chunks can define local variables and return values.

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

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

2.4.2 - Blocks

A block is a list of statements; syntactically, a block is equal to a chunk:
	block ::= chunk

A block may be explicitly delimited to produce a single statement:

	stat ::= do block end
Explicit blocks are useful to control the scope of variable declarations. Explicit blocks are also sometimes used to add a return or break statement in the middle of another block (see 2.4.4).

2.4.3 - Assignment

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

	stat ::= varlist1 `=´ explist1
	varlist1 ::= var {`,´ var}
	explist1 ::= exp {`,´ exp}
Expressions are discussed in __EXPRESSION__s">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 function call enter in the list of values, before the adjustment (except when the call is enclosed in parentheses; see __EXPRESSION__s">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.

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

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

	stat ::= return [explist1]

The break statement can be 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.

For syntactic reasons, 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. In practice, those idioms are only used during debugging.

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 var = 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
           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.
  • _limit and _step are invisible variables. The names are here for explanatory purposes only.
  • The behavior is undefined if you assign to var inside the block.
  • If the third expression (the step) is absent, then a step of 1 is used.
  • You can use break to exit a for loop.
  • The loop variable var is local to the statement; you cannot use its value after the for ends or is broken. If you need the value of the loop variable var, then assign it to another variable before breaking or exiting the loop.

The generic for statement works over functions, called iterators. For each iteration, it calls its iterator function to produce a new value, stopping when the new value is nil. The generic for loop has the following syntax:

	stat ::= for Name {`,´ Name} in explist1 do block end
A for statement like
       for var_1, ..., var_n in explist do block end
is equivalent to the code:
       do
         local _f, _s, var_1 = explist
         local var_2, ... , var_n
         while true do
           var_1, ..., var_n = _f(_s, var_1)
           if var_1 == 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 and _s are invisible variables. The names are here for explanatory purposes only.
  • The behavior is undefined if you assign to var_1 inside the block.
  • You can use break to exit a for loop.
  • The loop variables var_i are local to the statement; 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 2.5.7.

2.4.7 - Local Declarations

Local variables may be declared anywhere inside a block. The declaration may include an initial assignment:
	stat ::= local namelist [`=´ explist1]
	namelist ::= Name {`,´ Name}
If present, an initial assignment has the same semantics of a multiple assignment (see 2.4.3). Otherwise, all variables are initialized with nil.

A chunk is also a block (see 2.4.1), so local variables can be declared in a chunk outside any explicit block. Such local variables die when the chunk ends.

The visibility rules for local variables are explained in 2.6.

__EXPRESSION__s">

2.5 - Expressions

The basic expressions in Lua are the following:

	exp ::= prefixexp
	exp ::= nil | false | true
	exp ::= Number
	exp ::= Literal
	exp ::= function
	exp ::= tableconstructor
	prefixexp ::= var | functioncall | `(´ exp `)´

Numbers and literal strings are explained in 2.1; variables are explained in 2.3; function definitions are explained in 2.5.8; function calls are explained in 2.5.7; table constructors are explained in 2.5.6.

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

Expressions can also be built with arithmetic operators, relational operators, and logical operators, all of which are explained below.

2.5.1 - Arithmetic Operators

Lua supports the usual arithmetic operators: the binary + (addition), - (subtraction), * (multiplication), / (division), 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 except exponentiation have the usual meaning. Exponentiation calls a global function __pow; otherwise, an appropriate metamethod is called (see 2.8). The standard mathematical library defines function __pow, giving the expected meaning to exponentiation (see 5.5).

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

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

2.5.3 - Logical Operators

The logical operators in Lua are
       and   or    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 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. For example,

       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

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 - Precedence

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

2.5.6 - Table Constructors

Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. Constructors can be used to create empty tables, or to create a table and initialize some of its fields. The general syntax for constructors is
	tableconstructor ::= `{´ [fieldlist] `}´
	fieldlist ::= field {fieldsep field} [fieldsep]
	field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp
	fieldsep ::= `,´ | `;´

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

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

If the last field in the list has the form exp and the expression is a function call, then all values returned by the call enter the list consecutively (see 2.5.7). To avoid this, enclose the function call in parentheses (see __EXPRESSION__s">2.5).

