Split a string into valid words, if it is possible to do so

wordsplit.hs

import Control.Monad

import Data.Char
import Data.Functor
import Data.List
import Data.Maybe
import Data.Set (Set, fromList, member)

import System.Environment
import System.Exit

main = do args <- getArgs
          when (null args) (error "No argument")
          words <- splitSentence (map toLower $ head args) <$> getWordSet
          when (null words) exitFailure
          putStrLn $ unwords words

getWordSet :: IO (Set String)
getWordSet = fromList . filter okWord . lines . map toLower
         <$> readFile "/usr/share/dict/words"

okWord :: String -> Bool
okWord s | s `elem` ["i", "a"] = True
         | length s <= 1 = False
         | not $ all isAlpha s = False
         | all isUpper s = False
         | any (`elem`['a', 'e', 'i', 'o', 'u']) s = True
         | otherwise = False

splitSentence :: String -> Set String -> [String]
splitSentence s = go [] (length s) s
  where
    go consumed _ "" _ = reverse consumed
    go consumed atMost s words =
      case takeWord words $ take atMost s of
        Nothing -> case consumed of
          [] -> []
          lastWord:others ->
            go others (length lastWord - 1) (lastWord ++ s) words
        Just w ->
          go (w:consumed) (length s - length w) (drop (length w) s) words

takeWord :: Set String -> String -> Maybe String
takeWord words = listToMaybe . reverse . filter (`member`words) . inits