Lua 5.1 Reference Manual - The Language


The reference manual is the official definition of the Lua language. For a complete introduction to Lua programming, see the book Programming in Lua.

This manual is also available as a book:

Lua 5.1 Reference Manual
by R. Ierusalimschy, L. H. de Figueiredo, W. Celes, August 2006
ISBN 85-903798-3-3

Buy a copy of this book and help to support the Lua project.

SIMION Specific Note:
Some details specific to SIMION have been included. These
are set off from the main text in “SIMION Specific Notes”
such as this one.

1 - Introduction

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

Being an extension language, Lua has no notion of a “main” program: it only works embedded in a host client, called the embedding program or simply the host. This host program can invoke functions to execute a piece of Lua code, can write and read Lua variables, and can register C functions to be called by Lua code. Through the use of C functions, Lua can be augmented to cope with a wide range of different domains, thus creating customized programming languages sharing a syntactical framework. The Lua distribution includes a sample host program called lua, which uses the Lua library to offer a complete, stand-alone Lua interpreter.

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

SIMION Specific Note:
A copy of Lua 5.1.5 is embedded in the SIMION
program (simion.exe).

Like any other reference manual, this document is dry in places. For a discussion of the decisions behind the design of Lua, see the technical papers available at Lua’s web site. For a detailed introduction to programming in Lua, see Roberto’s book, Programming in Lua (Second Edition).

2 - The Language

This section describes the lexis, the syntax, and the semantics of Lua. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean.

The language constructs will be explained using the usual extended BNF notation, in which {a} means 0 or more a’s, and [a] means an optional a. Non-terminals are shown like non-terminal, keywords are shown like ‘kword’, and other terminal symbols are shown like ‘=’. The complete syntax of Lua can be found in 8. The Complete Syntax of Lua at the end of this manual.

2.1. Lexical Conventions

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

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

and       break     do        else      elseif
end       false     for       function  if
in        local     nil       not       or
repeat    return    then      true      until     while

Lua is a case-sensitive language: and is a reserved word, but And and AND are two different, valid names. As a convention, names starting with an underscore followed by uppercase letters (such as _VERSION) are reserved for internal global variables used by Lua.

The following strings denote other tokens:

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

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

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

3   3.0   3.1416   314.16e-2   0.31416E1   0xff   0x56

A comment starts with a double hyphen (--) anywhere outside a string. If the text immediately after -- is not an opening long bracket, the comment is a short comment, which runs until the end of the line. Otherwise, it is a long comment, which runs until the corresponding closing long bracket. Long comments are frequently used to disable code temporarily.

2.2. Values and Types

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

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

There are eight basic types in Lua: nil, boolean, number, string, function, userdata, thread, and table. Nil is the type of the value nil, whose main property is to be different from any other value; it usually represents the absence of a useful value. Boolean is the type of the values false and true. Both nil and false make a condition false; any other value makes it true. Number represents real (double-precision floating-point) numbers. (It is easy to build Lua interpreters that use other internal representations for numbers, such as single-precision float or long integers; see file luaconf.h.) String represents arrays of characters. Lua is 8-bit clean: strings can contain any 8-bit chracter, including embedded zeros (‘\0’) (see 2.1. Lexical Conventions).

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

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. Metatables). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.

The type thread represents independent threads of execution and it is used to implement coroutines (see 2.11. Coroutines). 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 as syntactic sugar for a["name"]. There are several convenient ways to create tables in Lua (see 2.5.7. Table Constructors).

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. Function Definitions).

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

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

2.2.1. Coercion

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

2.3. Variables

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

A single name can denote a global variable or a local variable (or a function’s formal parameter, which is a particular kind of local variable):

var ::= Name

Name denotes identifiers, as defined in 2.1. Lexical Conventions.

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

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. Metatables for a complete description of the gettable_event function. This function is not defined or callable in Lua. We use it here only for explanatory purposes.)

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

var ::= prefixexp '.' Name

All global variables live as fields in ordinary Lua tables, called environment tables or simply environments (see 2.9. 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. To get the environment table of a Lua function, you call getfenv(). To replace it, you call setfenv(). (You can only manipulate the environment of C functions through the debug library; (see debug).)

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. Metatables for a complete description of the gettable_event function. This function is not defined or callable in Lua. Similarly, the _env variable is not defined in Lua. We use them here only for explanatory purposes.)

