Lua 5.0 参考手册：轻量级编程语言详解

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“Lua manual” Lua 是一种强大的轻量级编程语言，主要设计用于扩展应用程序，同时也常被用作通用的独立编程语言。它的语法简洁，类似Pascal，但结合了基于关联数组的强大数据描述结构和可扩展的语义。Lua是动态类型语言，通过字节码解释执行，并具有自动内存管理功能，包括垃圾收集机制，这使得它在配置、脚本和快速原型开发中非常理想。这份文档描述了Lua编程语言5.0版本以及允许Lua程序与其宿主C程序交互的应用程序编程接口（API）。 Lua 5.0参考手册最后修订于2003年11月25日，由Tecgraf, PUC-Rio版权所有。该软件授权免费使用，但包含了一些条件，比如在所有副本或实质性的软件部分中必须包含版权通知和许可通知。该软件“按原样”提供，不提供任何形式的保证，包括但不限于适销性、特定用途的适用性和非侵权性的保证。作者或版权所有者对因软件或使用软件而产生的任何索赔、损害或其他责任概不负责。手册中详细涵盖了Lua语言的基础概念，如变量、控制结构、函数、表（作为其主要的数据结构）、模块化、错误处理、元编程等。表格在Lua中扮演了核心角色，它们是关联数组，可以用来表示集合、对象、哈希表等多种数据结构。Lua还支持闭包、匿名函数（也称为lambda函数）和递归，这些特性使其在处理复杂问题时保持灵活性。关于API部分，文档会介绍如何在C语言中集成Lua，包括加载和运行Lua脚本、创建和调用Lua函数、在C与Lua之间传递数据以及注册C函数到Lua环境中供Lua调用。Lua的API提供了多种函数，如luaL_newstate用于创建新的Lua状态（即解释器实例），luaL_loadbuffer用于加载Lua代码到解释器，以及lua_pcall用于安全地调用Lua函数。此外，手册还会讲解如何使用Lua进行错误处理和调试，以及如何利用Lua的垃圾回收系统来管理内存。对于想要深入学习和利用Lua进行系统扩展或独立编程的开发者来说，这份文档提供了全面的指导和参考资料。

exchanges the values of x and y.

The m eaning of assignments to global variables and table ﬁelds can be changed via metatables.

An assignment to an indexed variable t[i] = val is equivalent to settable_event(t,i,val).

(See §2.8 for a complete description of the settable_event function. This function is not deﬁned

or callable in Lua. We use it here only for explanatory purposes.)

An assignment to a global variable x = val is equivalent to the assignment _env.x = val,

which in turn is equivalent to

settable_event(_env, "x", val)

where _env is the environment of the running function. (The _env variable is not deﬁned in Lua.

We use it here only for explanatory purposes.)

2.4.4 Control Structures

The control structures if, while, and repeat have the usual meaning and familiar s yntax:

stat → while exp do block end

stat → repeat block until exp

stat → if exp then block { elseif exp then block } [ else block ] end

Lua also has a for statement, in two ﬂavors (see §2.4.5).

The condition expression exp of a control structure may return any value. Both false and nil

are considered false. All values diﬀerent from nil and false are considered true (in particular, the

number 0 and the empty string are also true).

The return statement is used to return values from a function or from a chunk. Functions and

chunks may return more than one value, so the syntax for the return statement is

stat → return [ explist1 ]

The break statement can be used to terminate the execution of a while, repeat, or for loop,

skipping to the next statement after the loop:

stat → break

A break ends the innermost enclosing lo op.

For syntactic reasons, return and break statements can only be written as the last statement of

a block. If it is really necessary to return or break in the middle of a block, then an explicit inner

block can be used, as in the idioms ‘do return end’ and ‘do break end’, because now return

and break are the last statements in their (inner) blocks. In practice, those idioms are only used

during debugging.

2.4.5 For Statement

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

The numeric for loop repeats a block of code while a control variable runs through an arithmetic

progression. It has the following syntax:

stat → for Name ‘=’ exp ‘,’ exp [ ‘,’ exp ] do block end

The block is repeated for name starting at the value of the ﬁrst exp, until it passes the second exp

by steps of the third exp. More precisely, a for statement like

for var = e1, e2, e3 do block end

• The behavior is undeﬁned if you assign to var_1 inside the block.

