Lua (programming language)

Lua (/ˈluːə/ LOO-ə; from Portuguese: lua [ˈlu(w)ɐ] meaning moon) is a lightweight, high-level, multi-paradigm programming language designed primarily for ==embedded== use in applications. Lua is cross-platform, since the interpreter of compiled bytecode is written in ANSI C, and Lua has a relatively simple C API to embed it into applications.
— Wikipedia

Learn Lua in Y minutes

Source: Learn Lua in Y Minutes.

-- 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  -- Numbers can be integer or floating point.
 
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.
 
--]]