Duncan Coutts
Perhaps I misunderstood but I think Alexey means that he wants to accumulate several different histograms (ie different arrays) but to only make one pass over the input data.
This is precicely my problem, too.
The form of accumArray does not make that possible (unless one managed to pack the different histograms into different parts of the same array).
This would have been a good match for my case. Of course, that is a bit of a hack, and it would be much nicer and safer to do an accumArray with tuple elements. But I need to use an UArray in order to avoid the array filling up with unevaluated thunks, and - correct me if I'm wrong - there's no way to do a 'seq' before accumArray's insertion for complex types. So the stack blows up. /me wishes for s/Data.Array.Unboxed/Data.Array.Strict/g and instances for arbitrary types. Anyway, I thought of using offsets in the same UArray too late, and chose to finish my ST-based implementation instead.
If a fold using a pure persistent map type really is too slow then it should still be possible to do an ST implementation in a reasonably elegant way using a foldr on the input list, accumulating an ST action.
Or must mapM_ an insert operation (in ST) over the input data? -k -- If I haven't seen further, it is by standing in the footprints of giants