(Poorly written) Haskell programme to find out 20 most common words in text

h.hs

import Text.ParserCombinators.Parsec hiding (spaces)
import System.Environment
import Data.List (foldl')

import qualified Data.Map.Strict as M

main = interact getWords

getWords input = 
  case parse parseWords "words parser" input of
    Left err -> "Error: " ++ show err
    Right val -> (show . getMost . countWords) val ++ "\n"

parseWords = many parseSingleWord
  where
    parseSingleWord = do
      parseBlank
      x <- many1 (noneOf symbolOrBlank)
      parseBlank
      return x
    parseBlank = many (oneOf symbolOrBlank)
    symbolOrBlank = ",. \n\r!?-:;\"/'=$()[]{}`"

countWords = foldl' (\d w -> M.insertWith (+) w 1 d) M.empty

getMost :: M.Map String Integer -> [(Integer, String)]
getMost = (take 20 . M.toDescList . getMost')
  where getMost' = M.foldlWithKey' (\d w c -> M.insert c w d) M.empty