2.4. Statements

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

2.4.1. Chunks

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

chunk ::= {stat [';']}

There are no empty statements and thus ‘;;’ is not legal.

Lua handles a chunk as the body of an anonymous function with a variable number of arguments (see 2.5.9. Function Definitions). As such, chunks can define local variables, receive arguments, and return values.

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

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

2.4.2. Blocks

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

block ::= chunk

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

stat ::= 'do' block 'end'

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

2.4.3. Assignment

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

stat ::= varlist '=' explist
varlist ::= var {',' var}
explist ::= exp {',' exp}

Expressions are discussed in 2.5. Expressions.

Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many nil’s as needed. If the list of expressions ends with a function call, then all values returned by this call enter the list of values, before the adjustment (except when the call is enclosed in parentheses; see 2.5. Expressions).

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. Metatables 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. For Statement).

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, so the syntax for the return statement is

stat ::= 'return' [explist]

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

stat ::= 'break'

A break ends the innermost enclosing loop.

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

2.4.5. For Statement

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

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

stat ::= 'for' namelist '=' explist '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:

  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
    var = var + step

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' explist1 '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:

  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

Note the following:

explist is evaluated only once. Its results are an iterator function,
a state, and an initial value for the first iterator variable.

f, s, and var are invisible variables. The names are here for explanatory purposes only.

You can use break to exit a for loop.

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

2.4.6. Function Calls as Statements

To allow possible side-effects, function calls can be executed as statements:

stat ::= functioncall

In this case, all returned values are thrown away. Function calls are explained in 2.5.8. Function Calls.

2.4.7. Local Declarations

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

stat ::= 'local' namelist ['=' explist]

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

A chunk is also a block (see 2.4.1. Chunks), 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. Visibility Rules.

2.5. Expressions

The basic expressions in Lua are the following:

exp ::= prefixexp
exp ::= 'nil' | 'false' | 'true'
exp ::= Number
exp ::= String
exp ::= function
exp ::= tableconstructor
exp ::= '...'
exp ::= exp binop exp
exp ::= unop exp
prefixexp ::= var | functioncall | '(' exp ')'

Numbers and literal strings are explained in 2.1. Lexical Conventions; variables are explained in 2.3. Variables; function definitions are explained in 2.5.9. Function Definitions; function calls are explained in 2.5.8. Function Calls; table constructors are explained in 2.5.7. Table Constructors. Vararg expressions, denoted by three dots (‘...’), can only be used when directly inside a vararg function; they are explained in 2.5.9. Function Definitions.

Binary operators comprise arithmetic operators (see 2.5.1. Arithmetic Operators), relational operators (see 2.5.2. Relational Operators), logical operators (see 2.5.3. Logical Operators), and the concatenation operator (see 2.5.4. Concatenation). Unary operators comprise the unary minus (see 2.5.1. Arithmetic Operators), the unary not (see 2.5.3. Logical Operators), and the unary length operator (see 2.5.5. The Length Operator).

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

Here are some examples:

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

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

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

2.5.1. Arithmetic Operators

Lua supports the usual arithmetic operators: the binary + (addition), - (subtraction), \* (multiplication), / (division), % (modulo), and ^ (exponentiation); and unary - (negation). If the operands are numbers, or strings that can be converted to numbers (see 2.2.1. Coercion), then all operations have the usual meaning. Exponentiation works for any exponent. For instance, x^(-0.5) computes the inverse of the square root of x. Modulo is defined as

a % b == a - math.floor(a/b)*b

That is, it is the remainder of a division that rounds the quotient towards minus infinity.

2.5.2. Relational Operators

The relational operators in Lua are

==    ~=    <     >     <=    >=

These operators always result in false or true.

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

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

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

2.5.3. Logical Operators

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

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

10 or 20            --> 10
10 or error()       --> 10
nil or "a"          --> "a"
nil and 10          --> nil
false and error()   --> false
false and nil       --> false
false or nil        --> nil
10 and 20           --> 20

(In this manual, --> indicates the result of the preceding expression.)

2.5.4. Concatenation

The string concatenation operator in Lua is denoted by two dots (‘..’). If both operands are strings or numbers, then they are converted to strings according to the rules mentioned in 2.2.1. Coercion. Otherwise, the “concat” metamethod is called (see 2.8. Metatables).

