Based on Lua 5.1 by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes
Copyright © 2006–2012 Lua.org, PUC-Rio. Freely available under the terms of the Lua license.
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Teliva is free software, and is provided as usual with no guarantees, as stated in its license.
This manual is based on the Lua manual. Lua features absent in Teliva should be absent in this manual as well. Features added to Teliva above and beyond Lua will be described in text like this.
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).
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 and threads 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
.
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, 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, thread, or function),
this new object is different from any previously existing object.
You can change the way that Lua compares tables 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
.
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.
Tables have individual metatables (although multiple tables 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. 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.
Behavior similar to the "add" operation.
%
operation.
Behavior similar to the "add" operation,
with the operation
o1 - floor(o1/o2)*o2
as the primitive operation.
^
(exponentiation) operation.
Behavior similar to the "add" operation,
with the function pow
(from the C math library)
as the primitive operation.
-
operation.
function unm_event (op) local o = tonumber(op) if o then -- operand is numeric? return -o -- '-' here is the primitive 'unm' else -- the operand is not numeric. -- Try to get a handler from the operand local h = metatable(op).__unm if h then -- call the handler with the operand return (h(op)) else -- no handler available: default behavior error(···) end end end
..
(concatenation) operation.
function concat_event (op1, op2) if (type(op1) == "string" or type(op1) == "number") and (type(op2) == "string" or type(op2) == "number") then return op1 .. op2 -- primitive string concatenation else local h = getbinhandler(op1, op2, "__concat") if h then return (h(op1, op2)) else error(···) end end end
#
operation.
function len_event (op) if type(op) == "string" then return strlen(op) -- primitive string length elseif type(op) == "table" then return #op -- primitive table length else local h = metatable(op).__len if h then -- call the handler with the operand return (h(op)) else -- no handler available: default behavior error(···) end end end
See §2.5.5 for a description of the length of a table.
==
operation.
The function getcomphandler
defines how Lua chooses a metamethod
for comparison operators.
A metamethod only is selected when both objects
being compared have the same type
and the same metamethod for the selected operation.
function getcomphandler (op1, op2, event) if type(op1) ~= type(op2) then return nil end local mm1 = metatable(op1)[event] local mm2 = metatable(op2)[event] if mm1 == mm2 then return mm1 else return nil end end
The "eq" event is defined as follows:
function eq_event (op1, op2) if type(op1) ~= type(op2) then -- different types? return false -- different objects end if op1 == op2 then -- primitive equal? return true -- objects are equal end -- try metamethod local h = getcomphandler(op1, op2, "__eq") if h then return (h(op1, op2)) else return false end end
a ~= b
is equivalent to not (a == b)
.
<
operation.
function lt_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 < op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 < op2 -- lexicographic comparison else local h = getcomphandler(op1, op2, "__lt") if h then return (h(op1, op2)) else error(···) end end end
a > b
is equivalent to b < a
.
<=
operation.
function le_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 <= op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 <= op2 -- lexicographic comparison else local h = getcomphandler(op1, op2, "__le") if h then return (h(op1, op2)) else h = getcomphandler(op1, op2, "__lt") if h then return not h(op2, op1) else error(···) end end end end
a >= b
is equivalent to b <= a
.
Note that, in the absence of a "le" metamethod,
Lua tries the "lt", assuming that a <= b
is
equivalent to not (b < a)
.
table[key]
.
function gettable_event (table, key) local h if type(table) == "table" then local v = rawget(table, key) if v ~= nil then return v end h = metatable(table).__index if h == nil then return nil end else h = metatable(table).__index if h == nil then error(···) end end if type(h) == "function" then return (h(table, key)) -- call the handler else return h[key] -- or repeat operation on it end end
table[key] = value
.
function settable_event (table, key, value) local h if type(table) == "table" then local v = rawget(table, key) if v ~= nil then rawset(table, key, value); return end h = metatable(table).__newindex if h == nil then rawset(table, key, value); return end else h = metatable(table).__newindex if h == nil then error(···) end end if type(h) == "function" then h(table, key,value) -- call the handler else h[key] = value -- or repeat operation on it end end
function function_event (func, ...) if type(func) == "function" then return func(...) -- primitive call else local h = metatable(func).__call if h then return h(func, ...) else error(···) end end end
Besides metatables, objects of types thread and function 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. Nested Lua functions are created sharing the environment of the creating Lua function.
