# cheat-sheet-lua

Here is the IO-Project cheat sheet to quickly learn the "Lua" programming language.
I suggest you to consult the documentation of lua for more information on the language, it is available by following [this link](https://www.lua.org/manual/5.3/).

## Table of contents

Use this table of contents to travel more easily through this cheat sheet.

- [cheat-sheet-lua](#cheat-sheet-lua)
  - [Table of contents](#table-of-contents)
  - [Basics](#basics)
    - [Code comments](#code-comments)
    - [Variables and loop](#variables-and-loop)
    - [Functions](#functions)
  - [Advanced](#advanced)
    - [Tables, Array, dict..](#tables-array-dict)
      - [Metatables and metamethods](#metatables-and-metamethods)
      - [Class-like tables and inheritance.](#class-like-tables-and-inheritance)
    - [Coroutine](#coroutine)
    - [Error handling](#error-handling)
    - [Modules](#modules)

## Basics

> We'll now introduce the basics of lua, starting with comments, variables, loops and functions.

### Code comments

````lua
-- Two dashes start a one-line comment.

--[[
     by adding two "[" opening and two "]" closing,
     you define a multi-line commentary. ;-)
--]]
````

### Variables and loop

Introduction to variables, basic conditions, some loops ([examples available here](support/while.lua)) and the equivalent of the ternary operator.

````lua
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 in 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 ==; 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 false; 0 and '' are true.
if not aBoolValue then print('that 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
````

### Functions

Introduction to function definition, recursion with lua and chain assignment followed by closure function.

````lua
-- The famous Fibonacci sequence.
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.

print(type(x))  --> number
-- 'type()' function allow to detemindthe  type of a variable.

-- '...' is an elipse parameter, retrievable in context by '...'.
function e(...) print(...) end
e(2, 4, 8, 6)  --> prints "2    4    8    6"

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

-- (local) 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.
````

## Advanced

> Let's move on to more advanced notions. With the notion of table, class, module, coroutine, meta-programming and module.

### Tables, Array, dict..

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

````lua
-- 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.
````

#### Metatables and metamethods

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

````lua
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 in the section 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.
-- Main list. Here is a table with the metamethods.

-- __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(...)
````

#### Class-like tables and inheritance.

Classes aren't built in, there are different ways to emulate them with tables and metatables.
The different ways to define a class in Lua are not easy to understand, so I suggest you to look at the [following document](support/class.lua) implementing 3 types of class definition. The last one being the one I chose (my preferred method).

````lua
-- 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
````

### Coroutine

Let's turn now to coroutines. Coroutines are functions that can be suspended and resumed at a later time. They are used to implement iterators, generators and event loops and represent a line of execution with its own stack. In other words, they can be compared to threads.

````lua
-- Create a coroutine that prints 'Hello' and then stops.
coHi = coroutine.create(function () print('Hello') end)
print(coHi)  -- thread: 0x7f9c0c00a0c0

-- Coroutine status can be 'suspended', 'running' or 'dead'.
-- The coroutine is created in the 'suspended' state.

-- Resume the coroutine. It will print 'Hello' and stop.
coroutine.resume(coHi)  -- return 'true'
print(coroutine.status(coHi))  -- 'dead'

-- We can also pass arguments to the coroutine.
-- The arguments of the first resume are passed to the
-- function of the coroutine. The following arguments
-- are passed to the yield function.
routine = coroutine.create(function (a, b, c)
  print('first print: ', a, b, c)
  print('yield1: ', coroutine.yield())
  print('yield2: ', coroutine.yield('a variable'))
  return(a+b+c)
end)

-- will run the coroutine until the first yield.
coroutine.resume(routine, 1, 2, 3)
-- run the coroutine until the second yield passing
-- the arguments 4, 5 and 6 to the 1er yield and
-- retrieve the return value of the second yield.
print('out routine: ' , coroutine.resume(routine, 4, 5, 6))
-- this will run out the second yield and made the
-- adition of 'a+b+c'  and kill the coroutine.
print(coroutine.resume(routine, 7, 8, 9)) -- 1+2+3 = '6'

-- All these steps will print:
--[[
    first print:    1       2       3
    yield1:         4       5       6
    out routine:    true    a variable
    yield2:         7       8       9
    true    6
--]]
````

### Error handling

Lua allows low-level error handling with the `error` function and high-level error handling with the `assert` function. The `error` function raises an error and handles it with the `pcall` or `xpcall` function. The `assert` function checks a condition and raises an error if the condition is not met.

````lua



-- Throw an error if the first argument is false
-- the second argument is the error message.
assert(type(firstvariable) = 'string', 'not a string')

-- TODO to continue

````

### Modules

Modules are a way to organize your code. They are a way to group functions and variables together in a single file. You can then use the module in other files by using the `require` function.

````lua
-- 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  -- Return the table 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()  -- Says hello to 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.

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

````