lua,
which uses the Lua library to offer a complete, stand-alone Lua interpreter.
Lua is free software,
and is provided as usual with no guarantees,
as stated in its license.
The implementation described in this manual is available
at Lua's official web site, www.lua.org.
Like any other reference manual, this document is dry in places. For a discussion of the decisions behind the design of Lua, see the technical papers available at Lua's web site. For a detailed introduction to programming in Lua, see Roberto's book, Programming in Lua (Second Edition).
This section describes the lexis, the syntax, and the semantics of Lua. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean.
The language constructs will be explained using the usual extended BNF notation, in which {a} means 0 or more a's, and [a] means an optional a. Non-terminals are shown like non-terminal, keywords are shown like kword, and other terminal symbols are shown like `=´. The complete syntax of Lua can be found in §8 at the end of this manual.
Names (also called identifiers) in Lua can be any string of letters, digits, and underscores, not beginning with a digit. This coincides with the definition of names in most languages. (The definition of letter depends on the current locale: any character considered alphabetic by the current locale can be used in an identifier.) Identifiers are used to name variables and table fields.
The following keywords are reserved and cannot be used as names:
and break do else elseif
end false for function if
in local nil not or
repeat return then true until while
Lua is a case-sensitive language:
and is a reserved word, but And and AND
are two different, valid names.
As a convention, names starting with an underscore followed by
uppercase letters (such as _VERSION)
are reserved for internal global variables used by Lua.
The following strings denote other tokens:
+ - * / % ^ #
== ~= <= >= < > =
( ) { } [ ]
; : , . .. ...
Literal strings
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
'\a' (bell),
'\b' (backspace),
'\f' (form feed),
'\n' (newline),
'\r' (carriage return),
'\t' (horizontal tab),
'\v' (vertical tab),
'\\' (backslash),
'\"' (quotation mark [double quote]),
and '\'' (apostrophe [single quote]).
Moreover, a backslash followed by a real newline
results in a newline in the string.
A character in a string can also be specified by its numerical value
using the escape sequence \ddd,
where ddd is a sequence of up to three decimal digits.
(Note that if a numerical escape is to be followed by a digit,
it must be expressed using exactly three digits.)
Strings in Lua can contain any 8-bit value, including embedded zeros,
which can be specified as '\0'.
Literal strings can also be defined using a long format
enclosed by long brackets.
We define an opening long bracket of level n as an opening
square bracket followed by n equal signs followed by another
opening square bracket.
So, an opening long bracket of level 0 is written as [[,
an opening long bracket of level 1 is written as [=[,
and so on.
A closing long bracket is defined similarly;
for instance, a closing long bracket of level 4 is written as ]====].
A long string starts with an opening long bracket of any level and
ends at the first closing long bracket of the same level.
Literals in this bracketed form can run for several lines,
do not interpret any escape sequences,
and ignore long brackets of any other level.
They can contain anything except a closing bracket of the proper level.
For convenience,
when the opening long bracket is immediately followed by a newline,
the newline is not included in the string.
As an example, in a system using ASCII
(in which 'a' is coded as 97,
newline is coded as 10, and '1' is coded as 49),
the five literal strings below denote the same string:
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
A numerical constant can be written with an optional decimal part
and an optional decimal exponent.
Lua also accepts integer hexadecimal constants,
by prefixing them with 0x.
Examples of valid numerical constants are
3 3.0 3.1416 314.16e-2 0.31416E1 0xff 0x56
A comment starts with a double hyphen (--)
anywhere outside a string.
If the text immediately after -- is not an opening long bracket,
the comment is a short comment,
which runs until the end of the line.
Otherwise, it is a long comment,
which runs until the corresponding closing long bracket.
Long comments are frequently used to disable code temporarily.
Lua is a dynamically typed language. This means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.
All values in Lua are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.
There are eight basic types in Lua:
nil, boolean, number,
string, function, userdata,
thread, and table.