Environments associated with threads are called global environments. They are used as the default environment for threads.
Environments associated with Lua functions are used to resolve all accesses to global variables within the function (see §2.3). They are used as the default environment for nested Lua functions created by the function.
You can change the environment of a Lua function or the
running thread by calling setfenv
.
You can get the environment of a Lua function or the running thread
by calling getfenv
.
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, 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.
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.
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
,
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) end 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" 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
The standard Lua libraries provide useful functions
that are implemented directly in C.
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
).
Currently, Lua has the following standard libraries:
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.
The basic library provides some core functions to Lua. If you do not include this library in your application, you should check carefully whether you need to provide implementations for some of its facilities.
assert (v [, message])
v
is false (i.e., nil or false);
otherwise, returns all its arguments.
message
is an error message;
when absent, it defaults to "assertion failed!"
collectgarbage ([opt [, arg]])
This function is a generic interface to the garbage collector.
It performs different functions according to its first argument, opt
:
arg
(larger values mean more steps) in a non-specified way.
If you want to control the step size
you must experimentally tune the value of arg
.
Returns true if the step finished a collection cycle.
arg
as the new value for the pause of
the collector (see §2.10).
Returns the previous value for pause.
arg
as the new value for the step multiplier of
the collector (see §2.10).
Returns the previous value for step.
error (message [, level])
message
as the error message.
Function error
never returns.
Usually, error
adds some information about the error position
at the beginning of the message.
The level
argument specifies how to get the error position.
With level 1 (the default), the error position is where the
error
function was called.
Level 2 points the error to where the function
that called error
was called; and so on.
Passing a level 0 avoids the addition of error position information
to the message.
_G
_G._G = _G
).
Lua itself does not use this variable;
changing its value does not affect any environment,
nor vice-versa.
(Use setfenv
to change environments.)
getfenv ([f])
f
can be a Lua function or a number
that specifies the function at that stack level:
Level 1 is the function calling getfenv
.
If the given function is not a Lua function,
or if f
is 0,
getfenv
returns the global environment.
The default for f
is 1.
getmetatable (object)
If object
does not have a metatable, returns nil.
Otherwise,
if the object's metatable has a "__metatable"
field,
returns the associated value.
Otherwise, returns the metatable of the given object.
ipairs (t)
Returns three values: an iterator function, the table t
, and 0,
so that the construction
for i,v in ipairs(t) do body end
will iterate over the pairs (1,t[1]
), (2,t[2]
), ···,
up to the first integer key absent from the table.
next (table [, index])
Allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
next
returns the next index of the table
and its associated value.
When called with nil as its second argument,
next
returns an initial index
and its associated value.
When called with the last index,
or with nil in an empty table,
next
returns nil.
If the second argument is absent, then it is interpreted as nil.
In particular,
you can use next(t)
to check whether a table is empty.
The order in which the indices are enumerated is not specified,
even for numeric indices.
(To traverse a table in numeric order,
use a numerical for or the ipairs
function.)
The behavior of next
is undefined if,
during the traversal,
you assign any value to a non-existent field in the table.
You may however modify existing fields.
In particular, you may clear existing fields.
pairs (t)
Returns three values: the next
function, the table t
, and nil,
so that the construction
for k,v in pairs(t) do body end
will iterate over all key–value pairs of table t
.
See function next
for the caveats of modifying
the table during its traversal.
print (···)
stdout
,
using the tostring
function to convert them to strings.
print
is not intended for formatted output,
but only as a quick way to show a value,
typically for debugging.
For formatted output, use string.format
.
rawequal (v1, v2)
v1
is equal to v2
,
without invoking any metamethod.
Returns a boolean.
rawget (table, index)
table[index]
,
without invoking any metamethod.
table
must be a table;
index
may be any value.
rawset (table, index, value)
table[index]
to value
,
without invoking any metamethod.
table
must be a table,
index
any value different from nil,
and value
any Lua value.
This function returns table
.
select (index, ···)
If index
is a number,
returns all arguments after argument number index
.
Otherwise, index
must be the string "#"
,
and select
returns the total number of extra arguments it received.
setfenv (f, table)
Sets the environment to be used by the given function.
f
can be a Lua function or a number
that specifies the function at that stack level:
Level 1 is the function calling setfenv
.
setfenv
returns the given function.