2.5.5. The Length Operator

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

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

2.5.6. Precedence

Operator precedence in Lua follows the table below, from lower to higher priority:

<     >     <=    >=    ~=    ==
+     -
*     /     %
not   #     - (unary)

As usual, you can use parentheses to change the precedences of an expression. The concatenation (‘..’) and exponentiation (‘^’) operators are right associative. All other binary operators are left associative.

2.5.7. Table Constructors

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

tableconstructor ::= '{' [fieldlist] '}'
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= '[' exp ']' '=' exp | Name '=' exp | exp
fieldsep ::= ',' | ';'

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

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

is equivalent to

  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

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. Function Calls). To avoid this, enclose the function call or the vararg expression in parentheses (see 2.5. Expressions).

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

2.5.8. Function Calls

A function call in Lua has the following syntax:

functioncall ::= prefixexp args

In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then this function is called with the given arguments. Otherwise, the prefixexp “call” metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see 2.8. Metatables).

The form

functioncall ::= prefixexp ':' Name args

can be used to call “methods”. A call v:name(args) is syntactic sugar for,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

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

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

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

2.5.9. Function Definitions

The syntax for function definition is

function ::= 'function' funcbody
funcbody ::= '(' [parlist] ')' block 'end'

The following syntactic sugar simplifies function definitions:

stat ::= 'function' funcname funcbody
stat ::= 'local' 'function' Name funcbody
funcname ::= Name {'.' Name} [':' Name]

The statement

function f () <body> end

translates to

f = function () <body> end

The statement

function t.a.b.c.f () <body> end

translates to

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

The statement

local function f () <body> end

translates to

local f; f = function () <body> end

not to

local f = function () <body> end

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

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

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

parlist ::= namelist [',' '...'] | '...'

When a function is called, the list of arguments is adjusted to the length of the list of parameters, unless the function is a variadic or vararg function, which is indicated by three dots (‘...’) at the end of its parameter list. A vararg function does not adjust its argument list; instead, it collects all extra arguments and supplies them to the function through a vararg expression, which is also written as three dots. The value of this expression is a list of all actual extra arguments, similar to a function with multiple results. If a vararg expression is used inside another expression or in the middle of a list of expressions, then its return list is adjusted to one element. If the expression is used as the last element of a list of expressions, then no adjustment is made (unless that last expression is enclosed in parentheses).

As an example, consider the following definitions:

function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end

Then, we have the following mapping from arguments to parameters and to the vararg expression:


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

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

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

is syntactic sugar for

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

2.6. Visibility Rules

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

x = 10                -- global variable
do                    -- new block
  local x = x         -- new 'x', with value 10
  print(x)            --> 10
  x = x+1
  do                  -- another block
    local x = x+1     -- another 'x'
    print(x)          --> 12
  print(x)            --> 11
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

The loop creates ten closures (that is, ten instances of the anonymous function). Each of these closures uses a different y variable, while all of them share the same x.

2.7. Error Handling

Because Lua is an embedded extension language, all Lua actions start from C code in the host program calling a function from the Lua library (see lua_pcall()).

Whenever an error occurs during Lua compilation or execution, control returns to C, which can take appropriate measures (such as printing an error message).

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

2.8. Metatables

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

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

You can query the metatable of any value through the getmetatable() function.

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

Tables and 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, and for all strings, etc.

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

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

The code shown here in Lua is only illustrative; the real behavior is hard coded in the interpreter and it is much more efficient than this simulation. All functions used in these descriptions (rawget(), tonumber(), etc.) are described in base - Basic standard library functions. In particular, to retrieve the metamethod of a given object, we use the expression


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).


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]

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


the - operation.

Behavior similar to the “add” operation.


the \* operation.

Behavior similar to the “add” operation.


the / operation.

Behavior similar to the “add” operation.


the % operation.

Behavior similar to the “add” operation, with the operation

o1 - floor(o1/o2)\*o2 as the primitive operation.


the ^ (exponentiation) operation.

Behavior similar to the “add” operation, with the function pow (from the C math library) as the primitive operation.


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


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
    local h = getbinhandler(op1, op2, "__concat")
    if h then
      return (h(op1, op2))


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
    local h = metatable(op).__len
    if h then
      -- call the handler with the operand
      return (h(op))
    else  -- no handler available: default behavior

See 2.5.5. The Length Operator for a description of the length of a table.


