David Benbennick wrote:
Support, though I'd tweak the code in your patch a little:
subsequences :: [a] -> [[a]] subsequences [] = [[]] subsequences (x:xs) = let s = subsequences xs in s ++ map (x:) s
This will cause a space leak, as 's' will be kept around while its first copy is consumed. (1) subsequences (x:xs) = let s = subsequences xs in concatMap (\xs -> [xs, x:xs]) s behaves better in that regard, but changes the order of the result lists. This version can also be made to work for infinite lists with a small modification, (2) subsequences (x:xs) = let s = subsequences xs in [] : tail (concatMap (\ys -> [ys, x:ys]) s) 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)) I prever both (2) and (3) over (1) and the original, and I'm leaning towards (3). (As an example where (3) is superior to (2), consider subsequences (1:2:undefined) (2) gives []:[1]:undefined, while (3) gives []:[1]:[2]:[1,2]:undefined) The only problem I see it that the output order is changed - does anybody feel that it is particularily important? Bertram