As a special case, when f
is 0 setfenv
changes
the environment of the running thread.
In this case, setfenv
returns no values.
setmetatable (table, metatable)
Sets the metatable for the given table.
(You cannot change the metatable of other types.)
If metatable
is nil,
removes the metatable of the given table.
If the original metatable has a "__metatable"
field,
raises an error.
This function returns table
.
tonumber (e [, base])
tonumber
returns this number;
otherwise, it returns nil.
An optional argument specifies the base to interpret the numeral.
The base may be any integer between 2 and 36, inclusive.
In bases above 10, the letter 'A
' (in either upper or lower case)
represents 10, 'B
' represents 11, and so forth,
with 'Z
' representing 35.
In base 10 (the default), the number can have a decimal part,
as well as an optional exponent part (see §2.1).
In other bases, only unsigned integers are accepted.
tostring (e)
string.format
.
If the metatable of e
has a "__tostring"
field,
then tostring
calls the corresponding value
with e
as argument,
and uses the result of the call as its result.
type (v)
nil
" (a string, not the value nil),
"number
",
"string
",
"boolean
",
"table
",
"function
",
"thread
",
and "userdata
".
unpack (list [, i [, j]])
return list[i], list[i+1], ···, list[j]
except that the above code can be written only for a fixed number
of elements.
By default, i
is 1 and j
is the length of the list,
as defined by the length operator (see §2.5.5).
_VERSION
Lua 5.1
".
The operations related to coroutines comprise a sub-library of
the basic library and come inside the table coroutine
.
See §2.11 for a general description of coroutines.
coroutine.create (f)
Creates a new coroutine, with body f
.
f
must be a Lua function.
Returns this new coroutine,
an object with type "thread"
.
coroutine.resume (co [, val1, ···])
Starts or continues the execution of coroutine co
.
The first time you resume a coroutine,
it starts running its body.
The values val1
, ··· are passed
as the arguments to the body function.
If the coroutine has yielded,
resume
restarts it;
the values val1
, ··· are passed
as the results from the yield.
If the coroutine runs without any errors,
resume
returns true plus any values passed to yield
(if the coroutine yields) or any values returned by the body function
(if the coroutine terminates).
If there is any error,
resume
returns false plus the error message.
coroutine.running ()
Returns the running coroutine, or nil when called by the main thread.
coroutine.status (co)
Returns the status of coroutine co
, as a string:
"running"
,
if the coroutine is running (that is, it called status
);
"suspended"
, if the coroutine is suspended in a call to yield
,
or if it has not started running yet;
"normal"
if the coroutine is active but not running
(that is, it has resumed another coroutine);
and "dead"
if the coroutine has finished its body function,
or if it has stopped with an error.
coroutine.wrap (f)
Creates a new coroutine, with body f
.
f
must be a Lua function.
Returns a function that resumes the coroutine each time it is called.
Any arguments passed to the function behave as the
extra arguments to resume
.
Returns the same values returned by resume
,
except the first boolean.
In case of error, propagates the error.
coroutine.yield (···)
Suspends the execution of the calling coroutine.
The coroutine cannot be running a primitive implemented in
C, a metamethod, or an iterator.
Any arguments to yield
are passed as extra results to resume
.
This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.
The string library provides all its functions inside the table
string
.
It also sets a metatable for strings
where the __index
field points to the string
table.
Therefore, you can use the string functions in object-oriented style.
For instance, string.byte(s, i)
can be written as s:byte(i)
.
The string library assumes one-byte character encodings.
string.byte (s [, i [, j]])
s[i]
,
s[i+1]
, ···, s[j]
.
The default value for i
is 1;
the default value for j
is i
.
Note that numerical codes are not necessarily portable across platforms.
string.char (···)
Note that numerical codes are not necessarily portable across platforms.
string.find (s, pattern [, init [, plain]])
pattern
in the string s
.
If it finds a match, then find
returns the indices of s
where this occurrence starts and ends;
otherwise, it returns nil.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and can be negative.
A value of true as a fourth, optional argument plain
turns off the pattern matching facilities,
so the function does a plain "find substring" operation,
with no characters in pattern
being considered "magic".
Note that if plain
is given, then init
must be given as well.
If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.
string.format (formatstring, ···)
printf
family of
standard C functions.
The only differences are that the options/modifiers
*
, l
, L
, n
, p
,
and h
are not supported
and that there is an extra option, q
.
