Hello What do you think of this? There is perhaps a recursive call that should be made tail recursive but it seems to work. The 'group' function takes the list of pairs as input and outputs a list of maps from key to sums. The nth element of the list of maps corresponds to the grouping applied for the elements 0....n of the input list of pairs. Thus, that also works on infinite list. Unless I am missing sth... import Data.Map (Map) import qualified Data.Map as Map group :: [(Int,Int)] -> Map Int Int -> [Map Int Int] group [] amap = [] group ((k, v):t) amap = newmap : group t newmap where newmap = (Map.insertWith (+) k v amap) l = [(1,1), (2,10), (1,2), (2,11), (1,3), (2,12)] r = group l Map.empty rr = take 2 r li = concat [ [(1,i), (2, 10*i)] | i <- [0..] ] ri = group li Map.empty rri = take 20 ri Regards J-C
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