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

[Achim Schneider]

Stefan O'Rear <stefanor at cox.net> wrote:

> On Tue, Jan 08, 2008 at 09:10:59PM +0100, Achim Schneider wrote:
> > 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.
> 

So it would be, instead of 

---$<---

idle :: State -> IdleCallback
idle state = do    
    (bpx, bpy) <- get $ ballPos state
    (bvx, bvy) <- get $ ballVel state
    
    ballPos state $= (bpx + bvx*td, bpy + bvy*td)   

---$<---

type StateRef = IORef State'

idle' :: StateRef -> IdleCallback
idle' st = do
    state <- get st
    let (bpx, bpy) = ballPos' state
        (bvx, bvy) = ballVel' state

    st $= state {ballPos' = (bpx+bvx, bpy+bvy)}

---$<---
or, while I'm at it
---$<---

moveBall :: State' -> State'
moveBall state =  state {ballPos' = (bpx+bvx, bpy+bvy)}
    where (bpx, bpy) = ballPos' state
          (bvx, bvy) = ballVel' state

idle'' :: StateRef -> IdleCallback
idle'' st = st $~ moveBall

---$<---

With the multiple IORef-Model, moveBall looks like this:

moveBall :: Vec2 -> Vec2 -> Vec2
moveBall (bpx, bpy) (bvx,bvy) = (bpx+bvx, bpy+bvy)

which is IMHO pure Haskell.

On the other hand, with the one IORef-Model, the draw function could
not possibly mangle any state, which would be a good thing, doing such
things can result in keyboards being smashed over heads by team
colleagues.

I'll think about it and then let you know how to properly tackle the
awkward GLpong squad.

Generally, in game programming you can have so many state-related bugs
that it's infeasible to detect them all statically. You also cheat
much, doing stage magic instead of the real stuff, and trading off
things all the time.

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