Nil is the type of the value nil,
whose main property is to be different from any other value;
it usually represents the absence of a useful value.
Boolean is the type of the values false and true.
Both nil and false make a condition false;
any other value makes it true.
Number represents real (double-precision floating-point) numbers.
(It is easy to build Lua interpreters that use other
internal representations for numbers,
such as single-precision float or long integers;
see file luaconf.h.)
String represents arrays of characters.
Lua is 8-bit clean:
strings can contain any 8-bit character,
including embedded zeros ('\0') (see §2.1).
Lua can call (and manipulate) functions written in Lua and functions written in C (see §2.5.8).
The type userdata is provided to allow arbitrary C data to be stored in Lua variables. This type corresponds to a block of raw memory and has no pre-defined operations in Lua, except assignment and identity test. However, by using metatables, the programmer can define operations for userdata values (see §2.8). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.
The type thread represents independent threads of execution and it is used to implement coroutines (see §2.11). Do not confuse Lua threads with operating-system threads. Lua supports coroutines on all systems, even those that do not support threads.
The type table implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except nil).
Tables can be heterogeneous;
that is, they can contain values of all types (except nil).
Tables are the sole data structuring mechanism in Lua;
they can be used to represent ordinary arrays,
symbol tables, sets, records, graphs, trees, etc.
To represent records, Lua uses the field name as an index.
The language supports this representation by
providing a.name as syntactic sugar for a["name"].
There are several convenient ways to create tables in Lua
(see §2.5.7).
Like indices, the value of a table field can be of any type (except nil). In particular, because functions are first-class values, table fields can contain functions. Thus tables can also carry methods (see §2.5.9).
Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.
The library function type returns a string describing the type
of a given value.
Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
this string to a number, following the usual conversion rules.
Conversely, whenever a number is used where a string is expected,
the number is converted to a string, in a reasonable format.
For complete control over how numbers are converted to strings,
use the format function from the string library
(see string.format).
Variables are places that store values. There are three kinds of variables in Lua: global variables, local variables, and table fields.
A single name can denote a global variable or a local variable (or a function's formal parameter, which is a particular kind of local variable):
var ::= Name
Name denotes identifiers, as defined in §2.1.
Any variable is assumed to be global unless explicitly declared as a local (see §2.4.7). Local variables are lexically scoped: local variables can be freely accessed by functions defined inside their scope (see §2.6).
Before the first assignment to a variable, its value is nil.
Square brackets are used to index a table:
var ::= prefixexp `[´ exp `]´
The meaning of accesses to global variables
and table fields can be changed via metatables.
An access to an indexed variable t[i] is equivalent to
a call gettable_event(t,i).
(See §2.8 for a complete description of the
gettable_event function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
The syntax var.Name is just syntactic sugar for
var["Name"]:
var ::= prefixexp `.´ Name
All global variables live as fields in ordinary Lua tables,
called environment tables or simply
environments (see §2.9).
Each function has its own reference to an environment,
so that all global variables in this function
will refer to this environment table.
When a function is created,
it inherits the environment from the function that created it.
To get the environment table of a Lua function,
you call getfenv.
To replace it,
you call setfenv.
(You can only manipulate the environment of C functions
through the debug library; (see §5.9).)
An access to a global variable x
is equivalent to _env.x,
which in turn is equivalent to
gettable_event(_env, "x")
where _env is the environment of the running function.
(See §2.8 for a complete description of the
gettable_event function.
This function is not defined or callable in Lua.
Similarly, the _env variable is not defined in Lua.
We use them here only for explanatory purposes.)
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.
The unit of execution of Lua is called a chunk. A chunk is simply a sequence of statements, which are executed sequentially. Each statement can be optionally followed by a semicolon:
chunk ::= {stat [`;´]}
There are no empty statements and thus ';;' is not legal.
