Well, this approach has the problem that the running sum of key k blocks until a new value for that k arrives in the input stream. If you wanted to know the sum of the values of each key after you got nelements in the input stream, we could change the valuesWithKey inner function into: 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 (\(k,v) -> if k==key then v else 0) then map (!!n) on the result of runningSumsOfValuesPerKey gives you the sum after n elements arrived. I think if you now generalize this so you don't use 0 but mempty, mconcat and other Monoid methods, that you might get something like Luke's trie solution, not sure, Luke is a fair bit smarter than I am :-) But this code is very inefficient I'm afraid, I guess the blueprint stuff that was posted really builds a map incrementally, but I don't understand that yet. Ik spreek Nederlands ja ('t is te zeggen, "Antwerps"). Yes I'm still learning Haskell, but I think with Haskell this is a never ending process, since there's soo much stuff to discover and the language evolves (which makes it both exciting and frustrating, but that's the dilemma of knowledge anyway, the more you know the better you realize the vast amount of knowledge that you don't know yet :-) On Fri, Mar 27, 2009 at 12:53 AM, Guenther Schmidt wrote:

Dear Peter,

wow, thanks, this is a very ... interesting ... approach, I hadn't thought about that yet ;)

Ben je nederlands?

In case you'd be interested to share the "road to Haskell" experience with another newbie just ask.

Günther

Peter Verswyvelen schrieb:

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')