I'm also learning Haskell so the solution below might be (1) inefficient and (2) incorrect, but hey, let's give it a try :-) For simplicity, in the testing code, I assume an infinite list of key/value pairs where they keys are of type Char between 'a' and 'z' and the values are Integers (the code also seems to work for keys with just a lower bound but no upper bound) I think the code speaks for itself import System.Random runningSumsOfValuesPerKey :: (Eq k, Num v) => [k] -> [(k, v)] -> [[v]] runningSumsOfValuesPerKey allPossibleKeys = runningSums . allValuesPerKey where runningSums = map (scanl (+) 0) allValuesPerKey pairs = [ valuesWithKey key pairs | key <- allPossibleKeys ] valuesWithKey key = map snd . filter ((==key) . fst) -- Testing randomPairs :: [(Char, Integer)] randomPairs = zip keys values where keys = randomRs ('a','z') rnd1 values = randomRs (0,9) rnd2 (rnd1,rnd2) = split (mkStdGen 0) test = map (take 10) [rs `atKey` 'c', rs `atKey` 'z'] where rs = runningSumsOfValuesPerKey ['a'..] randomPairs xs `atKey` k = xs !! (fromEnum k - fromEnum 'a')