learnxinyminutes-docs/lua.html.markdown

language	contributors	filename
Lua	Tyler Neylon http://tylerneylon.com/	learnlua.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('twas 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'

-- BEWARE: this only acts as a ternary if the value returned when the condition 
-- evaluates to true is not `false` or Nil
iAmNotFalse = (not aBoolValue) and false or true --> true
iAmAlsoNotFalse = (not aBoolValue) and true or false --> true

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 n 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 = function(x) return math.sin(x) end
-- Equivalent to local function g(x)..., except referring to g in the function
-- body won't work as expected.
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.

-- Calls with one table param don't need parens either (more on tables below):
print {} -- Works fine too.

--------------------------------------------------------------------------------
-- 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
-- behaviour. Later we'll see how metatables support js-prototype behaviour.

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)
  local 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.
  local 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" is 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()
  local 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 = Dog". So
--    seymour.key will equal 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()
  local 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()  -- 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')  --> Hi!
dofile('mod2')  --> Hi! (runs again, unlike require)

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

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

--]]

References

I was excited to learn Lua so I could make games with the Love 2D game engine. That's the why.

I started with BlackBulletIV's Lua for programmers. Next I read the official Programming in Lua book. That's the how.

It might be helpful to check out the Lua short reference on lua-users.org.

The main topics not covered are standard libraries:

• string library
• table library
• math library
• io library
• os library

By the way, the entire file is valid Lua; save it as learn.lua and run it with "lua learn.lua" !

This was first written for tylerneylon.com, and is also available as a github gist. Have fun with Lua!