[Haskell-cafe] Patterns for processing large but finite streams

[Eugene Kirpichov]

I meant the average of the whole list - given a sumS and lengthS ("S"
for "Stream"), write meanS as something like liftS2 (/) sumS lengthS.

Or is that possible with lazy lists too?

(looks like arrows actually - which arrow is appropriate here?)

2011/7/1 Malcolm Wallace <malcolm.wallace at me.com>:
> Sure you can.
>
> runningAverage :: Int -> [Double] -> [Double]
> runningAverage n xs = let chunk = take n xs
>                      in (sum chunk / length chunk) : runningAverage (tail xs)
>
> Lazy lists are absolutely ideal for this purpose.
> Regards,
>    Malcolm
>
> On 1 Jul 2011, at 07:33, Eugene Kirpichov wrote:
>
>> Plain old lazy lists do not allow me to combine multiple concurrent
>> computations, e.g. I cannot define average from sum and length.
>>
>> 2011/7/1 Heinrich Apfelmus <apfelmus at quantentunnel.de>:
>>> Eugene Kirpichov wrote:
>>>>
>>>> I'm rewriting timeplot to avoid holding the whole input in memory, and
>>>> naturally a problem arises:
>>>>
>>>> How to represent large but finite streams and functions that process
>>>> them, returning other streams or some kinds of aggregate values?
>>>>
>>>> Examples:
>>>> * Adjacent differences of a stream of numbers
>>>> * Given a stream of numbers with times, split it into buckets by time
>>>> of given width and produce a stream of (bucket, 50%,75% and 90%
>>>> quantiles in this bucket)
>>>> * Sum a stream of numbers
>>>>
>>>> Is this, perhaps, what comonads are for? Or iteratees?
>>>
>>> Plain old lazy lists?
>>>
>>>
>>> Best regards,
>>> Heinrich Apfelmus
>>>
>>> --
>>> http://apfelmus.nfshost.com
>>>
>>>
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>>
>>
>>
>> --
>> Eugene Kirpichov
>> Principal Engineer, Mirantis Inc. http://www.mirantis.com/
>> Editor, http://fprog.ru/
>>
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>
>



-- 
Eugene Kirpichov
Principal Engineer, Mirantis Inc. http://www.mirantis.com/
Editor, http://fprog.ru/