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

16 Jun 2008

      But if there are some beautiful arithmetics that do not use something like
state, I won't say no to them.

-----邮件原件-----
发件人: haskell-cafe-bounces@haskell.org
[mailto:haskell-cafe-bounces@haskell.org] 代表 Achim Schneider
发送时间: 2008年6月16日 12:01
收件人: haskell-cafe@haskell.org
主题: [Haskell-cafe] Re: How to do this in FP way?

"Magicloud Magiclouds"  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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