Bertram Felgenhauer wrote:
finally, we could make it slightly more lazy, as follows:
(3) subsequences :: [a] -> [[a]] subsequences xs = [] : case xs of [] -> [] (x:xs) -> tail (concatMap (\ys -> [ys, x:ys]) (subsequences xs))
You can get rid of the ugly call to 'tail': subsequences :: [a] -> [[a]] subsequences xs = [] : subsequences' xs where subsequences' [] = [] subsequences' (x:xs) = [x] : concatMap (\ys -> [ys, x:ys]) (subsequences' xs)
The only problem I see it that the output order is changed - does anybody feel that it is particularily important?
I don't think it matters much. This new version starts with things from the start of the list, as opposed to the end: subsequencesOld "abc" == ["","c","b","bc","a","ac","ab","abc"] subsequencesNew "abc" == ["","a","b","ab","c","ac","bc","abc"] If anything, I think this makes more sense, especially since it works for infinite lists. Twan