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

The “eq” event is defined as follows:

function eq_event (op1, op2)
  if type(op1) ~= type(op2) then  -- different types?
    return false   -- different objects
  if op1 == op2 then   -- primitive equal?
    return true   -- objects are equal
  -- try metamethod
  local h = getcomphandler(op1, op2, "__eq")
  if h then
    return (h(op1, op2))
    return false

a ~= b is equivalent to not (a == b).


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
    local h = getcomphandler(op1, op2, "__lt")
    if h then
      return (h(op1, op2))

a > b is equivalent to b < a.


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
    local h = getcomphandler(op1, op2, "__le")
    if h then
      return (h(op1, op2))
      h = getcomphandler(op1, op2, "__lt")
      if h then
        return not h(op2, op1)

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).


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
    h = metatable(table).__index
    if h == nil then
  if type(h) == "function" then
    return (h(table, key))    -- call the handler
  else return h[key]          -- or repeat operation on it


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
    h = metatable(table).__newindex
    if h == nil then
  if type(h) == "function" then
    h(table, key,value)           -- call the handler
  else h[key] = value             -- or repeat operation on it


called when Lua calls a value.

function function_event (func, ...)
  if type(func) == "function" then
    return func(...)   -- primitive call
    local h = metatable(func).__call
    if h then
      return h(func, ...)

2.9. Environments

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

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

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

Environments associated with threads are called global environments. They are used as the default environment for threads and non-nested Lua functions created by the thread and can be directly accessed by C code (see 3.3. Pseudo-Indices).

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

Environments associated with Lua functions are used to resolve all accesses to global variables within the function (see 2.3. Variables). They are used as the default environment for nested Lua functions created by the function.

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

2.10. Garbage Collection

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

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

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 lua_gc() in C or collectgarbage() in Lua. With these functions you can also control the collector directly (e.g., stop and restart it).

2.10.1. Garbage-Collection Metamethods

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

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.

2.10.2. 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 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.

2.11. Coroutines

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

You create a coroutine with a call to coroutine.create(). 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 a thread returned by coroutine.create :func:`coroutine.create, the coroutine starts its execution, at the first line of its main function. Extra arguments passed to coroutine.resume() are passed on to the coroutine main function. After the coroutine starts running, it runs until it terminates or yields.

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

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

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

As an example, consider the following code:

function foo (a)
  print("foo", a)
  return coroutine.yield(2*a)

co = coroutine.create(function (a,b)
      print("co-body", a, b)
      local r = foo(a+1)
      print("co-body", r)
      local r, s = coroutine.yield(a+b, a-b)
      print("co-body", r, s)
      return b, "end"

print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))

When you run it, it produces the following output:

co-body 1       10
foo     2

main    true    4
co-body r
main    true    11      -9
co-body x       y
main    true    10      end
main    false   cannot resume dead coroutine

3-4. C API

These sections describe the interface between Lua and C.

3. The Application Program Interface

4. The Auxiliary Library

5. Standard Libraries

The standard Lua libraries provide useful functions that are implemented directly through the C API. Some of these functions provide essential services to the language (e.g., type() and getmetatable()); others provide access to “outside” services (e.g., I/O); and others could be implemented in Lua itself, but are quite useful or have critical performance requirements that deserve an implementation in C (e.g., table.sort()).

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

5.1. basic library

5.2. coroutine manipulation - coroutine

5.3. package library - package

5.4. string manipulation - string

5.5. table manipulation - table

5.6. mathematical functions (sin, log, etc.) - math

5.7. input and output - io

5.8. operating system facilities - os

5.9. debug facilities - debug

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

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

6. Lua Stand-alone

Although Lua has been designed as an extension language, to be embedded in a host C program, it is also frequently used as a stand-alone language. An interpreter for Lua as a stand-alone language, called simply lua, is provided with the standard distribution. The stand-alone interpreter includes all standard libraries, including the debug library. Its usage is:

lua [options] [script [args]]

SIMION Specific Note:
Lua can be invoked from the command line using
"simion --nogui lua [options] [script [args]]"
   rather than "lua [options] [script [args]]".

The options are:

  • -e statm : executes string stat;
  • -l mod : “requires” mod;
  • -i : enters interactive mode after running script;
  • -v : prints version information;
  • -- : stops handling options;
  • - : executes stdin as a file and stops handling options.

After handling its options, lua runs the given script, passing to it the given args as string arguments. When called without arguments,

lua behaves as lua -v -i when the standard input (stdin) is a terminal, and as lua - otherwise.

