Memory hog + Stack overflow

mem.hs

import qualified Data.Map as M -- cabal install unordered-containers
import qualified Data.ByteString.Char8 as B
import Data.List (foldl')

main = B.interact $ exhibit . foldl' updateCount M.empty . B.words

 where exhibit counts =  B.concat $ zipWith pretty keys_ counts_ where
                           counts_ = M.elems counts ; keys_ = M.keys counts
       pretty w count = w +++ " :: " +++ show count +++ "\n"
       updateCount m w = M.insertWith (+) w (1::Int) m
       (+++) x y = B.append x (B.pack y)

Here's how I would have done this if I were writing it from scratch, I couldn't tell if you needed more complicated IO. If the file has unicode you will need Text. If it is huge you might need Text.Lazy or ByteString.Char8.Lazy . If compiled with ghc --make -O2 hsprogram.hs then cat 40mb.txt | ./hsprogram has no difficulty