[Haskell-cafe] Grouping - Map / Reduce

Hi, 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. The result of this grouping operation, which is effectively another list of key/value pairs, just sums this time, needs to be further processed. 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. Is there another way of doing this, something more "streaming architecture" like? Is Googles "Map - Reduce" related to this? Günther