https://blog.fefe.de/?ts=a269eb94

wp.hs

import qualified Data.HashMap.Strict as M
import qualified Data.ByteString.Char8 as B
import Data.List (sortOn)
import Data.List.Split (chunksOf)
import Control.Parallel.Strategies (parMap, rdeepseq)

main = do
    words <- B.words <$> B.getContents
    let result = sortOn snd $ M.toList $ foldl (M.unionWith (+)) M.empty $ parMap rdeepseq (foldl (\acc x -> M.insertWith (+) x (1 :: Integer) acc) M.empty) (chunksOf 50000 words)
    B.putStr . B.concat $ result >>= (\(a,b) -> [B.pack $ show b, B.singleton '\t', a, B.singleton '\n'])