[Haskell-cafe] Re: Data types, opengl display loop and readIORef/writeIORef

[bbrown]

On Mon, 08 Oct 2007 17:41:12 +0200, apfelmus wrote
> bbrown wrote:
> > This is more an aesthetics question, I have a simple opengl application 
that 
> > has a singleton like object I need to pass around to the display loop and 
> > possibly to the keyboard action handler. (pseudo code)
> > 
> > data SimpleMech = SimpleMech {
> >       mechPos         :: !MVector,
> >       mechRot         :: !MVector
> >     } deriving (Read)
> > 
> > mainMech = initMech (0, 0, 0) (0, 0, 0)
> > mainMechRef <- newIORef(mainMech)
> > 
> > displayCallback $= displayGameLoop mainMechRef
> > 
> > 
> > rotateMech :: (Double, SimpleMech) -> SimpleMech
> > rotateMech (angle, mech) =
> >   -- Calculate a new rotation
> >   let (xrot,yrot,zrot) = (mechRot mech)
> >       newyrot = newyrot + angle
> 
>                 ^^^
> That should be
>          newyrot = yrot + angle
> 
> >   in SimpleMech { mechRot = (xrot, newyrot, zrot),
> >                   mechPos = (mechPos mech) }
> > 
> > 
> > displayGameLoop mechRef = do
> > mech <- get mechRef
> > mechRef $= (rotateMech (0.1, mech))
> > 
> > ....
> > ....
> > For some reason in that code above, the "mechRef" doesnt update the value 
I 
> > want to change.  It is like I am getting an instance of the initial 
valyue 
> > back.  Also, when I use writeIORef, eg writeIORef mechRef  (rotateMech 
(0.1, 
> > mech)
> > 
> > I get stack overflow exceptions.
> 
> The bug above is probably responsible for both the stack overflows 
> with writeIORef and for a $= that doesn't update (it wants to update,
>  but the program is trapped in an infinite loop).
> 
> Btw, you don't need parenthesis about function applications, you can 
> use record update syntax and functions with several arguments are 
> usually written in curried form:
> 
>        -- Calculate a new rotation
>    rotateMech :: Double -> SimpleMech -> SimpleMech
>    rotateMech angle mech =
>       mech { mechRot = (xrot, yrot + angle, zrot) }
>       where
>          (xrot,yrot,zrot) = mechRot mech
> 
>    mainMech = initMech (0, 0, 0) (0, 0, 0)
> 
>    main = do
>       mainMechRef <- newIORef mainMech
>       displayCallback $= displayGameLoop mainMechRef
> 
>    displayGameLoop mechRef = do
>       mech <- get mechRef
>       mechRef $= rotateMech 0.1 mech
> 
> The latter can also be written as
> 
>    displayGameLoop mechRef = do
>       mechRef $~ rotateMech 0.1
> 
> Regards,
> apfelmus
> 
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That should be 
        newyrot = yrot + angle 


Thanks, I can't believe I missed that bug.  I laughed out loud when I saw 
that.

--
Berlin Brown
[berlin dot brown at gmail dot com]
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