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 know this is an old post, but I saw no solutions that implemented good old fold.

First strip the leading white-space using dropWhile. Then, using foldl' and a simple closure, you can analyze the rest of the string in one pass, and based on that analysis, pass that informative parameter to take, without needing reverse:

import Data.Char (isSpace)
import Data.List (foldl')

trim :: String -> String
trim s = let
  s'    = dropWhile isSpace s
  trim' = foldl'
            (\(c,w) x -> if isSpace x then (c,w+1)
                         else (c+w+1,0)) (0,0) s'
  in
   take (fst trim') s'

Variable c keeps track of combined white and non white-space that should be absorbed, and variable w keeps track of right side white-space to be stripped.

Test Runs:

print $ trim "      a   b c    "
print $ trim "      ab c    "
print $ trim "    abc    "
print $ trim "abc"
print $ trim "a bc    "

Output:

"a   b c"
"ab c"
"abc"
"abc"
"a bc"