Currently, it's defined

stringify :: String -> String
stringify s = reverse .  dropWhile isSpace . reverse -- drop trailing space
              . dropWhile isSpace                    -- drop leading space
              . compressSpaces                       -- replace each span of
                                                     -- whitespace with a single space
              $ s
    where compressSpaces [] = []
          compressSpaces (a:as) | isSpace a = ' ' : compressSpaces (dropWhile isSpace as)
          compressSpaces (a:as) = a : compressSpaces as

If we're going to go to the trouble of doing this by hand, we might as well take the efficiency advantage doing so can give us:

stringify :: String -> String
-- Spec: stringify = unwords . words
stringify xs = go False (dropWhile isSpace xs)
  where
    go _haveSpace [] = []
    go  haveSpace (x:xs)
      | isSpace x = go True xs
      | otherwise = if haveSpace
                    then ' ' : x : go False xs
                    else x : go False xs