Before running any argument, the interpreter checks for an environment variable LUA_INIT. If its format is @filename, then lua executes the file. Otherwise, lua executes the string itself.

All options are handled in order, except -i. For instance, an invocation like

$ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print the value of a (which is ‘1’), and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)

Before starting to run the script, lua collects all arguments in the command line in a global table called arg. The script name is stored at index 0, the first argument after the script name goes to index 1, and so on. Any arguments before the script name (that is, the interpreter name plus the options) go to negative indices. For instance, in the call

$ lua -la b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table

arg = { [-2] = "lua", [-1] = "-la",
        [0] = "b.lua",
        [1] = "t1", [2] = "t2" }

and finally runs the file b.lua. The script is called with arg[1], arg[2], … as arguments; it can also access these arguments with the vararg expression ‘...’.

In interactive mode, if you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt.

If the global variable _PROMPT contains a string, then its value is used as the prompt. Similarly, if the global variable _PROMPT2 contains a string, its value is used as the secondary prompt (issued during incomplete statements). Therefore, both prompts can be changed directly on the command line or in any Lua programs by assigning to _PROMPT. See the next example:

$ lua -e"_PROMPT='myprompt> '" -i

(The outer pair of quotes is for the shell, the inner pair is for Lua.) Note the use of -i to enter interactive mode; otherwise, the program would just end silently right after the assignment to _PROMPT.

To allow the use of Lua as a script interpreter in Unix systems, the stand-alone interpreter skips the first line of a chunk if it starts with #. Therefore, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in


(Of course, the location of the Lua interpreter may be different in your machine. If lua is in your PATH, then

#!/usr/bin/env lua

is a more portable solution.)

7. Incompatibilities with the Previous Version

Here we list the incompatibilities that you may find when moving a program from Lua 5.0 to Lua 5.1. You can avoid most of the incompatibilities compiling Lua with appropriate options (see file luaconf.h). However, all these compatibility options will be removed in the next version of Lua.

7.1. Changes in the Language

The vararg system changed from the pseudo-argument arg with a table with the extra arguments to the vararg expression. (See compile-time option LUA_COMPAT_VARARG in luaconf.h.)

There was a subtle change in the scope of the implicit variables of the for statement and for the repeat statement.

The long string/long comment syntax ([[ string ]]) does not allow nesting. You can use the new syntax ([=[ string ]=]) in these cases. (See compile-time option LUA_COMPAT_LSTR in luaconf.h.)

7.2. Changes in the Libraries

Function string.gfind was renamed string.gmatch(). (See compile-time option LUA_COMPAT_GFIND in luaconf.h.)

When string.gsub() is called with a function as its third argument, whenever this function returns nil or false the replacement string is the whole match, instead of the empty string.

Function table.setn was deprecated. Function table.getn corresponds to the new length operator (#); use the operator instead of the function. (See compile-time option LUA_COMPAT_GETN in luaconf.h.)

Function loadlib was renamed package.loadlib(). (See compile-time option LUA_COMPAT_LOADLIB in luaconf.h.)

Function math.mod was renamed math.fmod(). (See compile-time option LUA_COMPAT_MOD in luaconf.h.)

Functions table.foreach and table.foreachi are deprecated. You can use a for loop with pairs() or ipairs() instead.

There were substantial changes in function func:require due to
the new module system. However, the new behavior is mostly compatible with the old, but require gets the path from package.path instead of from LUA_PATH.
Function collectgarbage() has different arguments. Function gcinfo is deprecated;
use collectgarbage("count") instead.

7.3. Changes in the API

The luaopen_\* functions (to open libraries) cannot be called directly,
like a regular C function. They must be called through Lua, like a Lua function.
Function lua_open was replaced by lua_newstate :c:func:lua_newstate to
allow the user to set a memory-allocation function. You can use luaL_newstate() from the standard library to create a state with a standard allocation function (based on realloc).
Functions luaL_getn and luaL_setn (from the auxiliary library) are deprecated.
Use lua_objlen() instead of luaL_getn and nothing instead of luaL_setn.

Function luaL_openlib was replaced by luaL_register :c:func:luaL_register.

Function luaL_checkudata now throws an error when the given value is not a userdata of the expected type.
(In Lua 5.0 it returned NULL.)

8. 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' | '#'

This document is from Lua version 5.1.5. Copyright (c) 2006-2012, PUC-Rio. Freely available under the terms of the Lua license.