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

[Stefan O'Rear]

On Tue, Jan 08, 2008 at 09:10:59PM +0100, Achim Schneider wrote:
> Don Stewart <dons at galois.com> wrote:
> 
> > 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)
> > 
> Yes, you see, that was my first Haskell program bigger than 20 lines,
> there's no possibility to get the state out of the IO Monad, at least
> without writing a high-level interface to glut and gl, and then there's
> this thing that _every_ game works with input callbacks, one, global,
> state update function (where it doesn't _really_ matter whether you're
> passing and returning a state or updating a state) and one function
> that translates the state into some graphics representation.
> 
> That said, I think it's not very Haskell-like to do something elegantly
> in 1000 lines when you can do it in 100 lines and still have it look
> nicer than C.

I would use IORef State.  Making illegal states unrepresentable greatly
helps with code prettiness; the original State allowed internal
aliasing, which is quite definitely silly.

I think this should be written somewhere as a general rule - when you
have a mutable structure, use a reference to a record, not a record of
references.

Stefan
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