The q
option formats a string in a form suitable to be safely read
back by the Lua interpreter:
the string is written between double quotes,
and all double quotes, newlines, embedded zeros,
and backslashes in the string
are correctly escaped when written.
For instance, the call
string.format('%q', 'a string with "quotes" and \n new line')
will produce the string:
"a string with \"quotes\" and \ new line"
The options c
, d
, E
, e
, f
,
g
, G
, i
, o
, u
, X
, and x
all
expect a number as argument,
whereas q
and s
expect a string.
This function does not accept string values
containing embedded zeros,
except as arguments to the q
option.
string.gmatch (s, pattern)
pattern
over string s
.
If pattern
specifies no captures,
then the whole match is produced in each call.
As an example, the following loop
s = "hello world from Lua" for w in string.gmatch(s, "%a+") do print(w) end
will iterate over all the words from string s
,
printing one per line.
The next example collects all pairs key=value
from the
given string into a table:
t = {} s = "from=world, to=Lua" for k, v in string.gmatch(s, "(%w+)=(%w+)") do t[k] = v end
For this function, a '^
' at the start of a pattern does not
work as an anchor, as this would prevent the iteration.
string.gsub (s, pattern, repl [, n])
s
in which all (or the first n
, if given)
occurrences of the pattern
have been
replaced by a replacement string specified by repl
,
which can be a string, a table, or a function.
gsub
also returns, as its second value,
the total number of matches that occurred.
If repl
is a string, then its value is used for replacement.
The character %
works as an escape character:
any sequence in repl
of the form %n
,
with n between 1 and 9,
stands for the value of the n-th captured substring (see below).
The sequence %0
stands for the whole match.
The sequence %%
stands for a single %
.
If repl
is a table, then the table is queried for every match,
using the first capture as the key;
if the pattern specifies no captures,
then the whole match is used as the key.
If repl
is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order;
if the pattern specifies no captures,
then the whole match is passed as a sole argument.
If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).
Here are some examples:
x = string.gsub("hello world", "(%w+)", "%1 %1") --> x="hello hello world world" x = string.gsub("hello world", "%w+", "%0 %0", 1) --> x="hello hello world" x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1") --> x="world hello Lua from" x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv) --> x="home = /home/roberto, user = roberto" local t = {name="lua", version="5.1"} x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t) --> x="lua-5.1.tar.gz"
string.len (s)
""
has length 0.
Embedded zeros are counted,
so "a\000bc\000"
has length 5.
string.lower (s)
string.match (s, pattern [, init])
pattern
in the string s
.
If it finds one, then match
returns
the captures from the pattern;
otherwise it returns nil.
If pattern
specifies no captures,
then the whole match is returned.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and can be negative.
string.rep (s, n)
n
copies of
the string s
.
string.reverse (s)
s
reversed.
string.sub (s, i [, j])
s
that
starts at i
and continues until j
;
i
and j
can be negative.
If j
is absent, then it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call string.sub(s,1,j)
returns a prefix of s
with length j
,
and string.sub(s, -i)
returns a suffix of s
with length i
.
string.upper (s)
A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
^$()%.[]*+-?
)
represents the character x itself.
.
: (a dot) represents all characters.%a
: represents all letters.%c
: represents all control characters.%d
: represents all digits.%l
: represents all lowercase letters.%p
: represents all punctuation characters.%s
: represents all space characters.%u
: represents all uppercase letters.%w
: represents all alphanumeric characters.%x
: represents all hexadecimal digits.%z
: represents the character with representation 0.%x
: (where x is any non-alphanumeric character)
represents the character x.
This is the standard way to escape the magic characters.
Any punctuation character (even the non magic)
can be preceded by a '%
'
when used to represent itself in a pattern.
[set]
:
represents the class which is the union of all
characters in set.
A range of characters can be specified by
separating the end characters of the range with a '-
'.
All classes %
x described above can also be used as
components in set.
All other characters in set represent themselves.
For example, [%w_]
(or [_%w]
)
represents all alphanumeric characters plus the underscore,
[0-7]
represents the octal digits,
and [0-7%l%-]
represents the octal digits plus
the lowercase letters plus the '-
' character.
The interaction between ranges and classes is not defined.
Therefore, patterns like [%a-z]
or [a-%%]
have no meaning.
[^set]
:
represents the complement of set,
where set is interpreted as above.
