我的Learn Lua in Y minutes

1. 又要让我学习新知识？

本文是我学习Lua语言的一个开端。至于又要学习一门新编程语言的动机，在《进化 neovim 到 lin.nvim 风味》中已经自述的很明白了：成年人的世界没有简单二字，要是有得选，谁愿意把自己逼成文武全才~ 囧rz

其实我心里特反感没完没了的学习。在我看来，对于一个已经不是学生身份的人来说，让自己沉溺于所谓的“学习”，是在用”战术的勤奋”掩盖”战略的懒惰”。 成年人的学习必须有输出结果，不然的话就不要学什么习了 。

正是基于此理念：（1）我通过研读本文初步掌握Lua语言之后，会应用Lua语言复刻《Steam Deck 掌机催我搞技术新基建》中shell脚本工具（附在文末）。（2）在学习的过程中，我会在下文的2. 速战速决Lua脚本语言中给出要点解读；同时为了验证我的理解，我会用Mathematica语言对该Lua程序重写。

这么大的工作量没有神灵庇护怎么行！必须得把我的神仙请出来，保佑我能废寝忘食地专心学习😍：

2. 速战速决Lua脚本语言

2.1. 原文：Learn Lua in Y minutes

原文链接是Learn X in Y minutes, Where X=Lua. 这篇短小的教程是Literate Programming的方式写成，全文如下:

-- https://learnxinyminutes.com/docs/lua/
--
-- Two dashes start a one-line comment.

--[[
     Adding two ['s and ]'s makes it a
     multi-line comment.
--]]

----------------------------------------------------
-- 1. Variables and flow control.
----------------------------------------------------

num = 42  -- All numbers are doubles.
-- Don't freak out, 64-bit doubles have 52 bits for
-- storing exact int values; machine precision is
-- not a problem for ints that need < 52 bits.

s = 'walternate'  -- Immutable strings like Python.
t = "double-quotes are also fine"
u = [[ Double brackets
       start and end
       multi-line strings.]]
t = nil  -- Undefines t; Lua has garbage collection.

-- Blocks are denoted with keywords like do/end:
while num < 50 do
  num = num + 1  -- No ++ or += type operators.
end

-- If clauses:
if num > 40 then
  print('over 40')
elseif s ~= 'walternate' then  -- ~= is not equals.
  -- Equality check is == like Python; ok for strs.
  io.write('not over 40\n')  -- Defaults to stdout.
else
  -- Variables are global by default.
  thisIsGlobal = 5  -- Camel case is common.

  -- How to make a variable local:
  local line = io.read()  -- Reads next stdin line.

  -- String concatenation uses the .. operator:
  print('Winter is coming, ' .. line)
end

-- Undefined variables return nil.
-- This is not an error:
foo = anUnknownVariable  -- Now foo = nil.

aBoolValue = false

-- Only nil and false are falsy; 0 and '' are true!
if not aBoolValue then print('it was false') end

-- 'or' and 'and' are short-circuited.
-- This is similar to the a?b:c operator in C/js:
ans = aBoolValue and 'yes' or 'no'  --> 'no'

karlSum = 0
for i = 1, 100 do  -- The range includes both ends.
  karlSum = karlSum + i
end

-- Use "100, 1, -1" as the range to count down:
fredSum = 0
for j = 100, 1, -1 do fredSum = fredSum + j end

-- In general, the range is begin, end[, step].

-- Another loop construct:
repeat
  print('the way of the future')
  num = num - 1
until num == 0


----------------------------------------------------
-- 2. Functions.
----------------------------------------------------

function fib(n)
  if n < 2 then return 1 end
  return fib(n - 2) + fib(n - 1)
end

-- Closures and anonymous functions are ok:
function adder(x)
  -- The returned function is created when adder is
  -- called, and remembers the value of x:
  return function (y) return x + y end
end
a1 = adder(9)
a2 = adder(36)
print(a1(16))  --> 25
print(a2(64))  --> 100

-- Returns, func calls, and assignments all work
-- with lists that may be mismatched in length.
-- Unmatched receivers are nil;
-- unmatched senders are discarded.

x, y, z = 1, 2, 3, 4
-- Now x = 1, y = 2, z = 3, and 4 is thrown away.

function bar(a, b, c)
  print(a, b, c)
  return 4, 8, 15, 16, 23, 42
end

x, y = bar('zaphod')  --> prints "zaphod  nil nil"
-- Now x = 4, y = 8, values 15...42 are discarded.

