On Wed, Sep 28, 2011 at 10:19 AM, Kazu Yamamoto
Hello,
Many languages provide the 'chop' or 'trip' function but Data.List does not. I would like to add a new function called 'tailDropWhile' so that people can easily implement 'chop' for String:
chop :: String -> String chop = tailDropWhile isSpace
The definition of tailDropWhile is as follows:
{- wren ng thornton's version. This is lazier than Aoe's one. Out and inP implement push-down automata. -} tailDropWhile :: (a -> Bool) -> [a] -> [a] tailDropWhile p = out where out [] = [] out (x:xs) | p x = inP [x] xs | otherwise = x : out xs inP _ [] = [] inP ss (x:xs) | p x = inP (x:ss) xs | otherwise = reverse ss ++ x : out xs
{- Mitsutoshi Aoe's version. This is faster is many cases but more strict. Just for reference. tailDropWhile :: (a -> Bool) -> [a] -> [a] tailDropWhile p = foldr go [] where go x xs | p x && null xs = [] | otherwise = x:xs -}
For more information, please read: http://www.mail-archive.com/haskell-cafe@haskell.org/msg93192.html
Discussion period: 2 weeks.
Regards,
--Kazu
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I'd implement it as: tailDropWhile :: (a -> Bool) -> [a] -> [a] tailDropWhile p xs = go id xs where go _ [] = [] go k (x:xs) | p x = go (k . (x:)) xs | otherwise = k (x : go id xs) which is as lazy as possible and doesn't involve any reversing. It's a function I've needed to use before, but I don't feel strongly about its inclusion particularly. While Ivan raises legitimate concerns, the version I've given above is a bit more streamy, and anyway many of us acknowledge the value of (!!) and related "bad" list functions in some circumstances.