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

2.5.7 - 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 that function is called with the given arguments. Otherwise, its "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(...) is syntactic sugar for v.name(v,...), except that v is evaluated only once.

Arguments have the following syntax:

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

Because a function can return any number of results (see 2.4.4), the number of results must be adjusted before they are used. If the function is called as a statement (see 2.4.6), then its return list is adjusted to zero elements, thus discarding all returned values. If the function is called inside another expression or in the middle of a list of expressions, then its return list is adjusted to one element, thus discarding all returned values except the first one. If the function is called as the last element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses).

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 values returned by f()
       a,b,c = f(), x     -- f() is adjusted to 1 result (and c gets nil)
       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 values returned by f()
       return x,y,f()     -- returns x, y, and all values returned by f()
       {f()}              -- creates a list with all values returned by f()
       {f(), nil}         -- f() is adjusted to 1 result

If you enclose a function call in parentheses, then it is adjusted to return exactly one value:

       return x,y,(f())   -- returns x, y, and the first value from f()
       {(f())}            -- creates a table with exactly one element

As an exception to the free-format syntax of Lua, you cannot put a line break before the `(´ in a function call. That restriction avoids some ambiguities in the language. If you write

       a = f
       (g).x(a)
Lua would read that as 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 returns exactly the returns of the called function. So, all the following examples are not 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

2.5.8 - Function Definitions

The syntax for function definition is

	function ::= function funcbody
	funcbody ::= `(´ [parlist1] `)´ block end

The following syntactic sugar simplifies function definitions:

	stat ::= function funcname funcbody
	stat ::= local function Name funcbody
	funcname ::= Name {`.´ Name} [`:´ Name]
The statement
       function f () ... end
translates to
       f = function () ... end
The statement
       function t.a.b.c.f () ... end
translates to
       t.a.b.c.f = function () ... end
The statement
       local function f () ... end
translates to
       local f; f = function () ... end

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

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

	parlist1 ::= namelist [`,´ `...´]
	parlist1 ::= `...´
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 into an implicit parameter, called arg. The value of arg is a table, with a field `n´ that holds the number of extra arguments and with the extra arguments at positions 1, 2, ..., n.

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:
       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, arg={n=0}
       g(3, 4)          a=3, b=4,   arg={n=0}
       g(3, 4, 5, 8)    a=3, b=4,   arg={5, 8; n=2}
       g(5, r())        a=5, b=1,   arg={2, 3; n=2}

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 (...) ... end
is syntactic sugar for
       t.a.b.c.f = function (self, ...) ... 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. For instance:

  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. For instance:

  local counter = 0
  function inc (x)
    counter = counter + x
    return counter
  end
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 extension language, all Lua actions start from C code in the host program calling a function from the Lua library (see 3.15). Whenever an error occurs during Lua compilation or execution, control returns to C, which can take appropriate measures (such as print an error message).

Lua code can explicitly generate an error by calling the error function (see 5.1). If you need to catch errors in Lua, you can use the pcall function (see 5.1).

2.8 - Metatables

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

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

You can query and change the metatable of an object through the set/getmetatable functions (see 5.1).

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

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

The code shown here in Lua is only illustrative; the real behavior is hard coded in the interpreter and it is much more efficient than this simulation. All functions used in these descriptions (rawget, tonumber, etc.) are described in 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(metatable(obj) or {}, event)
That is, the access to a metamethod does not invoke other metamethods, and the access to objects with no metatables does not fail (it simply results in nil).

  • "add": 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
    
    Using that 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.

  • "pow": the ^ (exponentiation) operation.
     function pow_event (op1, op2)
       local o1, o2 = tonumber(op1), tonumber(op2)
       if o1 and o2 then  -- both operands are numeric?
         return __pow(o1, o2)   -- call global `__pow'
       else  -- at least one of the operands is not numeric
         local h = getbinhandler(op1, op2, "__pow")
         if h then
           -- call the handler with both operands
           return h(op1, op2)
         else  -- no handler available: default behavior
           error("...")
         end
       end
      end
    

  • "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 and nil
           return h(op, nil)
         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
    

  • "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
    

  • "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
         return h(table, key,value)    -- call the handler
       else h[key] = value             -- or repeat operation on it
     end
    

  • "call": called when Lua calls a value.
     function function_event (func, ...)
       if type(func) == "function" then
         return func(unpack(arg))   -- primitive call
       else
         local h = metatable(func).__call
         if h then
           return h(func, unpack(arg))
         else
           error("...")
         end
       end
     end
    

2.9 - Garbage Collection

Lua does automatic memory management. That means that you do not have to worry about allocating memory for new objects and 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, those objects that are no longer accessible from Lua). All objects in Lua are subject to automatic management: tables, userdata, functions, threads, and strings.