-- Functions are first-class, may be local/global.
-- These are the same:
function f(x) return x * x end
f = function (x) return x * x end

-- And so are these:
local function g(x) return math.sin(x) end
local g; g  = function (x) return math.sin(x) end
-- the 'local g' decl makes g-self-references ok.

-- Trig funcs work in radians, by the way.

-- Calls with one string param don't need parens:
print 'hello'  -- Works fine.


----------------------------------------------------
-- 3. Tables.
----------------------------------------------------

-- Tables = Lua's only compound data structure;
--          they are associative arrays.
-- Similar to php arrays or js objects, they are
-- hash-lookup dicts that can also be used as lists.

-- Using tables as dictionaries / maps:

-- Dict literals have string keys by default:
t = {key1 = 'value1', key2 = false}

-- String keys can use js-like dot notation:
print(t.key1)  -- Prints 'value1'.
t.newKey = {}  -- Adds a new key/value pair.
t.key2 = nil   -- Removes key2 from the table.

-- Literal notation for any (non-nil) value as key:
u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
print(u[6.28])  -- prints "tau"

-- Key matching is basically by value for numbers
-- and strings, but by identity for tables.
a = u['@!#']  -- Now a = 'qbert'.
b = u[{}]     -- We might expect 1729, but it's nil:
-- b = nil since the lookup fails. It fails
-- because the key we used is not the same object
-- as the one used to store the original value. So
-- strings & numbers are more portable keys.

-- A one-table-param function call needs no parens:
function h(x) print(x.key1) end
h{key1 = 'Sonmi~451'}  -- Prints 'Sonmi~451'.

for key, val in pairs(u) do  -- Table iteration.
  print(key, val)
end

-- _G is a special table of all globals.
print(_G['_G'] == _G)  -- Prints 'true'.

-- Using tables as lists / arrays:

-- List literals implicitly set up int keys:
v = {'value1', 'value2', 1.21, 'gigawatts'}
for i = 1, #v do  -- #v is the size of v for lists.
  print(v[i])  -- Indices start at 1 !! SO CRAZY!
end
-- A 'list' is not a real type. v is just a table
-- with consecutive integer keys, treated as a list.

----------------------------------------------------
-- 3.1 Metatables and metamethods.
----------------------------------------------------

-- A table can have a metatable that gives the table
-- operator-overloadish behavior. Later we'll see
-- how metatables support js-prototype behavior.

f1 = {a = 1, b = 2}  -- Represents the fraction a/b.
f2 = {a = 2, b = 3}

-- This would fail:
-- s = f1 + f2

metafraction = {}
function metafraction.__add(f1, f2)
  sum = {}
  sum.b = f1.b * f2.b
  sum.a = f1.a * f2.b + f2.a * f1.b
  return sum
end

setmetatable(f1, metafraction)
setmetatable(f2, metafraction)

s = f1 + f2  -- call __add(f1, f2) on f1's metatable

-- f1, f2 have no key for their metatable, unlike
-- prototypes in js, so you must retrieve it as in
-- getmetatable(f1). The metatable is a normal table
-- with keys that Lua knows about, like __add.

-- But the next line fails since s has no metatable:
-- t = s + s
-- Class-like patterns given below would fix this.

-- An __index on a metatable overloads dot lookups:
defaultFavs = {animal = 'gru', food = 'donuts'}
myFavs = {food = 'pizza'}
setmetatable(myFavs, {__index = defaultFavs})
eatenBy = myFavs.animal  -- works! thanks, metatable

-- Direct table lookups that fail will retry using
-- the metatable's __index value, and this recurses.

