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diff --git a/lua.html.markdown b/lua.html.markdown new file mode 100644 index 00000000..b2e96c1e --- /dev/null +++ b/lua.html.markdown @@ -0,0 +1,421 @@ +--- +language: lua +author: Tyler Neylon +author_url: http://tylerneylon.com/ +--- + +```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' + +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-prototypey 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. +``` + +```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 + +-- 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. + +--]] + +---------------------------------------------------- +-- 5. 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, this 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! + +--]] +``` |