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

"Magicloud" 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@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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