-- An __index value can also be a function(tbl, key)
-- for more customized lookups.

-- Values of __index,add, .. are called metamethods.
-- Full list. Here a is a table with the metamethod.

-- __add(a, b)                     for a + b
-- __sub(a, b)                     for a - b
-- __mul(a, b)                     for a * b
-- __div(a, b)                     for a / b
-- __mod(a, b)                     for a % b
-- __pow(a, b)                     for a ^ b
-- __unm(a)                        for -a
-- __concat(a, b)                  for a .. b
-- __len(a)                        for #a
-- __eq(a, b)                      for a == b
-- __lt(a, b)                      for a < b
-- __le(a, b)                      for a <= b
-- __index(a, b)  <fn or a table>  for a.b
-- __newindex(a, b, c)             for a.b = c
-- __call(a, ...)                  for a(...)

----------------------------------------------------
-- 3.2 Class-like tables and inheritance.
----------------------------------------------------

-- Classes aren't built in; there are different ways
-- to make them using tables and metatables.

-- Explanation for this example is below it.

Dog = {}                                   -- 1.

function Dog:new()                         -- 2.
  newObj = {sound = 'woof'}                -- 3.
  self.__index = self                      -- 4.
  return setmetatable(newObj, self)        -- 5.
end

function Dog:makeSound()                   -- 6.
  print('I say ' .. self.sound)
end

mrDog = Dog:new()                          -- 7.
mrDog:makeSound()  -- 'I say woof'         -- 8.

-- 1. Dog acts like a class; it's really a table.
-- 2. function tablename:fn(...) is the same as
--    function tablename.fn(self, ...)
--    The : just adds a first arg called self.
--    Read 7 & 8 below for how self gets its value.
-- 3. newObj will be an instance of class Dog.
-- 4. self = the class being instantiated. Often
--    self = Dog, but inheritance can change it.
--    newObj gets self's functions when we set both
--    newObj's metatable and self's __index to self.
-- 5. Reminder: setmetatable returns its first arg.
-- 6. The : works as in 2, but this time we expect
--    self to be an instance instead of a class.
-- 7. Same as Dog.new(Dog), so self = Dog in new().
-- 8. Same as mrDog.makeSound(mrDog); self = mrDog.

----------------------------------------------------

-- Inheritance example:

LoudDog = Dog:new()                           -- 1.

function LoudDog:makeSound()
  s = self.sound .. ' '                       -- 2.
  print(s .. s .. s)
end

seymour = LoudDog:new()                       -- 3.
seymour:makeSound()  -- 'woof woof woof'      -- 4.

-- 1. LoudDog gets Dog's methods and variables.
-- 2. self has a 'sound' key from new(), see 3.
-- 3. Same as LoudDog.new(LoudDog), and converted to
--    Dog.new(LoudDog) as LoudDog has no 'new' key,
--    but does have __index = Dog on its metatable.
--    Result: seymour's metatable is LoudDog, and
--    LoudDog.__index = LoudDog. So seymour.key will
--    = seymour.key, LoudDog.key, Dog.key, whichever
--    table is the first with the given key.
-- 4. The 'makeSound' key is found in LoudDog; this
--    is the same as LoudDog.makeSound(seymour).

-- If needed, a subclass's new() is like the base's:
function LoudDog:new()
  newObj = {}
  -- set up newObj
  self.__index = self
  return setmetatable(newObj, self)
end

----------------------------------------------------
-- 4. Modules.
----------------------------------------------------


--[[ I'm commenting out this section so the rest of
--   this script remains runnable.

-- Suppose the file mod.lua looks like this:
local M = {}

local function sayMyName()
  print('Hrunkner')
end

function M.sayHello()
  print('Why hello there')
  sayMyName()
end

return M

-- Another file can use mod.lua's functionality:
local mod = require('mod')  -- Run the file mod.lua.

