Am Freitag, 30. September 2005 17:14 schrieb Henning Thielemann:
On Fri, 30 Sep 2005, gary ng wrote:
Once again, many thanks to all who taught me about this small little problem. Don't even know there is init/last and thought there is only head/tail.
http://www.haskell.org/ghc/docs/6.4/html/libraries/base/Data.List.html
But just for my curiosity, would the takeWhile still store the intermediate result till my result is reached ? If so, and my list is really very long(and I need to go to 1/2 of its length), I would still use a lot more memory than imperative method or even the foldl one(where in both case, I just take one element) ?
If you don't trust the compiler you can at least test if the rest of the list is ignored: Run with an infinite list.
E.g. last (takeWhile (<10) (scanl (+) 0 (repeat 1)))
I think Gary wanted to know whether the initial part of scanl's result is stored. I think, it shouldn't, because of the 'last'. However, when profiling your versions versus testFoldl :: (a -> Bool) -> (a -> b -> a) -> a -> [b] -> a testFoldl _ _ z [] = z testFoldl p _ z _ | p z = z testFoldl p f z (x:xs) = testFoldl p f (f z x) xs, I found that your head (dropWhile ... ) took about twice as long and allocated roughly 2.6 times as much memory as mine and last (takeWhile ... ) was a bit worse. Still, it didn't use near enough memory to store takeWhile (<= 5*10^7) (scanl (+) 0 (repeat 1)), all three used (if I interpret the profiling graphs [-hc, -hb] correctly) about 16.5 k of memory for practically the complete runtime. Besides, head (dropWhile (<10) (scanl (+) 0 (replicate 9 1))) will raise an error, as will last (takeWhile ...) if the starting value satisfies the break-condition. Cheers, Daniel