test.txt

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wc.hs

{-# LANGUAGE TupleSections #-}

import qualified Data.Text.Lazy as TL
import qualified Data.Text.Lazy.IO as TL
import           Data.Char (isLetter)
import           Data.List (sortBy)
import           Data.Function (on)
import qualified Data.HashMap.Strict as HM

import System.Environment (getArgs)

collect = sortBy (flip compare `on` snd)
        . HM.toList
        . HM.fromListWith (+)
        . map (,1)
        . filter (not . TL.null)
        . TL.split (not . isLetter)
        . TL.toLower

display (word, count) = do
    TL.putStr word
    putStr ": "
    print count

main = do
    [fname, count] <- getArgs
    text <- TL.readFile fname

    mapM_ display $ take (read count) (collect text)