In Haskell, how do you trim whitespace from the beginning and end of a string?

Question

How do you trim whitespace from the start and end of a string?

trim \"  abc \" 

=>

\"abc\"

Edit:

Ok, let me be a little cleare

Answer 1

This should be right about O(n), I believe:

import Data.Char (isSpace)

trim :: String -> String
-- Trimming the front is easy. Use a helper for the end.
trim = dropWhile isSpace . trim' []
  where
    trim' :: String -> String -> String
    -- When finding whitespace, put it in the space bin. When finding
    -- non-whitespace, include the binned whitespace and continue with an
    -- empty bin. When at the end, just throw away the bin.
    trim' _ [] = []
    trim' bin (a:as) | isSpace a = trim' (bin ++ [a]) as
                     | otherwise = bin ++ a : trim' [] as