[Haskell-cafe] An ugly zip3 problem..

[Michael Feathers]

Thanks.  That's interesting (but a little beyond me).  Seems like he's 
assuming that values beyond his range are zero whereas I'm trying to use 
the values at the edges of the range.

Is there anything I can do before I understand comonads? ;-)

Michael

Dan Weston wrote:
> There's an interesting blog post by Dan Piponi on the subject:
> 
> http://sigfpe.blogspot.com/2007/01/monads-hidden-behind-every-zipper.html
> 
> Summary: "convolution is comonadic"
> 
> Dan
> 
> 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.
>>
>> Thanks,
>>
>> Michael Feathers
>>
>>
> 
> 
> 


-- 
Now Playing: http://www.youtube.com/watch?v=SsnDdq4V8zg