-- require is the standard way to include modules.
-- require acts like:     (if not cached; see below)
local mod = (function ()
  <contents of mod.lua>
end)()
-- It's like mod.lua is a function body, so that
-- locals inside mod.lua are invisible outside it.

-- This works because mod here = M in mod.lua:
mod.sayHello() -- Prints: Why hello there Hrunkner

-- This is wrong; sayMyName only exists in mod.lua:
mod.sayMyName()  -- error

-- require's return values are cached so a file is
-- run at most once, even when require'd many times.

-- Suppose mod2.lua contains "print('Hi!')".
local a = require('mod2')  -- Prints Hi!
local b = require('mod2')  -- Doesn't print; a=b.

-- dofile is like require without caching:
dofile('mod2.lua')  --> Hi!
dofile('mod2.lua')  --> Hi! (runs it again)

-- loadfile loads a lua file but doesn't run it yet.
f = loadfile('mod2.lua')  -- Call f() to run it.

-- load is loadfile for strings.
-- (loadstring is deprecated, use load instead)
g = load('print(343)')  -- Returns a function.
g()  -- Prints out 343; nothing printed before now.

--]]

2.2. Mathematica复刻Lua代码

在这一部分，我将用Mathematica语言对上述的Lua代码重写；并将Lua代码的原文（包括行号）以Mathematica注释的形式给出。如下代码所示：

(* 
行号  Lua代码原文
15 num = 42  -- All numbers are doubles.
16 -- Don't freak out, 64-bit doubles have 52 bits for
17 -- storing exact int values; machine precision is
18 -- not a problem for ints that need < 52 bits.
19 
20 s = 'walternate'  -- Immutable strings like Python.
21 t = "double-quotes are also fine"
22 u = [[ Double brackets
23        start and end
24        multi-line strings.]]
25 t = nil  -- Undefines t; Lua has garbage collection.
26 
*)

num = 42

s = "walternate"
t = "double-quotes are also fine"
u = "Double brackets 
start and end 
multi-line strings."

t = Null

(*
行号  Lua代码原文
28 while num < 50 do
29   num = num + 1  -- No ++ or += type operators.
30 end
*)
While[num < 50, num = num + 1]

(*
行号  Lua代码原文
32 -- If clauses:
33 if num > 40 then
34   print('over 40')
35 elseif s ~= 'walternate' then  -- ~= is not equals.
36   -- Equality check is == like Python; ok for strs.
37   io.write('not over 40\n')  -- Defaults to stdout.
38 else
39   -- Variables are global by default.
40   thisIsGlobal = 5  -- Camel case is common.
41 
42   -- How to make a variable local:
43   local line = io.read()  -- Reads next stdin line.
44 
45   -- String concatenation uses the .. operator:
46   print('Winter is coming, ' .. line)
47 end
*)

If[num > 40,
 Print["over 40"],
 If[s != "walternate",
  Print["not over 40\n"],
  thisIsGlobal = 5;
   (* Wolfram Mathematica的局部变量是通过 `Module`，`With`等函数实现 *)
  ]]

(*
行号  Lua代码原文
62 karlSum = 0
63 for i = 1, 100 do  -- The range includes both ends.
64   karlSum = karlSum + i
65 end
66 
67 -- Use "100, 1, -1" as the range to count down:
68 fredSum = 0
69 for j = 100, 1, -1 do fredSum = fredSum + j end
70 
71 -- In general, the range is begin, end[, step].
72 
73 -- Another loop construct:
74 repeat
75   print('the way of the future')
76   num = num - 1
77 until num == 0
*)

karlSum = 0;
Do[karlSum = karlSum + 1, {i, 1, 100}]

fredSum = 0;
Do[fredSum = fredSum + j, {j, 100, 1, -1}]

num = 10;
Until[num == 0, Print["the way of the future"]; num = num - 1]

