[Haskell-cafe] How to implement a digital filter, using Arrows?

John Lask jvlask at hotmail.com
Thu Oct 20 00:02:19 CEST 2011

On 20/10/2011 5:11 AM, Captain Freako wrote:

for your use case then, the StateArrow seems more appropriate as it 
provides you with the final state. Ofcourse the Automaton arrow could 
also be used:

 > liftAu' f s0 = proc x -> do
 >    rec (y,s') <- arr f -< (x,s)
 >        s <- delay s0 -< s'
 >    returnA -< (y,s)

this explicitly returns the state with evry value. However, you then 
need to manually thread the state through.

note: the arrow "plumbing" is only used when you start composing 
filters, with arrow combinators. If your not doing any composition as
such then lifting your functions into the "arrows" is not buying you much.

> John Lask wrote:
>> This is literate code. It expounds on your initial question and provides
>> two solutions based either on the StateArrow or Automaton....
>> (Remainder omitted.)
> John,
> Thanks so much for your help!
> I'm going to study your example code and try to understand how the
> Automaton implicit plumbing is being used.
> (I'm determined to get my head around this arrow concept, damn it! ;-) )
> I wonder if I might pester you for the final piece of the puzzle here:
> When the processing of the input stream through the filter is
> complete, I need to pass the final filter state, along with the output
> stream, back to the calling program.
> (This is because the calling program is going to split the input
> signal into conveniently sized chunks, calling my filter routine
> several times, in order to process the entire input stream. It expects
> to be able to pass an initial filter state into my routine, which is
> just the final state of the filter that I passed back to it after the
> previous call. In this way, we avoid inserting any edge artifacts at
> the input stream segmentation boundaries.)
> So, is it possible to `fetch' the final state out of that Automaton,
> when it's finished processing the input list?
> Thanks!
> -db

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