piperchester · June 25, 2013 16:26 · Jun 24, 2013 · Jun 24, 2013
diff --git a/learn.lua b/learn.lua
@@ -117,7 +117,8 @@ 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
+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.
 

diff --git a/learn.lua b/learn.lua
@@ -0,0 +1,410 @@
+-- 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 = function (x) return math.sin(x) end
+
+-- 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.
+-- 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 Löve 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!
+
+--]]
+
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