Re: [Haskell-cafe] Re: Grouping - Map / Reduce

Dear Günther, the map can't be consumed while it is constructed. At any point during its construction you don't know for any key in the map if it will appear in the not cosumed rest of the list again. This means you can't process any entry of the map because it may change later. The only point when nothing will change anymore is when the map is completely constructed. Regards, Martin. GüŸnther Schmidt schrieb:

Hi Ketil,

Ketil Malde schrieb:

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"Gü?nther Schmidt" writes:

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let say I got an unordered lazy list of key/value pairs like

[('a', 99), ('x', 42), ('a', 33) ... ]

and I need to sum up all the values with the same keys.

So far I wrote a naive implementation, using Data.Map, foldl and insertWith.

Data.Map.fromListWith (+)

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The building of this map is of course a bottleneck, the successive processing needs to wait until the entire list is eventually consumed the Map is built and flattened again.

Sure this is not an artifact of the laziness of foldl?

well I can't really see how the map could be consumed *while* it's still being built, I just don't see it. (I'm using foldl' and insertWith', sry for not saying so initially).

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Is there another way of doing this, something more "streaming architecture" like?

I don't see how you can do this much better - for a small, fixed set of keys, you could use an (STU) array for the sums, but it depends if the added complexity is worth it. You're already doing a single pass over the data.

-k

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