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

[Erik Hesselink]

This sound exactly like what attribute grammars, like the system
developed at Utrecht University [1], are useful for.

Erik

[1] http://www.cs.uu.nl/wiki/HUT/AttributeGrammarSystem

On Fri, Jul 1, 2011 at 10:54, Eugene Kirpichov <ekirpichov at gmail.com> wrote:
> 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.skladnoy at gmail.com>:
>> On Fri, Jul 1, 2011 at 12:21 PM, Eugene Kirpichov <ekirpichov at 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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