MapReduce and token-counting in Haskell.

mrCount.hs

import Control.Arrow
import Data.Function
import Data.List
import Data.Ord

-- mapReduce key_func val_func reduce_func --
mapReduce :: Ord c => (b1 -> c) -> (b1 -> b) -> ([b] -> c1) -> [b1] -> [(c, c1)]
mapReduce k r v  = reduce' r . group' . map' k v
  where
    reduce' f = map (fst . head &&& f . map snd)
    group'    = groupBy (on (==) fst) . sortBy (comparing fst)
    map' k v  = map (k &&& v)

tokenCount :: (Num a, Ord b) => [b] -> [(b, a)]
tokenCount =  mapReduce id sum (const 1)