2023-02-05

Lua Tutorial, Example, Cheatsheet

Lua is one of the most popular embeddable language (other than Javascript that already embedded in browser), there's a lot of products that embed lua as it's scripting language (probably because the language and the C-API is simple, VM size is small, and there's a product called LuaJIT that is currently the fastest JIT implementation for scripting language. Current version of Lua is 5.4, but LuaJIT only support 5.1 and 5.2 partially. There's a lot of products that are built having Lua embedded inside:

Variables, Data Types, Comments, basic stdlib Functions

 
Today we're going to learn a bit about Lua syntax, to run a lua script just run lua yourscript.lua on terminal or use luajit also fine:

--[[ this is multiline comment
  types in Lua: boolean, number, string
    table, function, nil, thread, userdata
]]
-- this is single line comment, just like SQL

-- create variable
local foo = 'test' -- create a string, can escape \ same as with ""
local bar = 1 -- create a number

-- random value
math.randomseed(os.time()) -- set random table
math.random() -- get random float
math.random(10) -- get random int 0-9

-- print, concat
print(#foo) -- get length of string, same as string.len(foo)
foo .. 123 -- concat string, will convert non-string to string
print(1 + 2 ^ 3 * 4, 5) -- print 33 5 separated with tab

-- type and conversion
type(bar) -- get type (number)
tostring(1) -- convert to string
tonumber("123") -- convert to number

-- multiline string
print([[foo
bar]]) -- print a string, but preserve newlines, will not escape \

-- string functions
string.upper('abc') -- return uppercased string
string.sub('abcde', 2,4) -- get 2nd to 4th char (bcd)
string.char(65) -- return ascii of a byte
string.byte('A') -- return byte of a character
string.rep('x', 5, ' ') -- repeat x 5 times separated with space
string.format('%.2f %d %i', math.pi, 1, 4) -- smae as C's sprintf 
start, end = string.find('haystack', 'st') -- find index start-end of need on haystack, nil if not found, end is optional
string.gsub('abc','b','d') -- global substitution

Decision and Loop

There's one syntax for decision (if), and 3 syntax for loop (for, while-do, repeat-until) in Lua:

-- decision
if false == true then
  print(1)
elseif 15 then -- truthy
  print( 15 ~= 0 ) -- not equal, in another language: !=
else
  print('aaa')
end

-- can be on single line
if (x > 0) or (x < 0) then print(not true) end

-- counter loop
for i = 1, 5 do print(i) end -- 1 2 3 4 5
for i = 1, 4, 2 do print(i) end -- 1 3
for i = 4, 1, -2 do print(i) end -- 4 2

-- iterating array
local arr = {1, 4, 9} -- table
for i = 1, #arr do print(arr[i]) end

-- top-checked loop
while true do
  break -- break loop
end

-- bottom-checked loop
repeat
until false

IO, File, and OS

IO is for input output, OS is for operating system functions:

-- IO
local in = io.read() -- read until newline
io.write('foo') -- like print() but without newline
io.output('bla.txt') -- set bla.txt as stdout
io.close() -- make sure stdout is closed
io.input('foo.txt') -- set foo.txt as stdin
io.read(4) -- read 4 characters from stdin
io.read('*number') -- read as number
io.read('*line') -- read until newline
io.read('*all') -- read everything
local f = io.open('file.txt', 'w') -- create for write, a=append, r=read
f:write('bla') -- write to file
f:close() -- flush and close file

-- OS
os.time({year = 2023, month = 2, day = 4, hour = 12, min = 23,sec  = 34})
os.getenv('PATH') -- get environtment variables value
os.rename('a.txt','b.txt') -- rename a.txt to b.txt
os.rename('a.txt') -- erase file
os.execute('echo 1')  -- execute shell command
os.clock() -- get current second as float
os.exit() -- exit lua script

Table

Table is combination of list/dictionary/set/record/object, it can store anything, including a function.

-- as list
local arr = {1, true, "yes"} -- arr[4] is nil, #arr is 3

-- mutation functions
table.sort(arr) -- error, cannot sort if elements on different types
table.insert(arr, 2, 3.4) -- insert on 2nd position, shifting the rest
table.remove(arr, 3) -- remove 3rd element, shifting remaining left

-- non mutating
table.concat(arr, ' ') -- return string with spaces

-- nested
local mat = {
  {1,2,3},
  {4,5,6}, -- can end with comma
}

-- table as dictionary and record
local user = {
  name = 'Yui',
  age = 23,
}
user['salutations'] = 'Ms.' -- or user.salutations

Function

Function is block of code that contains logic.

-- function can receive parameter
local function bla(x)
  x = x or 'the default value'
  local y = 1 -- local scope variable
  return x
end

-- function can receive multivalue
local function foo()
  return 1,2
end
local x,y = foo()
local x = foo() -- only receive first one

-- function with internal state
local function lmd()
   local counter = 0
   return function()
     counter = counter + 1
    return counter
  end
end
local x = lmd()
print(x()) -- 1
print(x()) -- 2

-- variadic arguments
local function vargs(...)
  for k, v in pairs({...}) do print(k, v) end
end
vargs('a','b') --  1 a, 2 b 

Coroutine

Coroutine is resumable function

local rot1 = coroutine.create(function()
   for i = 1, 10 do
    print(i)
    if i == 5 then coroutine.yield() end -- suspend
   end
end)
if coroutine.status(rot1) == 'suspended' then
   -- running, [suspended], normal, dead
  coroutine.resume(rot1)  -- will resume after yield
end

Module

To organize a code, we can use module:

-- module, eg. mod1.lua
_G.mod1 = {}
function mod1.add(x,y)
  return x + y
end
return mod1

-- import a module
local mod1 = require('mod1')
local x = mod1.add(1,2)

Basic OOP

To simulate OOP-like features we can do things like in Javascript, just that to call a method of object we can use colon instead of dot (so self parameter will be passed):

-- constructor
local function NewUser(name, age)
  return {
    name = name,
    age = age,
    show = function(self) -- method example
      print(self.name, self.age)
    end
  }
end
local me = NewUser('Tzuyu', 21)
me:show() -- short version of: me.show(me)

-- inheritance, overriding
local function NewAdmin(name, age)
  local usr = NewUser(name, age)
  usr.isAdmin = true
  usr.setPerm = function(self, perms)
     self.perms = perms
  end
  local parent_show = usr.show
  usr.show = function(self)
    -- override parent method
    -- parent_show(self) to call parent
  end
  return usr
end
local adm = NewAdmin('Kis', 37)
adm:setPerm({'canErase','canAddNewAdmin'})
adm:show() -- does nothing

-- override operator
local obj = {whatever = 1}
setmetatable(obj, {
   __add = function(x,y)  -- overrides + operator
    return x.whatever + tonumber(y)
   end,
  -- __sub, __mul, __div, __mod, __pow,
  -- __concat, __len, __eq, __lt, __le, __gt, __ge
})
print(obj + "123")

For more information about modules you can visit https://luarocks.org/ 

For more information about the syntax you can check their doc: https://devdocs.io/lua~5.2/ or https://learnxinyminutes.com/docs/lua/ or this 220 minutes video

But as usual, don't use dynamic typed language for large project, it would be pain to maintain in the long run, especially if the IDE cannot jump properly to correct method or cannot suggest correct type hints/methods to call

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