For all classes represented by single letters (%a
, %c
, etc.),
the corresponding uppercase letter represents the complement of the class.
For instance, %S
represents all non-space characters.
The definitions of letter, space, and other character groups
depend on the current locale.
In particular, the class [a-z]
may not be equivalent to %l
.
A pattern item can be
*
',
which matches 0 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
+
',
which matches 1 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
-
',
which also matches 0 or more repetitions of characters in the class.
Unlike '*
',
these repetition items will always match the shortest possible sequence;
?
',
which matches 0 or 1 occurrence of a character in the class;
%n
, for n between 1 and 9;
such item matches a substring equal to the n-th captured string
(see below);
%bxy
, where x and y are two distinct characters;
such item matches strings that start with x, end with y,
and where the x and y are balanced.
This means that, if one reads the string from left to right,
counting +1 for an x and -1 for a y,
the ending y is the first y where the count reaches 0.
For instance, the item %b()
matches expressions with
balanced parentheses.
A pattern is a sequence of pattern items.
A '^
' at the beginning of a pattern anchors the match at the
beginning of the subject string.
A '$
' at the end of a pattern anchors the match at the
end of the subject string.
At other positions,
'^
' and '$
' have no special meaning and represent themselves.
A pattern can contain sub-patterns enclosed in parentheses;
they describe captures.
When a match succeeds, the substrings of the subject string
that match captures are stored (captured) for future use.
Captures are numbered according to their left parentheses.
For instance, in the pattern "(a*(.)%w(%s*))"
,
the part of the string matching "a*(.)%w(%s*)"
is
stored as the first capture (and therefore has number 1);
the character matching ".
" is captured with number 2,
and the part matching "%s*
" has number 3.
As a special case, the empty capture ()
captures
the current string position (a number).
For instance, if we apply the pattern "()aa()"
on the
string "flaaap"
, there will be two captures: 3 and 5.
A pattern cannot contain embedded zeros. Use %z
instead.
This library provides generic functions for table manipulation.
It provides all its functions inside the table table
.
Most functions in the table library assume that the table represents an array or a list. For these functions, when we talk about the "length" of a table we mean the result of the length operator.
table.concat (table [, sep [, i [, j]]])
table[i]..sep..table[i+1] ··· sep..table[j]
.
The default value for sep
is the empty string,
the default for i
is 1,
and the default for j
is the length of the table.
If i
is greater than j
, returns the empty string.
table.insert (table, [pos,] value)
Inserts element value
at position pos
in table
,
shifting up other elements to open space, if necessary.
The default value for pos
is n+1
,
where n
is the length of the table (see §2.5.5),
so that a call table.insert(t,x)
inserts x
at the end
of table t
.
table.maxn (table)
Returns the largest positive numerical index of the given table, or zero if the table has no positive numerical indices. (To do its job this function does a linear traversal of the whole table.)
table.remove (table [, pos])
Removes from table
the element at position pos
,
shifting down other elements to close the space, if necessary.
Returns the value of the removed element.
The default value for pos
is n
,
where n
is the length of the table,
so that a call table.remove(t)
removes the last element
of table t
.
table.sort (table [, comp])
table[1]
to table[n]
,
where n
is the length of the table.
If comp
is given,
then it must be a function that receives two table elements,
and returns true
when the first is less than the second
(so that not comp(a[i+1],a[i])
will be true after the sort).
If comp
is not given,
then the standard Lua operator <
is used instead.
The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.
This library is an interface to the standard C math library.
It provides all its functions inside the table math
.
math.abs (x)
Returns the absolute value of x
.
math.acos (x)
Returns the arc cosine of x
(in radians).
math.asin (x)
Returns the arc sine of x
(in radians).
math.atan (x)
Returns the arc tangent of x
(in radians).
math.atan2 (y, x)
Returns the arc tangent of y/x
(in radians),
but uses the signs of both parameters to find the
quadrant of the result.
