[Haskell-cafe] Looking for a more functional way to do this

[Adrian Neumann]

There is the State Monad which is build just for that kind of purpose, I 
believe:

http://www.haskell.org/all_about_monads/html/statemonad.html

That would safe you from passing around the State

Jefferson Heard schrieb:
> Working with HOpenGL and GLUT, I find myself approaching a common
> problem with a common solution that I don't really like all that much,
> as it reeks of procedural programming.  Basically the problem is that
> of complex program state, such that when the user provides input to
> the program in the form of a mouse click or a typed string or
> character, the program updates its internal state to reflect this,
> whether that's changing the rotation, scale, or position of a screen
> element, or deciding what data to have loaded from disc.
> 
> What I often do is something that looks like this:
> 
> data ProgramState  = ProgramState {
>     some_associative_data :: Map String String
>   , position :: GL.Vector3 Float
>   , look_at :: GL Vector3 Float
>   , selectables :: Map GLuint NamedObject
>   }
> 
> render :: IORef ProgramState -> IO ()
> render state = do
>   st <- readIORef state
>   ...
> 
> handleMouseClicks :: IORef ProgramState -> GLUT.KeyboardMouseHandler
> handleMouseClicks state ... = do
>   st <- readIORef state
>   ...
> 
> main = do
>   ...
>   let st = ProgramState { Map.empty ... }
>       render' = render st
>       mouse' = handleMouseClicks st
> 
>   GLUT.renderCallback $= render
>   GLUT.keyboardMouseCallback $= Just mouse'
> 
> and so on and so forth.   Generally there are not fewer than 5 and not
> more than about 32 variables that I have to track between mouse
> clicks, and there's all this boilerplate code as well.  I'm searching
> for a better way to do this, and I feel sure there is one.  I'm
> considering using Template Haskell or possibly SYB to generate this
> code, but it also seems like I also ought to be able to declare some
> kind of state monad or continuation monad that can encapsulate
> ProgramState without having to declare an explicit structure for it
> everytime.
> 
> For one thing, I'd like to genericize this code and write something
> akin to Processing for Haskell (http://www.processing.org).
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