Lua uses two numbers to control its garbage-collection cycles. One number counts how many bytes of dynamic memory Lua is using; the other is a threshold. When the number of bytes crosses the threshold, Lua runs the garbage collector, which reclaims the memory of all dead objects. The byte counter is adjusted, and then the threshold is reset to twice the new value of the byte counter.

Through the C API, you can query those numbers and change the threshold (see 3.7). Setting the threshold to zero actually forces an immediate garbage-collection cycle, while setting it to a huge number effectively stops the garbage collector. Using Lua code you have a more limited control over garbage-collection cycles, through the gcinfo and collectgarbage functions (see 5.1).

2.9.1 - 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).

Free 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.

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

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

2.10 - Coroutines

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

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

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

A coroutine can terminate its execution in two ways: Normally, when its main function returns (explicitly or implicitly, after the last instruction); and abnormally, if there is an unprotected error. In the first case, coroutine.resume returns true, plus any values returned by the coroutine main function. In case of errors, coroutine.resume returns false plus an error message.

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

The coroutine.wrap function creates a coroutine like coroutine.create, but instead of returning the coroutine itself, it returns a function that, when called, resumes the coroutine. Any arguments passed to that function go as extra arguments to resume. The function returns all the values returned by resume, except the first one (the boolean error code). Unlike coroutine.resume, this function does not catch errors; any error is propagated to the caller.

As an example, consider the next code:

function foo1 (a)
  print("foo", a)
  return coroutine.yield(2*a)
end
 
co = coroutine.create(function (a,b)
      print("co-body", a, b)
      local r = foo1(a+1)
      print("co-body", r)
      local r, s = coroutine.yield(a+b, a-b)
      print("co-body", r, s)
      return b, "end"
end)
       
a, b = coroutine.resume(co, 1, 10)
print("main", a, b)
a, b, c = coroutine.resume(co, "r")
print("main", a, b, c)
a, b, c = coroutine.resume(co, "x", "y")
print("main", a, b, c)
a, b = coroutine.resume(co, "x", "y")
print("main", a, b)
When you run it, it produces the following output:
co-body 1       10
foo     2
main    true    4
co-body r
main    true    11      -9
co-body x       y
main    true    10      end
main    false   cannot resume dead coroutine

3 - The Application Program Interface

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

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

3.1 - States

The Lua library is fully reentrant: it has no global variables. The whole state of the Lua interpreter (global variables, stack, etc.) is stored in a dynamically allocated structure of type lua_State. A pointer to this state must be passed as the first argument to every function in the library, except to lua_open, which creates a Lua state from scratch.

Before calling any API function, you must create a state by calling lua_open:

       lua_State *lua_open (void);

To release a state created with lua_open, call lua_close:

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

3.2 - The Stack and Indices

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

Whenever Lua calls C, the called function gets a new stack, which is independent of previous stacks and of stacks of C functions that are still active. That stack initially contains any arguments to the C function, and it is where the C function pushes its results to be returned to the caller (see 3.16).

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

At any time, you can get the index of the top element by calling lua_gettop:

       int lua_gettop (lua_State *L);
Because indices start at 1, the result of lua_gettop is equal to the number of elements in the stack (and so 0 means an empty stack).

When you interact with Lua API, you are responsible for controlling stack overflow. The function

       int lua_checkstack (lua_State *L, int extra);
grows the stack size to top + extra elements; it returns false if it cannot grow the stack to that size. This function never shrinks the stack; if the stack is already larger than the new size, it is left unchanged.

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

Most query functions accept as indices any value inside the available stack space, that is, indices up to the maximum stack size you have set through lua_checkstack. Such indices are called acceptable indices. More formally, we define an acceptable index as follows:

     (index < 0 && abs(index) <= top) || (index > 0 && index <= stackspace)
Note that 0 is never an acceptable index.

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

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