lua

Lua is a powerful, efficient, lightweight, embeddable scripting language.
Lua 即為脚本语言

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更新記錄

item	note
20160810	第一版

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目錄

• Lua
  • 直譯器比較表
  • JIT說明
• Lua command
  • Luac
• Lua Language
  • Lexical Conventions
  • Values and Types
• ex.如何定義-註解
• ex. 多重赋值
• 參考來源

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Lua

• Lua 并不是唯一的脚本语言，除了 Lua 以外，还有很多能够提供类似功能的脚本语言，例如Node Perl、Tcl、Ruby、Forth、Python 等
• lua is the stand-alone Lua interpreter. It loads and executes Lua programs, either in textual source form or in precompiled binary form
• Precompiled binaries are output by luac, the Lua compiler

直譯器比較表

• LuaJIT 比 Lua快了一倍以上
• LuaJIT 比 Node還快

LuaJIT > Node > Lua

出處：深入淺出 Nodje.js page 34

JIT說明

• JIT 是 just in time 的缩写, 也就是即时编译编译器
• 如果一段代码频繁的调用方法，或是一个循环，也就是这段代码被多次执行，那么编译就非常值得了
• 如同binary已被compile可直接執行而不是直譯(一行一行的解譯)，此時即為JIT的功用

出處：深入浅出 JIT 编译器

Lua command

• If no arguments are given, then “-v -i” is assumed when the standard input is a terminal

lua [ options ] [ script [ args ] ]

• OPTIONS
  • -i enter interactive mode after script is executed.
  • -v show version information.
  • -l name call require(‘name’) before executing script. Typically used to load libraries.

出處: lua-5.1.5/doc/lua.html

Luac

• Lua compiler
• It translates programs written in the Lua programming language into binary files that can be later loaded and executed.
• The main advantages of precompiling chunks are:
  • faster loading,
  • protecting source code from accidental user changes,
  • off-line syntax checking.
• luac produces a single output file containing the bytecodes for all source files given

luac [ options ] [ filenames ]

• OPTIONS

  • -o file output to file, instead of the default luac.out
  • -p load files but do not generate any output file
    Used mainly for syntax checking and for testing precompiled chunks: corrupted files will probably generate errors when loaded.
  • -s strip debug information before writing the output file.

• 使用luac -p好處是預先檢查語法是否有問題

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Lua Language

Lexical Conventions

• Lua is a case-sensitive language

• 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

• The following strings denote other tokens:

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

Values and Types

• Lua is a dynamically typed language
• All values in Lua are first-class values

• There are eight basic types in Lua: nil, boolean, number, string, function, userdata, thread, and table.

  • nil
    it usually represents the absence of a useful value
    value nil
  • boolean
    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
  • string
    String represents arrays of characters. Lua is 8-bit clean: strings can contain any 8-bit character,
  • userdata
    The type userdata is provided to allow arbitrary C data to be stored in Lua variables.
  • thread
    Lua supports coroutines, also called collaborative multithreading.
    ex. coroutine.create / coroutine.resume
  • table
    The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any value (except nil).

• Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them.

• There are three kinds of variables in Lua: global variables, local variables, and table fields.

> a = 10                                                                         
> print(type(a))                                                                 
number                                                                           
> a = "a string!!"                                                               
> print(type(a))                                                                 
string                                                                           
>  a = print                                                                     
> print(type(a))                                                                 
function                                                                         
>

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ex.如何定義-註解

• 單行
  Lua 的单行注释语句以双连接符“–”开头

• 多行

--[[ 
print(10)      -- no action (comment) 
--]]

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ex. 多重赋值

function foo0() end       -- returns no results 
function foo1() return 'a' end     -- returns 1 result 
function foo2() return 'a', 'b' end  -- returns 2 results

x, y = foo2()         -- x = 'a', y = 'b' 
x = foo2()        -- x = 'a', 'b' is discarded
x, y, z = 10, foo2()     -- x = 10, y = 'a', z = 'b'

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參考來源

• 深入浅出 JIT 编译器
• Lua 5.1 Reference Manual