(*
行号  Lua代码原文
84 function fib(n)
85   if n < 2 then return 1 end
86   return fib(n - 2) + fib(n - 1)
87 end
88 
89 -- Closures and anonymous functions are ok:
90 function adder(x)
91   -- The returned function is created when adder is
92   -- called, and remembers the value of x:
93   return function (y) return x + y end
94 end
95 a1 = adder(9)
96 a2 = adder(36)
97 print(a1(16))  --> 25
98 print(a2(64))  --> 100
*)

fib[n_] := Module[{}, If[n < 2, 1, fib[n - 2] + fib[n - 1]]]

addr[x_, y_] := Module[{}, x + y]
a1 = CurryApplied[addr, 2][9];
a2 = CurryApplied[addr, 2][36];
Print[a1[16]]
Print[a2[64]]

(*
行号  Lua代码原文
143 -- Dict literals have string keys by default:
144 t = {key1 = 'value1', key2 = false}
145 
146 -- String keys can use js-like dot notation:
147 print(t.key1)  -- Prints 'value1'.
148 t.newKey = {}  -- Adds a new key/value pair.
149 t.key2 = nil   -- Removes key2 from the table.
150 
151 -- Literal notation for any (non-nil) value as key:
152 u = {['@!#'] = 'qbert', [{}] = 1729, [6.28] = 'tau'}
153 print(u[6.28])  -- prints "tau"
154 
155 -- Key matching is basically by value for numbers
156 -- and strings, but by identity for tables.
157 a = u['@!#']  -- Now a = 'qbert'.
158 b = u[{}]     -- We might expect 1729, but it's nil:
*)

t = <|key1 -> "value1", key2 -> False|>;
Print[t[key1]]
t[newKey] = Null
t[key2] = Null;

u = <|"@!#" -> "qbert", Null -> 1729, 6.28 -> "tau"|>;
Print[u[6.28]]

a = u["@!#"] (* a = "qbert" *)
b = u[Null]     (* b = 1729 *)

(*
行号  Lua代码原文
168 for key, val in pairs(u) do  -- Table iteration.
169   print(key, val)
170 end
*)

KeyValueMap[{#1, #2} &, u]

2.3. Learn … minutes要点解读

100 -- Returns, func calls, and assignments all work
101 -- with lists that may be mismatched in length.
102 -- Unmatched receivers are nil;
103 -- unmatched senders are discarded.
104 
105 x, y, z = 1, 2, 3, 4
106 -- Now x = 1, y = 2, z = 3, and 4 is thrown away.
107 
108 function bar(a, b, c)
109   print(a, b, c)
110   return 4, 8, 15, 16, 23, 42
111 end
112 
113 x, y = bar('zaphod')  --> prints "zaphod  nil nil"
114 -- Now x = 4, y = 8, values 15...42 are discarded.

从上述代码可知：Lua的赋值语句是“多退少补”。

待实习的要点：

• 元表（metatable）类似于数据模板；元方法（metamethod）则是与某个元表绑定的函数。绑定的方法是setmetatable(sometable, metatable). 钻研材料Lua 元表(Metatable)。
• 面向对象。
• 模块（Module）的搜索路径。
• 模块（Module）的编写方法。

3. Lua复刻Shell脚本工具

Shell脚本所在位置：Steam Deck 掌机催我搞技术新基建。源代码如下：

#! /bin/zsh

echo "Input parent directory of shared forlds, without tailing '/':"
read parentDirectory

sharedFolders=($(ls $parentDirectory))

for itr in $sharedFolders
    ln -s "$parentDirectory/$itr" "$HOME/$itr"

用本文所学习的lua语言对上述脚本代码重写，得到：

print("Input parent directory of shared folders, with tailing '/':")
parentDirectory = io.read()

for itr in io.popen("ls " .. parentDirectory):lines() do
	io.popen("ln -s " .. parentDirectory .. itr .. "  $HOME/" .. itr)
end

上述shell脚本和lua脚本二者实现相同的功能。但是，shell的不同软件（譬如bash和zsh）的shell语法是有区别的；另一方面，lua脚本用到的系统指令在不同Linux和shell终端是通用的，所以lua脚本工具具有更好的通用性。