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

[Achim Schneider]

"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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