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

[Arlen Cuss]

> confusing bit... to make myself learn things I typed out the full type
> of the keyboardMouseCallback function:

(you'll actually do this in production code too! It's a good habit.)

>     myKeyMouse :: Key -> KeyState -> Modifiers -> Position -> IO ()
> 
> and slotted it into the code like so:
> 
>     keyboardMouseCallback $= Just myKeyMouse

Okay -- so here we're seeing that keyboardMouseCallback itself is an IORef:

> keyboardMouseCallback :: IORef (Maybe (Key -> KeyState -> Modifiers ->
Position -> IO ()))

Keep in mind also:

> ($=) :: IORef a -> a -> IO ()

> 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)

Now let's look at the reason it works.

Your types are:

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

Now, what's the type of the expression `myKeyMouse gameState'? Well,
you've assigned the first argument -- all functions are curried -- so
you get a partial application:

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

Now you wrap that in Just:

> Just (myKeyMouse gameState) :: Maybe (Key -> KeyState -> Modifiers ->
Position -> IO ())

What was the type of keyboardMouseCallback again?

> keyboardMouseCallback :: IORef (Maybe (Key -> KeyState -> Modifiers ->
Position -> IO ()))

So the partial application returns a function with the first argument
fixed -- and the resulting function's type is that which is used for the
callbacks.

HTH a little.

A