• You can use break to exit a for loop.

• The loop variables var_i are local to the statement; you cannot use their values after the for

ends. If you need these values, then assign them to other variables before breaking or exiting

the loop.

2.4.6 Function Calls as Statements

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

stat → functioncall

In this case, all returned values are thrown away. Function calls are explained in §2.5.7.

2.4.7 Local Declarations

Local variables may be declared anywhere inside a blo ck. The declaration may include an initial

assignment:

stat → local namelist [ ‘=’ explist1 ]

namelist → Name { ‘,’ Name }

If present, an initial assignment has the same semantics of a multiple assignment (see §2.4.3).

Otherwise, all variables are initialized with nil.

A chunk is also a block (see §2.4.1), so local variables can be declared in a chunk outside any

explicit block. Such local variables die when the chunk ends.

The visibility rules for local variables are explained in §2.6.

2.5 Expressions

The basic expressions in Lua are the following:

exp → preﬁxexp

exp → nil | false | true

exp → Number

exp → Literal

exp → function

exp → tableconstructor

preﬁxexp → var | functioncall | ‘(’ exp ‘)’

Numbers and literal strings are explained in §2.1; variables are explained in §2.3; function

deﬁnitions are explained in §2.5.8; function calls are explained in §2.5.7; table constructors are

explained in §2.5.6.

An expression enclosed in parentheses always results in only one value. Thus, (f(x,y,z)) is

always a single value, e ven if f returns several values. (The value of (f(x,y,z)) is the ﬁrst value

returned by f or nil if f does not return any values.)

Expressions can also be built with arithmetic operators, relational operators, and logical oper-

ators, all of which are explained below.

2.5.1 Arithmetic Operators

Lua supports the usual arithmetic operators: the binary + (addition), - (subtraction), * (multipli-

cation), / (division), and ^ (exponentiation); and unary - (negation). If the operands are numbers,

or strings that can be converted to numbers (see §2.2.1), then all operations except exponentiation

have the usual meaning. Exponentiation calls a global function __pow; otherwise, an appropriate

metamethod is called (see §2.8). The standard mathematical library deﬁnes function __pow, giving

the expected meaning to exponentiation (see §5.5).

2.5.2 Relational Operators

The relational operators in Lua are

== ~= < > <= >=

These operators always result in false or true.

Equality (==) ﬁrst compares the type of its operands. If the types are diﬀerent, then the result

is false. Otherwise, the values of the operands are compared. Numbers and strings are compared

in the usual way. Objects (tables, userdata, threads, and functions) are compared by reference:

Two objects are considered equal only if they are the same object. Every time you create a new

object (a table, userdata, or function), this new object is diﬀerent from any previously existing

object.

You can change the way that Lua compares tables and userdata using the “eq” metamethod

(see §2.8).

The conversion rules of §2.2.1 do not apply to equality comparisons. Thus, "0"==0 evaluates

to false, and t[0] and t["0"] denote diﬀerent entries in a table.

The operator ~= is exactly the negation of equality (==).

The order operators work as follows. If both arguments are numbers, then they are compared

as such. Otherwise, if both arguments are strings, then their values are compared according to the

current locale. Otherwise, Lua tries to call the “lt” or the “le” metamethod (see §2.8).

2.5.3 Logical Operators

The logical operators in Lua are

and or not

Like the control structures (see §2.4.4), all logical operators consider both false and nil as false

and anything else as true.

The operator not always return false or true.

The conjunction op erator and returns its ﬁrst argument if this value is false or nil; otherwise,

and returns its second argument. The disjunction operator or returns its ﬁrst argument if this

value is diﬀerent from nil and false; otherwise, or returns its second argument. Both and and or

use short-cut evaluation, that is, the second operand is evaluated only if necessary. For example,

10 or error() -> 10

nil or "a" -> "a"

nil and 10 -> nil

false and error() -> false

false and nil -> false

false or nil -> nil

10 and 20 -> 20

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