[Haskell-beginners] OpenGL keyboardMouseCallback ...confused over type

[Sean Charles]

I asked a question the other day about passing application state around 
an OpenGL program and I've consulted RWH and the inter-web extensively 
and I think I am on the right lines so far!

In my main program, I use Data.IORef like this:

     gameState <- newIORef $ newGameState

and GameState contains all the stuff to do with the game model. Next the 
confusing bit... to make myself learn things I typed out the full type 
of the keyboardMouseCallback function:

     myKeyMouse :: Key -> KeyState -> Modifiers -> Position -> IO ()

and slotted it into the code like so:

     keyboardMouseCallback $= Just myKeyMouse

In order to be able to pass the "IORef GameState" into the keyboard 
handler I have to re-type my keyboardMouseCallback function like so:

     myKeyMouse :: IORef GameState -> Key -> KeyState -> Modifiers -> 
Position -> IO ()

and modify the assignment in main to this:

     keyboardMouseCallback $= Just (myKeyMouse gameState)

But I haven't really understood *why* that is the case. The 
displayCallback is also similarly affected as it is now:

     myDisplay :: IORef GameState -> IO ()
     ... displayCallback $= (myDisplay gameState)

and again I am not sure what magic is happening here to have made it 
compile and work! My real source of confusion is over my apparent 
victory over the type system in that I appear to have extended the type 
signature (is that the correct term?) for both of the callbacks be 
prepending "IORef GameState" to them and yet it all compiles and works. 
Is "prepending" the key ? I feel a tingling of Eureka! organ but I am 
not sure yet.

Explanations are welcome as I am sure there is something "important" 
here that will really advance my understanding of the type system. It 
smells like curry but I don't think... hang on... the keyboard one is 
wrapped in the Maybe monad... therefore(?) the function I am handing in 
can be *any* type so long as they actually match types! Is that it then? 
I can smell partial application and curry now.
HELP!
Thanks,
Sean.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20110630/c3ef26dc/attachment.htm>