| > power_list :: [a] -> [[a]] | > power_list = foldr (\x ~(_:xs) -> []:xs >>= \ys -> [ys, x:ys]) [[]] | | I loved how short and sweet this version is, but sadly with GHC it's | noticeably slower than Bertram's first, more directly coded, version | (1.32 seconds vs 0.55 seconds for power_list [1..24]). | | The two-line variant below is just over 25% faster than the above | oneliner under GHC, but at 1.04 seconds, it's still bested by the | explicit version: | | power_list :: [a] -> [[a]] | power_list [] = [[]] | power_list (x:xs) = [] : tail [y | ps <- power_list xs, y <- [ps, | x:ps]] | | Anyway, we're now far from our original topic, but thanks to Bertram, | we can see that power_list can be coded in a way that is memory | efficient, lazy-list friendly, and (relatively) easy to read. I wonder if it'd be worth writing up this thread into a Wiki page? Some neat programming here! Simon