OpenWest 2014/Lua

Script up your application with Lua!

Ryan Erickson (@ryan_erickson)

"Come learn about Lua, the embeddable language that's used in everything from game scripting to Wikipedia, in mobile to web development. In this presentation, I'll give a short introduction to Lua, how it can be used to extend a C/C++ application, to allow complex scripting without losing performance. I'll also show some interesting Lua language features. Lastly, I'll give a live demonstration of the ease of embedding Lua into a C application."

Script up your application with Lua

• Ryan Erickson
• http://www.untestedhacks.com
• Works at Control4

Ryan's home automation - http://ericksonfamily.com/

The Programming Language Lua - http://www.lua.org/

Lua: getting started - http://www.lua.org/start.html

Installation:

yum install lua

curl -R -O http://www.lua.org/ftp/lua-5.2.3.tar.gz
tar zxf lua-5.2.3.tar.gz
cd lua-5.2.3
make linux test

History:

• lua.org
• created in 1993 at PUC-RIO
• Predecessors: DEL and Sol (merged to Lua)
• Lua is Portuguese for Moon, not an acronym (play on words from Sol)

Focus:

• Simplicity for non-programmer audience
• Portability
• Designed to be easily embedded, extended
• Clean ANSI C code
• Garbage Collected

Why:

• Size (tiny) < 100kb DLL/LIB
• Runs on mobile / embedded devices
• Performance - interpreted faster than Pyuthon, Ruby, Perl, PHP
  • Need faster? LuaJIT
  • LuaJIT is C++ / Java 6 territory (luajit.org)

Momentum:

• Used a lot in games
• Adobe Lightroom and Photoshop
• First interpreted lanugage allowed on iOS
• Angry Birds / World of Warcraft
• Control4 uses

Safety:

• Code runs in sandbox
• Embedder chooses which modules to expose
• Host application can provide APIs / primitives to Lua engine

Language:

• Dynamically typed
• Whitespace not significant
  • spaces, linebreaks, tabs, what you like
• Semicolons not required, and discouraged
• single line comments
• variables are global by default, 'local' keyword

Types:

• number, string, boolean, nil, table, function, userdata
• numbers are double by default
• can represent floats and integers
• no i++, i+=2. use i =i +1

Strings:

• single/double quote
• backslash to escape
• string concatenation uses '..' (not +)

nil:

• empty value
• evaluates to false
• frees item for garbage collection

tables:

• lua's single data structure
• simultaneous array and hashmap

a = {"apple", "banana"}
b = {lua = "cool", java = "sucks"}
c = a
print(c[3], b.fred, b.["java"], a.grape)
  # orange 3 sucks nul

• arrays in Lua are 1 based

Comments:

-- this is a comment
--[[ this is a multi
line comment ]]

Functions:

• first class objects

function add(a, b)
   print(a + b)
end
multiply = function(a, b) print(a * b) end
plus = add
plus(3,5)

Control:

if ... then ... elseif ... else ... End
do
  print("one")
end
for i = 1, 10 do print(i) end
for k, v in pairs({"}} ???

C Programmer Hangups:

• no curly braces - uses begin... end
• No +=, ++
• Not equals is "~="
• Not is "not"
• arrays start at 1
• #array is not always right (if empty items in the middle)

C interface API

• set of functions allow C to interact with Lua
• functions to read/write lua global variables
• functions to call lua functions
• functions to register c functions to call within lua
• stack based parameter passing

Book:

• "Programming in Lua" - http://lua.org/pil

Reference PDF:

• http://lua-users.org/wiki/LuaShortReference

Demo:

-- comment
print("test" .. os.date())

function rirc(user, message)
  local no_err, errmsg = pcall(parseIRC, user, message)
  if (no_error == false) then SendIRC("ryan", errmsg) end
end

function parseIRC(user, message)
  message = message:gsub("%s", "") -- trim white space
  if (message == "|reload") then
    if (user == "ryan") then
      dofile("default.lua")
    end
  end
  if (message == "|echo") then
      SendIRC(user, "hello")
  end
  if (message:find("|giflet") ~= nil) then
      local figstr = message:sub(8)
      local sd = io.popen("figlet " .. figstr)
      while(line) do
            SendIRC(user, line)
      end
      sd:close()
  end

end