Re: [Haskell-cafe] reversing big list with constant heap space used

On Wed, 16 May 2007, Evan Laforge wrote:

...

I think that in every particular case you have to find out how to avoid 'reverse'. Especially if you have two 'reverse's like in reverse . dropWhile p . reverse there are more efficient solutions.

Just from curiosity, what *is* an efficient way to do rDropWhile? Here's something which at least avoids 2 reverses:

rDropWhile = doRDropWhile []

doRDropWhile accum f [] = [] doRDropWhile accum f (x:xs) | f x = doRDropWhile (x:accum) f xs | otherwise = reverse (x:accum) ++ doRDropWhile [] f xs

http://www.haskell.org/pipermail/libraries/2005-August/004217.html dropWhileRev :: (a -> Bool) -> [a] -> [a] dropWhileRev p = foldr (\x xs -> if p x && null xs then [] else x:xs) []