Alexey, your definition of "mean" does not look like "liftS2 (/) sum
length" - you have to manually "fuse" these computations.
I'm asking for a formalism that does this fusion automatically (and
guaranteedly).
2011/7/1 Alexey Khudyakov <alexey.sklad...@gmail.com>:
> On Fri, Jul 1, 2011 at 12:21 PM, Eugene Kirpichov <ekirpic...@gmail.com>
> wrote:
>> 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?
>>
> Sure you can. Sum, length and mean could be calculated as left
> fold. If you need to calculate more that one statistic at time you
> can combine accumulators
>
>> sum = foldl (+) 0
>> length = foldl (\n _ -> n+1) 0
>> data Mean Double Int
>> mean = foldl (\(Mean m n) x -> Mean (m + (x - m) / fromIntegral (n+1))
>> (n+1)) (Mean 0 0)
>
> AFAIU iteratees basically use same technique.
>
--
Eugene Kirpichov
Principal Engineer, Mirantis Inc. http://www.mirantis.com/
Editor, http://fprog.ru/
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