Lua handles a chunk as the body of an anonymous function with a variable number of arguments (see §2.5.9). As such, chunks can define local variables, receive arguments, and return values.
A chunk can be stored in a file or in a string inside the host program. To execute a chunk, Lua first pre-compiles the chunk into instructions for a virtual machine, and then it executes the compiled code with an interpreter for the virtual machine.
Chunks can also be pre-compiled into binary form;
see program luac for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.
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).
Lua allows multiple assignments. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas:
stat ::= varlist `=´ explist
varlist ::= var {`,´ var}
explist ::= exp {`,´ exp}
Expressions are discussed in §2.5.
Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many nil's as needed. If the list of expressions ends with a function call, then all values returned by that call enter the list of values, before the adjustment (except when the call is enclosed in parentheses; see §2.5).
The assignment statement first evaluates all its expressions and only then are the assignments performed. Thus the code
i = 3
i, a[i] = i+1, 20
sets a[3] to 20, without affecting a[4]
because the i in a[i] is evaluated (to 3)
before it is assigned 4.
Similarly, the line
x, y = y, x
exchanges the values of x and y,
and
x, y, z = y, z, x
cyclically permutes the values of x, y, and z.
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable t[i] = val is equivalent to
settable_event(t,i,val).
(See §2.8 for a complete description of the
settable_event function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
An assignment to a global variable x = val
is equivalent to the assignment
_env.x = val,
which in turn is equivalent to
settable_event(_env, "x", val)
where _env is the environment of the running function.
(The _env variable is not defined in Lua.
We use it here only for explanatory purposes.)
The control structures if, while, and repeat have the usual meaning and familiar syntax:
stat ::= while exp do block end
stat ::= repeat block until exp
stat ::= if exp then block {elseif exp then block} [else block] end
Lua also has a for statement, in two flavors (see §2.4.5).
The condition expression of a control structure can return any value. Both false and nil are considered false. All values different from nil and false are considered true (in particular, the number 0 and the empty string are also true).
In the repeat–until loop, the inner block does not end at the until keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block.
The return statement is used to return values from a function or a chunk (which is just a function). Functions and chunks can return more than one value, and so the syntax for the return statement is
stat ::= return [explist]
The break statement is used to terminate the execution of a while, repeat, or for loop, skipping to the next statement after the loop:
stat ::= break
A break ends the innermost enclosing loop.
The return and break
statements can only be written as the last statement of a block.
If it is really necessary to return or break in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
do return end and do break end,
because now return and break are the last statements in
their (inner) blocks.
The for statement has two forms: one numeric and one generic.
The numeric for loop repeats a block of code while a control variable runs through an arithmetic progression. It has the following syntax:
stat ::= for Name `=´ exp `,´ exp [`,´ exp] do block end
The block is repeated for name starting at the value of the first exp, until it passes the second exp by steps of the third exp. More precisely, a for statement like
for v = e1, e2, e3 do block end
is equivalent to the code:
do
local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and limit and step) then error() end
while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
local v = var
block
var = var + step
end
end
Note the following:
var, limit, and step are invisible variables.
The names shown here are for explanatory purposes only.
v is local to the loop;
you cannot use its value after the for ends or is broken.
If you need this value,
assign it to another variable before breaking or exiting the loop.
The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The generic for loop has the following syntax:
stat ::= for namelist in explist do block end
namelist ::= Name {`,´ Name}
A for statement like
for var_1, ···, var_n in explist do block end
is equivalent to the code:
do
local f, s, var = explist
while true do
local var_1, ···, var_n = f(s, var)
var = var_1
if var == nil then break end
block
end
end
Note the following:
explist is evaluated only once.
Its results are an iterator function,
a state,
and an initial value for the first iterator variable.
f, s, and var are invisible variables.
The names are here for explanatory purposes only.
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.
To allow possible side-effects, function calls can be executed as statements:
stat ::= functioncall
In this case, all returned values are thrown away. Function calls are explained in §2.5.8.