(It also handles correctly the case of x
being zero.)
math.ceil (x)
Returns the smallest integer larger than or equal to x
.
math.cos (x)
Returns the cosine of x
(assumed to be in radians).
math.cosh (x)
Returns the hyperbolic cosine of x
.
math.deg (x)
Returns the angle x
(given in radians) in degrees.
math.exp (x)
Returns the value ex.
math.floor (x)
Returns the largest integer smaller than or equal to x
.
math.fmod (x, y)
Returns the remainder of the division of x
by y
that rounds the quotient towards zero.
math.frexp (x)
Returns m
and e
such that x = m2e,
e
is an integer and the absolute value of m
is
in the range [0.5, 1)
(or zero when x
is zero).
math.huge
The value HUGE_VAL
,
a value larger than or equal to any other numerical value.
math.ldexp (m, e)
Returns m2e (e
should be an integer).
math.log (x)
Returns the natural logarithm of x
.
math.log10 (x)
Returns the base-10 logarithm of x
.
math.max (x, ···)
Returns the maximum value among its arguments.
math.min (x, ···)
Returns the minimum value among its arguments.
math.modf (x)
Returns two numbers,
the integral part of x
and the fractional part of x
.
math.pi
The value of pi.
math.pow (x, y)
Returns xy.
(You can also use the expression x^y
to compute this value.)
math.rad (x)
Returns the angle x
(given in degrees) in radians.
math.random ([m [, n]])
This function is an interface to the simple
pseudo-random generator function rand
provided by ANSI C.
(No guarantees can be given for its statistical properties.)
When called without arguments,
returns a uniform pseudo-random real number
in the range [0,1).
When called with an integer number m
,
math.random
returns
a uniform pseudo-random integer in the range [1, m].
When called with two integer numbers m
and n
,
math.random
returns a uniform pseudo-random
integer in the range [m, n].
math.randomseed (x)
Sets x
as the "seed"
for the pseudo-random generator:
equal seeds produce equal sequences of numbers.
math.sin (x)
Returns the sine of x
(assumed to be in radians).
math.sinh (x)
Returns the hyperbolic sine of x
.
math.sqrt (x)
Returns the square root of x
.
(You can also use the expression x^0.5
to compute this value.)
math.tan (x)
Returns the tangent of x
(assumed to be in radians).
math.tanh (x)
Returns the hyperbolic tangent of x
.
start_reading (fs, filename)
This function opens a file exclusively for reading, and returns a channel (NOT a file as regular Lua) or nil on error. recv() from the channel to read a line at a time from the file.
(The fs parameter is currently unused. It will be used to pass in fake file systems for tests.)
character_by_character(chanin, chanout)
This function converts a channel that can recv() a line at a time from a file into a channel that can recv() a character at a time. Don't try to mix by-line reads with by-character reads. Once a channel is passed into character_by_character, calling code should stop trying to use it.
start_writing(fs, filename)
This function opens a file exclusively for writing, and returns a channel (NOT a file as regular Lua) or nil on error. send() to the channel will write to the file. close() on the channel will persist the changes and make them externally visible. All writes are hidden until close().
(The fs parameter is currently unused. It will be used to pass in fake file systems for tests.)
Unless otherwise stated, all File I/O functions return nil on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from nil on success.
io.close (file)
Equivalent to file:close()
.
io.open (filename [, mode])
This function opens a file,
in the mode specified in the string mode
.
It returns a new file handle,
or, in case of errors, nil plus an error message.
The mode
string can be any of the following:
The mode
string can also have a 'b
' at the end,
which is needed in some systems to open the file in binary mode.
This string is exactly what is used in the
standard C function fopen
.
io.tmpfile ()
Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.
io.type (obj)
Checks whether obj
is a valid file handle.
Returns the string "file"
if obj
is an open file handle,
"closed file"
if obj
is a closed file handle,
or nil if obj
is not a file handle.
file:close ()
Closes file
.
Note that files are automatically closed when
their handles are garbage collected,
but that takes an unpredictable amount of time to happen.
file:flush ()
Saves any written data to file
.
file:lines ()
Returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in file:lines() do body end
will iterate over all lines of the file.
(Unlike io.lines
, this function does not close the file
when the loop ends.)
file:read (···)
Reads the file file
,
according to the given formats, which specify what to read.
For each format,
the function returns a string (or a number) with the characters read,
or nil if it cannot read data with the specified format.
When called without formats,
it uses a default format that reads the entire next line
(see below).
The available formats are
file:seek ([whence] [, offset])
Sets and gets the file position,
measured from the beginning of the file,
to the position given by offset
plus a base
specified by the string whence
, as follows:
In case of success, function seek
returns the final file position,
measured in bytes from the beginning of the file.
If this function fails, it returns nil,
plus a string describing the error.
