[Haskell-cafe] Re: Is there anyone out there who can translate C# generics into Haskell?

[Don Stewart]

jules:
> Achim Schneider wrote:
> >things like
> >
> >
> >data State = State 
> >    { winSize   :: IORef Size
> >    , t         :: IORef Int
> >    , fps       :: IORef Float
> >    , showFPS   :: IORef Bool
> >    , showHelp  :: IORef Bool
> >    , grabMouse :: IORef Bool
> >    , mousePos  :: IORef (Maybe Position)
> >    , mouseDelta :: IORef Position
> >    , viewRot   :: IORef Vec3
> >    , angle'    :: IORef GLfloat
> >    , ballPos   :: IORef Vec2
> >    , ballVel   :: IORef Vec2
> >    }
> 
> 
> Yuck!
> 
> I'm not sure whether this is a real example or not, but if it's real, 
> get rid of all those IORefs. Make State a simple type, and use (IORef 
> State) as needed for callbacks, and hide that fact in other code.

I agree, this is very-unHaskelly :)

The State type should be a simple purely functional structured, threaded
through your code via a StateT or some such. Not a bunch of pointers
in IO.

See xmonad for examples of this in highly effectful programs,

    http://code.haskell.org/xmonad/XMonad/Core.hs

    newtype X a = X (ReaderT XConf (StateT XState IO) a)

(Carries read-only and updatable state components)

-- Don