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

16 Jun 2008

      On Mon, 2008-06-16 at 10:19 +0200, Ketil Malde wrote:
...
"Magicloud Magiclouds"  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

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

Michał Pałka