Delete duplicate files in Haskell

nub_files.hs

#!/usr/bin/runhaskell

import Data.List

import Control.Monad

import System.Process (readProcess)
import System.Environment (getArgs)
import System.Directory (doesFileExist, removeFile)

data HashedFile = HashedFile { path :: String, hash :: String }
	deriving Show

instance Eq HashedFile where
	HashedFile _ h1 == HashedFile _ h2 = h1 == h2

mkHashedFile file = do
	hash <- (last . words) `fmap` readProcess "md5" [file] ""
	return $ HashedFile file hash

main = do
	hfiles <- getArgs >>= filterM doesFileExist >>= mapM mkHashedFile
	let dups = map path . concatMap tail . group $ hfiles
	mapM_ putStrLn dups
	mapM_ removeFile dups