JJ's problem

gistfile1.hs

import Data.Char

main = interact unindent

unindent = unlines . unindentLines . lines

unindentLines ls = map (drop commonPrefixLength) ls
  where -- the number of characters to drop from each line
        commonPrefixLength = length (commonPrefix prefixes)
        -- whitespace-only prefixes of non-empty lines of ls
        prefixes = map (takeWhile isSpace) nonEmptyLines
        -- the non-empty lines of ls
        nonEmptyLines = filter (not . all isSpace) ls

-- the common prefix of a list of lists
commonPrefix []  = []
commonPrefix xss = foldr1 commonPrefix' xss
  where -- the common prefix of two lists
        commonPrefix' xs ys = map fst (takeWhile (uncurry (==)) (zip xs ys))