Editing OpenGLTutorial1 (section)

==Hello World==
A minimal OpenGL program will need to load the OpenGL libraries and open a window. This is all you need to get an OpenGL program running.
This is the skeleton that we'll be building on for the rest of this tutorial:

<haskell>
import Graphics.UI.GLUT
 
main :: IO ()
main = do
  (_progName, _args) <- getArgsAndInitialize
  _window <- createWindow "Hello World"
  displayCallback $= display
  mainLoop
 
display :: DisplayCallback
display = do
  clear [ ColorBuffer ]
  flush
</haskell>

Save it to HelloWorld.hs and load it into GHCi or compile it with GHC via <code>ghc --make HelloWorld.hs</code>. When you run the program, a new blank window open with the title "Hello World" will open.

This code creates a window and sets the main display function.
<hask>getArgsAndInitialize</hask> initializes the OpenGL systems.
<hask>createWindow</hask> opens the window; its argument is the title of the window.
<hask>displayCallback</hask> controls the main display function for the current window.

We use <hask>($=)</hask> to set it to our <hask>display</hask> function.
<hask>mainLoop</hask> is where GLUT takes over, using our <hask>displayCallback</hask> to draw the contents of the window.

This defines a function <hask>display</hask> that calls a few OpenGL functions.
<hask>clear</hask> clears out the graphics color state (so we get a blank canvas).
<hask>flush</hask> pushes our OpenGL commands down to the system graphics for actual display.

===<code>displayCallback $= display</code>===
We don't call <hask>display</hask> directly. In fact, we don't call any graphics drawing functions directly. 
Instead we set a display callback, and then call <hask>mainLoop</hask>. 
In <hask>mainLoop</hask>, GLUT takes over. 
It handles all the details of interacting with the OS, refreshing our window, and calling our <hask>displayCallback</hask> to draw graphics.

<hask>displayCallback</hask> is a globally defined StateVar (mutable state variable), which we set using a call to <hask>($=)</hask>.
In the OpenGL [http://hackage.haskell.org/packages/archive/OpenGL/latest/doc/html/Graphics-Rendering-OpenGL-GL-StateVar.html StateVar] module, there is a HasSetter type class and an StateVar implementation providing functions <hask>($=)</hask> (assignment) and <hask>get</hask> to facilitate interactions with these state variables.

IORefs are StateVars, too:

<haskell>
do height <- newIORef 1.0
   currentHeight <- get height
   height $= 1.5
</haskell>