Local variables can be declared anywhere inside a block. The declaration can include an initial assignment:
stat ::= local namelist [`=´ explist]
If present, an initial assignment has the same semantics of a multiple assignment (see §2.4.3). Otherwise, all variables are initialized with nil.
A chunk is also a block (see §2.4.1), and so local variables can be declared in a chunk outside any explicit block. The scope of such local variables extends until the end of the chunk.
The visibility rules for local variables are explained in §2.6.
The basic expressions in Lua are the following:
exp ::= prefixexp exp ::= nil | false | true exp ::= Number exp ::= String exp ::= function exp ::= tableconstructor exp ::= `...´ exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | `(´ exp `)´
Numbers and literal strings are explained in §2.1;
variables are explained in §2.3;
function definitions are explained in §2.5.9;
function calls are explained in §2.5.8;
table constructors are explained in §2.5.7.
Vararg expressions,
denoted by three dots ('...'), can only be used when
directly inside a vararg function;
they are explained in §2.5.9.
Binary operators comprise arithmetic operators (see §2.5.1), relational operators (see §2.5.2), logical operators (see §2.5.3), and the concatenation operator (see §2.5.4). Unary operators comprise the unary minus (see §2.5.1), the unary not (see §2.5.3), and the unary length operator (see §2.5.5).
Both function calls and vararg expressions can result in multiple values. If an expression is used as a statement (only possible for function calls (see §2.4.6)), then its return list is adjusted to zero elements, thus discarding all returned values. If an expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, discarding all values except the first one.
Here are some examples:
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all results from f()
a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)
a,b = ... -- a gets the first vararg parameter, b gets
-- the second (both a and b can get nil if there
-- is no corresponding vararg parameter)
a,b,c = x, f() -- f() is adjusted to 2 results
a,b,c = f() -- f() is adjusted to 3 results
return f() -- returns all results from f()
return ... -- returns all received vararg parameters
return x,y,f() -- returns x, y, and all results from f()
{f()} -- creates a list with all results from f()
{...} -- creates a list with all vararg parameters
{f(), nil} -- f() is adjusted to 1 result
Any expression enclosed in parentheses always results in only one value.
Thus,
(f(x,y,z)) is always a single value,
even if f returns several values.
(The value of (f(x,y,z)) is the first value returned by f
or nil if f does not return any values.)
Lua supports the usual arithmetic operators:
the binary + (addition),
- (subtraction), * (multiplication),
/ (division), % (modulo), and ^ (exponentiation);
and unary - (negation).
If the operands are numbers, or strings that can be converted to
numbers (see §2.2.1),
then all operations have the usual meaning.
Exponentiation works for any exponent.
For instance, x^(-0.5) computes the inverse of the square root of x.
Modulo is defined as
a % b == a - math.floor(a/b)*b
That is, it is the remainder of a division that rounds the quotient towards minus infinity.
The relational operators in Lua are
== ~= < > <= >=
These operators always result in false or true.
Equality (==) first compares the type of its operands.
If the types are different, then the result is false.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Objects (tables, userdata, threads, and functions)
are compared by reference:
two objects are considered equal only if they are the same object.
Every time you create a new object
(a table, userdata, thread, or function),
this new object is different from any previously existing object.
You can change the way that Lua compares tables and userdata by using the "eq" metamethod (see §2.8).
The conversion rules of §2.2.1
do not apply to equality comparisons.
Thus, "0"==0 evaluates to false,
and t[0] and t["0"] denote different
entries in a table.
The operator ~= is exactly the negation of equality (==).
The order operators work as follows.
If both arguments are numbers, then they are compared as such.
Otherwise, if both arguments are strings,
then their values are compared according to the current locale.
Otherwise, Lua tries to call the "lt" or the "le"
metamethod (see §2.8).
A comparison a > b is translated to b < a
and a >= b is translated to b <= a.