The default value for whence
is "cur"
,
and for offset
is 0.
Therefore, the call file:seek()
returns the current
file position, without changing it;
the call file:seek("set")
sets the position to the
beginning of the file (and returns 0);
and the call file:seek("end")
sets the position to the
end of the file, and returns its size.
file:setvbuf (mode [, size])
Sets the buffering mode for an output file. There are three available modes:
flush
the file
(see io.flush
)).
For the last two cases, size
specifies the size of the buffer, in bytes.
The default is an appropriate size.
file:write (···)
Writes the value of each of its arguments to
the file
.
The arguments must be strings or numbers.
To write other values,
use tostring
or string.format
before write
.
This library is implemented through table os
.
os.clock ()
Returns an approximation of the amount in seconds of CPU time used by the program.
os.date ([format [, time]])
Returns a string or a table containing date and time,
formatted according to the given string format
.
If the time
argument is present,
this is the time to be formatted
(see the os.time
function for a description of this value).
Otherwise, date
formats the current time.
If format
starts with '!
',
then the date is formatted in Coordinated Universal Time.
After this optional character,
if format
is the string "*t
",
then date
returns a table with the following fields:
year
(four digits), month
(1--12), day
(1--31),
hour
(0--23), min
(0--59), sec
(0--61),
wday
(weekday, Sunday is 1),
yday
(day of the year),
and isdst
(daylight saving flag, a boolean).
If format
is not "*t
",
then date
returns the date as a string,
formatted according to the same rules as the C function strftime
.
When called without arguments,
date
returns a reasonable date and time representation that depends on
the host system and on the current locale
(that is, os.date()
is equivalent to os.date("%c")
).
os.difftime (t2, t1)
Returns the number of seconds from time t1
to time t2
.
In POSIX, Windows, and some other systems,
this value is exactly t2
-t1
.
os.execute ([command])
This function is equivalent to the C function system
.
It passes command
to be executed by an operating system shell.
It returns a status code, which is system-dependent.
If command
is absent, then it returns nonzero if a shell is available
and zero otherwise.
os.exit ([code])
Calls the C function exit
,
with an optional code
,
to terminate the host program.
The default value for code
is the success code.
os.getenv (varname)
Returns the value of the process environment variable varname
,
or nil if the variable is not defined.
os.remove (filename)
Deletes the file or directory with the given name. Directories must be empty to be removed. If this function fails, it returns nil, plus a string describing the error.
os.rename (oldname, newname)
Renames file or directory named oldname
to newname
.
If this function fails, it returns nil,
plus a string describing the error.
os.setlocale (locale [, category])
Sets the current locale of the program.
locale
is a string specifying a locale;
category
is an optional string describing which category to change:
"all"
, "collate"
, "ctype"
,
"monetary"
, "numeric"
, or "time"
;
the default category is "all"
.
The function returns the name of the new locale,
or nil if the request cannot be honored.
If locale
is the empty string,
the current locale is set to an implementation-defined native locale.
If locale
is the string "C
",
the current locale is set to the standard C locale.
When called with nil as the first argument, this function only returns the name of the current locale for the given category.
os.time ([table])
Returns the current time when called without arguments,
or a time representing the date and time specified by the given table.
This table must have fields year
, month
, and day
,
and may have fields hour
, min
, sec
, and isdst
(for a description of these fields, see the os.date
function).
The returned value is a number, whose meaning depends on your system.
In POSIX, Windows, and some other systems, this number counts the number
of seconds since some given start time (the "epoch").
In other systems, the meaning is not specified,
and the number returned by time
can be used only as an argument to
date
and difftime
.
os.tmpname ()
Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and explicitly removed when no longer needed.
On some systems (POSIX), this function also creates a file with that name, to avoid security risks. (Someone else might create the file with wrong permissions in the time between getting the name and creating the file.) You still have to open the file to use it and to remove it (even if you do not use it).
When possible,
you may prefer to use io.tmpfile
,
which automatically removes the file when the program ends.
This library provides the functionality of the debug interface to Lua programs. You should exert care when using this library. The functions provided here should be used exclusively for debugging and similar tasks, such as profiling. Please resist the temptation to use them as a usual programming tool: they can be very slow. Moreover, several of these functions violate some assumptions about Lua code (e.g., that variables local to a function cannot be accessed from outside or that userdata metatables cannot be changed by Lua code) and therefore can compromise otherwise secure code.
