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

I don't know anything about the runtime or efficiency but what about this:

-- entirely input is to be trimmed
trim :: String -> String
trim = Prelude.filter (not . isSpace')

-- just the left and the right side of the input is to be trimmed
lrtrim :: String -> String
lrtrim = \xs -> rtrim $ ltrim xs
  where
    ltrim = dropWhile (isSpace')
    rtrim xs
      | Prelude.null xs = []
      | otherwise = if isSpace' $ last xs
                    then rtrim $ init xs
                    else xs 

-- returns True if input equals ' '
isSpace' :: Char -> Bool
isSpace' = \c -> (c == ' ')

A solution without using any other module or library than the Prelude.

Some tests:

>lrtrim ""
>""

>lrtrim "       "
>""

>lrtrim "haskell       "
>"haskell"

>lrtrim "      haskell       "
>"haskell"

>lrtrim "     h  a  s k e   ll       "
>"h  a  s k e   ll"

It could be runtime O(n).

But I actually don't know it because I don't know the runtimes of the functions last and init. ;)