The logical operators in Lua are and, or, and not. Like the control structures (see §2.4.4), all logical operators consider both false and nil as false and anything else as true.
The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation; that is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10
10 or error() --> 10
nil or "a" --> "a"
nil and 10 --> nil
false and error() --> false
false and nil --> false
false or nil --> nil
10 and 20 --> 20
(In this manual,
--> indicates the result of the preceding expression.)
The string concatenation operator in Lua is
denoted by two dots ('..').
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in §2.2.1.
Otherwise, the "concat" metamethod is called (see §2.8).
The length operator 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).
Operator precedence in Lua follows the table below, from lower to higher priority:
or
and
< > <= >= ~= ==
..
+ -
* / %
not # - (unary)
^
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation ('..') and exponentiation ('^')
operators are right associative.
All other binary operators are left associative.
Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. A constructor can be used to create an empty table or to create a table and initialize some of its fields. The general syntax for constructors is
tableconstructor ::= `{´ [fieldlist] `}´
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp
fieldsep ::= `,´ | `;´
Each field of the form [exp1] = exp2 adds to the new table an entry
with key exp1 and value exp2.
A field of the form name = exp is equivalent to
["name"] = exp.
Finally, fields of the form exp are equivalent to
[i] = exp, where i are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
is equivalent to
do
local t = {}
t[f(1)] = g
t[1] = "x" -- 1st exp
t[2] = "y" -- 2nd exp
t.x = 1 -- t["x"] = 1
t[3] = f(x) -- 3rd exp
t[30] = 23
t[4] = 45 -- 4th exp
a = t
end
If the last field in the list has the form exp
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
(see §2.5.8).
To avoid this,
enclose the function call or the vararg expression
in parentheses (see §2.5).
The field list can have an optional trailing separator, as a convenience for machine-generated code.
A function call in Lua has the following syntax:
functioncall ::= prefixexp args
In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then this function is called with the given arguments. Otherwise, the prefixexp "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see §2.8).
The form
functioncall ::= prefixexp `:´ Name args
can be used to call "methods".
A call v:name(args)
is syntactic sugar for v.name(v,args),
except that v is evaluated only once.
Arguments have the following syntax:
args ::= `(´ [explist] `)´ args ::= tableconstructor args ::= String
All argument expressions are evaluated before the call.
A call of the form f{fields} is
syntactic sugar for f({fields});
that is, the argument list is a single new table.
A call of the form f'string'
(or f"string" or f[[string]])
is syntactic sugar for f('string');
that is, the argument list is a single literal string.
As an exception to the free-format syntax of Lua,
you cannot put a line break before the '(' in a function call.
This restriction avoids some ambiguities in the language.
If you write
a = f
(g).x(a)
Lua would see that as a single statement, a = f(g).x(a).
So, if you want two statements, you must add a semi-colon between them.
If you actually want to call f,
you must remove the line break before (g).
A call of the form return functioncall is called
a tail call.
Lua implements proper tail calls
(or proper tail recursion):
in a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the return has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
So, none of the following examples are tail calls:
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1
The syntax for function definition is
function ::= function funcbody funcbody ::= `(´ [parlist] `)´ block end
The following syntactic sugar simplifies function definitions:
stat ::= function funcname funcbody
stat ::= local function Name funcbody
funcname ::= Name {`.´ Name} [`:´ Name]
The statement
function f () body end
translates to
f = function () body end
The statement
function t.a.b.c.f () body end
translates to
t.a.b.c.f = function () body end
The statement
local function f () body end
translates to
local f; f = function () body end
not to
local f = function () body end
(This only makes a difference when the body of the function
contains references to f.)
A function definition is an executable expression, whose value has type function. When Lua pre-compiles a chunk, all its function bodies are pre-compiled too. Then, whenever Lua executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression. Different instances of the same function can refer to different external local variables and can have different environment tables.
