[Haskell-cafe] An ugly zip3 problem..
westondan at imageworks.com
Thu Mar 20 22:07:05 EDT 2008
There's an interesting blog post by Dan Piponi on the subject:
Summary: "convolution is comonadic"
Michael Feathers wrote:
> I'm working on something and it's looking rather ugly.. essentially, it
> it's an application of a low pass filer to a dataset.
> type Dataset = [Double]
> type FilterWindow3 = (Double,Double,Double)
> internalList :: [a] -> [a]
> internalList = tail . init
> lowPass3 :: FilterWindow3 -> Double
> lowPass3 (i, j, k) = (i + 2 * j + k) / 4.0
> filter3 :: (FilterWindow3 -> Double) -> Dataset -> Dataset
> filter3 f3 ds = [(f3 x) | x <- formWindows ds]
> iterFilter :: (Dataset -> Dataset) -> Int -> Dataset -> Dataset
> iterFilter f n ds
> | n > 0 = iterFilter f (n - 1) (f ds)
> | otherwise = ds
> smooth :: Int -> Dataset -> Dataset
> smooth = iterFilter $ filter3 lowPass3
> formWindows :: Dataset -> [FilterWindow3]
> formWindows ds =
> internalList $ zip3 x y z
> where c0 = [head ds]
> cn = [last ds]
> x = ds ++ cn ++ cn
> y = c0 ++ ds ++ cn
> z = c0 ++ c0 ++ ds
> The key idea is that I can take care of edge conditions with that last
> function. It lets me build a list of 3-tuples, each of which is reduced
> to a single point in the next rewrite of the dataset. I used zip3 to
> build up that list, and I take care to keep the lists the same length by
> duplicating the head and last elements as necessary. Has anyone done
> this sort of thing before?
> Any and all style advice welcome.
> Michael Feathers
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