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| -rw-r--r-- | lua.html.markdown | 169 |
1 files changed, 85 insertions, 84 deletions
diff --git a/lua.html.markdown b/lua.html.markdown index 32174a81..0a7c4f00 100644 --- a/lua.html.markdown +++ b/lua.html.markdown @@ -12,13 +12,15 @@ filename: learnlua.lua 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. +-- 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" @@ -58,15 +60,10 @@ 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: +-- '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 @@ -84,19 +81,20 @@ repeat num = num - 1 until num == 0 --------------------------------------------------------------------------------- + +---------------------------------------------------- -- 2. Functions. --------------------------------------------------------------------------------- +---------------------------------------------------- function fib(n) - if n < 2 then return n end + 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: + -- 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) @@ -104,9 +102,10 @@ 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. +-- 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. @@ -119,15 +118,13 @@ 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: +-- 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. @@ -136,16 +133,15 @@ local g; g = function (x) return math.sin(x) end -- 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. +-- 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: @@ -161,13 +157,14 @@ t.key2 = nil -- Removes key2 from the table. 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. +-- 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. +-- 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 @@ -187,15 +184,16 @@ 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. +-- 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. +-- 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} @@ -205,7 +203,7 @@ f2 = {a = 2, b = 3} metafraction = {} function metafraction.__add(f1, f2) - local sum = {} + sum = {} sum.b = f1.b * f2.b sum.a = f1.a * f2.b + f2.a * f1.b return sum @@ -216,9 +214,10 @@ 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. +-- 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 @@ -230,12 +229,11 @@ 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. +-- 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. +-- 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. @@ -256,19 +254,19 @@ eatenBy = myFavs.animal -- works! thanks, metatable -- __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. +-- 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. + newObj = {sound = 'woof'} -- 3. self.__index = self -- 4. return setmetatable(newObj, self) -- 5. end @@ -281,59 +279,62 @@ 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. +-- 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. +-- 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. +-- 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. + 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 +-- 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)". +-- 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 = {} + 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. +--[[ I'm commenting out this section so the rest of +-- this script remains runnable. ``` ```lua @@ -359,8 +360,8 @@ local mod = require('mod') -- Run the file mod.lua. 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. +-- 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. @@ -368,19 +369,19 @@ 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. +-- 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) +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') -- Calling f() runs mod2.lua. +f = loadfile('mod2.lua') -- Call f() to run it. -- loadstring is loadfile for strings. g = loadstring('print(343)') -- Returns a function. |