Oops, just realised that I didn't include the mailing list in my original reply to Florian -- here was my reply: --------- Hi, I would write it as one of the following, ---- keys [] = [[]] keys (x:xs) = [0..x] >>= (\k -> map (k:) (keys xs)) -- or, keys' [] = [[]] keys' (x:xs) = do { k <- [0..x] ; map (k:) (keys' xs) } -- or, keys'' [] = [[]] keys'' (x:xs) = concat [map (k:) (keys'' xs) | k <- [0..x]] -- or, keys''' [] = [[]] keys''' (x:xs) = concat (map (\k -> map (k:) (keys''' xs)) [0..x]) ---- Each of which does what I think you want. The first two use the fact that lists are a monad, using bind and do notation respectively. (You might have a look at http://www.haskell.org/hawiki/MonadsAsContainers for more information and intuition about what's going on there.) The third makes use of a list comprehension, and the fourth is written using just list functions without any special syntax. The general idea is that to construct keys (x:xs) you add each of [0..x] to the front of each list produced by keys xs. Which of these is most elegant is up to you :) - Cale