[Haskell-cafe] Re: How to do this in FP way?

[Achim Schneider]

"Magicloud" <magicloud.magiclouds at gmail.com> wrote:

> I think if I do not use a state, and the function would be called for
> many times, it would waste memory, if using something like loop,
> right?
> 
nope, at least not in general.

update :: MyState -> Int -> MyState
draw :: MyState -> IO ()

mainLoop :: MyState -> Int -> IO ()
mainLoop st old = do
	now <- getTimeOfDay
	let td = now - old
	    st' = update st td
	draw st'
	mainLoop st' now

runs in constant space. Look up "tail recursion" in wikipedia.

> -----邮件原件-----
> 发件人: haskell-cafe-bounces at haskell.org
> [mailto:haskell-cafe-bounces at haskell.org] 代表 Achim Schneider
> 发送时间: 2008年6月16日 12:01
> 收件人: haskell-cafe at haskell.org
> 主题: [Haskell-cafe] Re: How to do this in FP way?
> 
> "Magicloud Magiclouds" <magicloud.magiclouds at gmail.com> wrote:
> 
> > static int old;
> > int diff (int now) { /* this would be called once a second */
> >   int ret = now - old;
> >   old = now;
> >   return ret;
> > }
> >
> You do it with variables, of course. This is out of some GLUT code,
> using IORef's:
> 
> idle :: State -> IdleCallback
> idle state = do
>     t0 <- get $ t state
>     t1 <- get elapsedTime
>     t state $= t1
>     let td = fromIntegral t1 - fromIntegral t0
>     fps state $= 1/td * 1000
> 
>     angle' state $~! (+2)
>     
>     (bpx, bpy) <- get $ ballPos state
>     (bvx, bvy) <- get $ ballVel state
>     
>     ballPos state $= (bpx + bvx*td, bpy + bvy*td)   
>     postRedisplay Nothing
> 
> One half of all Haskell coders will tell you that mutable state isn't
> a good starting point to learn Haskell, the other half will tell you
> the same because they want to be cool kids, too. 
> 


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