Parameters act as local variables that are initialized with the argument values:
parlist ::= namelist [`,´ `...´] | `...´
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters,
unless the function is a variadic or vararg function,
which is
indicated by three dots ('...') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a vararg expression,
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless that last expression is enclosed in parentheses).
As an example, consider the following definitions:
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
Then, we have the following mapping from arguments to parameters and to the vararg expression:
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, ... --> (nothing)
g(3, 4) a=3, b=4, ... --> (nothing)
g(3, 4, 5, 8) a=3, b=4, ... --> 5 8
g(5, r()) a=5, b=1, ... --> 2 3
Results are returned using the return statement (see §2.4.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.
The colon syntax
is used for defining methods,
that is, functions that have an implicit extra parameter self.
Thus, the statement
function t.a.b.c:f (params) body end
is syntactic sugar for
t.a.b.c.f = function (self, params) body end
Lua is a lexically scoped language. The scope of variables begins at the first statement after their declaration and lasts until the end of the innermost block that includes the declaration. Consider the following example:
x = 10 -- global variable
do -- new block
local x = x -- new 'x', with value 10
print(x) --> 10
x = x+1
do -- another block
local x = x+1 -- another 'x'
print(x) --> 12
end
print(x) --> 11
end
print(x) --> 10 (the global one)
Notice that, in a declaration like local x = x,
the new x being declared is not in scope yet,
and so the second x refers to the outside variable.
Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function.
Notice that each execution of a local statement defines new local variables. Consider the following example:
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different y variable,
while all of them share the same x.
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.
Every value in Lua can have a metatable.
This metatable is an ordinary Lua table
that defines the behavior of the original value
under certain special operations.
You can change several aspects of the behavior
of operations over a value by setting specific fields in its metatable.
For instance, when a non-numeric value is the operand of an addition,
Lua checks for a function in the field "__add" in its metatable.
If it finds one,
Lua calls this function to perform the addition.
We call the keys in a metatable events
and the values metamethods.
In the previous example, the event is "add"
and the metamethod is the function that performs the addition.
You can query the metatable of any value
through the getmetatable function.
You can replace the metatable of tables
through the setmetatable
function.
You cannot change the metatable of other types from Lua
(except by using the debug library);
you must use the C API for that.
Tables and full userdata have individual metatables (although multiple tables and userdata can share their metatables). Values of all other types share one single metatable per type; that is, there is one single metatable for all numbers, one for all strings, etc.
A metatable controls how an object behaves in arithmetic operations, order comparisons, concatenation, length operation, and indexing. A metatable also can define a function to be called when a userdata is garbage collected. For each of these operations Lua associates a specific key called an event. When Lua performs one of these operations over a value, it checks whether this value has a metatable with the corresponding event. If so, the value associated with that key (the metamethod) controls how Lua will perform the operation.
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores, '__';
for instance, the key for operation "add" is the
string "__add".
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes the operation.
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter
and it is much more efficient than this simulation.
All functions used in these descriptions
(rawget, tonumber, etc.)
are described in §5.1.
In particular, to retrieve the metamethod of a given object,
we use the expression
metatable(obj)[event]
This should be read as
rawget(getmetatable(obj) or {}, event)
That is, the access to a metamethod does not invoke other metamethods, and the access to objects with no metatables does not fail (it simply results in nil).
+ operation.
The function getbinhandler below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.
function getbinhandler (op1, op2, event)
return metatable(op1)[event] or metatable(op2)[event]
end
By using this function,
the behavior of the op1 + op2 is
function add_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric?
return o1 + o2 -- '+' here is the primitive 'add'
else -- at least one of the operands is not numeric
local h = getbinhandler(op1, op2, "__add")
if h then
-- call the handler with both operands
return (h(op1, op2))
else -- no handler available: default behavior
error(···)
end
end
end
- operation.
Behavior similar to the "add" operation.
* operation.
Behavior similar to the "add" operation.
/ operation.
B