Words.hs

words'' :: String -> [String]
words'' s
  | s == "" = []
  | otherwise = takeWhile (/= ' ') s : words'' (dropWhile (== ' ') (dropWhile (/= ' ') s))

words' :: String -> [String]
words' s
  | w /= ""   = w : words' rest
  | otherwise = []
  where
    w    = takeWhile (/= ' ') . dropWhile (== ' ') $ s
    rest = dropWhile (/= ' ') . dropWhile (== ' ') $ s

demo :: (String -> [String]) -> [[String]]
demo f = map f
  [ ""
  , " "
  , "   "
  , " abc"
  , "def "
  , "   ghi"
  , "jkl   "
  , "mno pqr"
  , " stu vwx "
  , "   a   b    cd    efg    h  i   "
  ]

test :: (String -> [String]) -> Bool
test f = demo f == demo words