All functions in this library are provided
inside the debug
table.
All functions that operate over a thread
have an optional first argument which is the
thread to operate over.
The default is always the current thread.
debug.debug ()
Enters an interactive mode with the user,
running each string that the user enters.
Using simple commands and other debug facilities,
the user can inspect global and local variables,
change their values, evaluate expressions, and so on.
A line containing only the word cont
finishes this function,
so that the caller continues its execution.
Note that commands for debug.debug
are not lexically nested
within any function, and so have no direct access to local variables.
debug.getfenv (o)
o
.
debug.gethook ([thread])
Returns the current hook settings of the thread, as three values:
the current hook function, the current hook mask,
and the current hook count
(as set by the debug.sethook
function).
debug.getinfo ([thread,] function [, what])
Returns a table with information about a function.
You can give the function directly,
or you can give a number as the value of function
,
which means the function running at level function
of the call stack
of the given thread:
level 0 is the current function (getinfo
itself);
level 1 is the function that called getinfo
;
and so on.
If function
is a number larger than the number of active functions,
then getinfo
returns nil.
The returned table can contain all the fields returned by lua_getinfo
,
with the string what
describing which fields to fill in.
The default for what
is to get all information available,
except the table of valid lines.
If present,
the option 'f
'
adds a field named func
with the function itself.
If present,
the option 'L
'
adds a field named activelines
with the table of
valid lines.
For instance, the expression debug.getinfo(1,"n").name
returns
a table with a name for the current function,
if a reasonable name can be found,
and the expression debug.getinfo(print)
returns a table with all available information
about the print
function.
debug.getlocal ([thread,] level, local)
This function returns the name and the value of the local variable
with index local
of the function at level level
of the stack.
(The first parameter or local variable has index 1, and so on,
until the last active local variable.)
The function returns nil if there is no local
variable with the given index,
and raises an error when called with a level
out of range.
(You can call debug.getinfo
to check whether the level is valid.)
Variable names starting with '(
' (open parentheses)
represent internal variables
(loop control variables, temporaries, and C function locals).
debug.getmetatable (object)
Returns the metatable of the given object
or nil if it does not have a metatable.
debug.getregistry ()
Returns the registry table (see §3.5).
debug.getupvalue (func, up)
This function returns the name and the value of the upvalue
with index up
of the function func
.
The function returns nil if there is no upvalue with the given index.
debug.setfenv (object, table)
Sets the environment of the given object
to the given table
.
Returns object
.
debug.sethook ([thread,] hook, mask [, count])
Sets the given function as a hook.
The string mask
and the number count
describe
when the hook will be called.
The string mask may have the following characters,
with the given meaning:
"c"
: the hook is called every time Lua calls a function;"r"
: the hook is called every time Lua returns from a function;"l"
: the hook is called every time Lua enters a new line of code.
With a count
different from zero,
the hook is called after every count
instructions.
When called without arguments,
debug.sethook
turns off the hook.
When the hook is called, its first parameter is a string
describing the event that has triggered its call:
"call"
, "return"
(or "tail return"
,
when simulating a return from a tail call),
"line"
, and "count"
.
For line events,
the hook also gets the new line number as its second parameter.
Inside a hook,
you can call getinfo
with level 2 to get more information about
the running function
(level 0 is the getinfo
function,
and level 1 is the hook function),
unless the event is "tail return"
.
In this case, Lua is only simulating the return,
and a call to getinfo
will return invalid data.
debug.setlocal ([thread,] level, local, value)
This function assigns the value value
to the local variable
with index local
of the function at level level
of the stack.
The function returns nil if there is no local
variable with the given index,
and raises an error when called with a level
out of range.
(You can call getinfo
to check whether the level is valid.)
Otherwise, it returns the name of the local variable.
debug.setmetatable (object, table)
Sets the metatable for the given object
to the given table
(which can be nil).
debug.setupvalue (func, up, value)
This function assigns the value value
to the upvalue
with index up
of the function func
.
The function returns nil if there is no upvalue
with the given index.
Otherwise, it returns the name of the upvalue.
debug.traceback ([thread,] [message [, level]])
Returns a string with a traceback of the call stack.
An optional message
string is appended
at the beginning of the traceback.
An optional level
number tells at which level
to start the traceback
(default is 1, the function calling traceback
).
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 | `#´