On Mon, 2008-06-16 at 10:19 +0200, Ketil Malde wrote:
> "Magicloud Magiclouds" <[EMAIL PROTECTED]> writes:
>
> > static int old;
> > int diff (int now) { /* this would be called once a second */
> > int ret = now - old;
> > old = now;
> > return ret;
> > }
>
> > Because there is no "variable" in Haskell. So how to do this in a FP
> > way?
>
> I would claim the FP way is like this:
>
> -- | Repeatedly subtract values from a baseline, returning a list
> -- containing each intermediate result
> diff :: Int -> [Int] -> [Int]
> diff = scanl (-)
>
> Prelude> diff 100 [1..10]
> [100,99,97,94,90,85,79,72,64,55,45]
Better yet, you could create a recursive type that reflects what you are
actually doing:
newtype Step a = Step (a -> (a, Step a))
diff' :: Int -> Step Int
diff' x = Step (\a -> let r = a - x in (r, diff' r))
This way it will be easier to resume previous diff computations.
Best,
Michał
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