Editing WoW:AddOn programming tutorial/Introduction (section)

=== The .lua or Lua code file ===
The .lua files are where the main "what to do" instructions for the add-on reside. You will see a variety of terms for this such as logic and executable code (or simply "code"). Lua logic, or executable code or a script, does its thing in response to something that happens in the game. Things that happen in the game are called Events.

==== Events ====
There are two basic kinds of events. The first is when something happens in the game. This might be somebody saying something, something happening to your character, or another character's stats changing. Nearly everything that happens in the game causes events.

The second kind of event is when you do something to a UI Element (a UI Element is something on the screen and is affectionately called a widget. We'll get to that more in the next section). This second kind of event might be clicking on something in your bags or button bar. There is a technical difference between the two types of events, and we will discuss that as the tutorial progresses. For more detail on the first kind of event see [[Events (API)]] and for the second see [[Widget handlers]].

These concepts are extremely important because absolutely nothing happens in the game except in response to an event. Furthermore, should you happen to write a piece of code that runs for an extended time (perhaps forever), absolutely nothing new will happen in the game. Your screen will be frozen and nothing will move. That would be classified as "not good".

So, how do you tell WoW that you are interested in a particular event? There are two ways: first, you can tell WoW which code to run when a particular event happens. This is called registering your event. Second, you can tell the XML to run a piece of code when a UI Element is manipulated (such as clicking on it or moving your mouse over it). Pieces of code that run in response to events are called ''functions''.

==== Functions ====
Functions are groupings of code that accomplish a specific purpose. There are numerous predefined functions provided by WoW (called API functions), or you can make your own user-defined functions. While there are multiple ways to create functions in Lua, the easiest to understand 
looks like this:

 <local> function ''function_name''(<zero or more arguments>)
    ... code ...
 end

*The <local> is an optional keyword that limits the visibility of the function to a particular scope. Scope will be covered in more depth shortly.
*The ''function_name'' is simply a name you make up so you can reference your function from other parts of your AddOn.
*The <zero or more arguments> are ways to pass information into the function. This is what gives functions their power. Each time you call the function, you can supply a different set of arguments and get different results based upon them.
*The ... code ... is where the work gets done in a function. Here is where you do calculations, comparisons, call other functions, etc. to get the task of the function done.
*The end simply marks the end of the definition of the function.

Note that this only defines or declares the function. The function is not actually run until some other piece of code references (or calls) it.

For more information on functions, see the [http://www.lua.org/manual/5.1/ Lua 5.1 Reference Manual] or [http://www.lua.org/pil/ Programming in Lua (first edition)]. Also see the [[Lua]] page which lists more Lua resources.

==== HelloWorld.lua ====
Now to continue with our Hello, World code example. Place the following into your HelloWorld.lua file and save it:

 function HelloWorld() 
   print("Hello, World!"); 
 end

You should understand everything in here by now. This function is named HelloWorld and it has zero arguments. The code part is simply the 'print("Hello, World!");' portion. And it ends with "end".

This is a fine piece of code, but by itself is useless unless something calls the function. Onward to UI Elements (aka Widgets).