Haskell grep simplification
I see these areas where your code could be improved:


processFiles can be expressed very simply using mapM_ from Control.Monad:

processFiles :: String -&gt; [String] -&gt; IO ()
processFiles pattern = mapM_ (processFile pattern)

All your functions are in the IO monad. This goes a bit against Haskell's philosophy to keep side effects to minimum.
parseLines requires the whole file to be read into the memory. This could be solved by using lazy IO, but I'd strongly discourage you from doing so.


One possibility to solve 2. and 3. is to use conduits. This may seem as somewhat complex subject, but the idea is actually very intuitive. A conduit is something that reads input a produces output, using some particular monad. This allows to break your program into very small, reusable components, each doing a single particular task. This makes it easier to debug, test and maintain.

For example, your code could be refactored as follows. (First some required imports.)

import Control.Monad
import Control.Monad.IO.Class
import Data.ByteString (unpack)
import Data.Conduit
import qualified Data.Conduit.Binary as C
import qualified Data.Conduit.List as C
import Data.List (isInfixOf)
import System.Environment (getArgs)
import System.Directory (doesFileExist)
import System.IO

sourceFileLines :: (MonadResource m) =&gt; FilePath -&gt; Source m String
sourceFileLines file = bracketP (openFile file ReadMode) hClose loop
  where
    loop h = do
        eof &lt;- liftIO (hIsEOF h)
        unless eof (liftIO (hGetLine h) &gt;&gt;= yield &gt;&gt; loop h)


This function takes a file name and creates a Source - a conduit that takes no input, but produces output. It reads a file line by line and sends each line down the pipeline using yield. Using bracketP we ensure that the file will get closed no matter what happens to the pipeline.

numLines :: (Monad m) =&gt; Conduit a m (Int, a)
numLines = C.scanl step 1
  where
    step x n = (n + 1, (n, x))


This component built using scanl is very simple. It just sends its input to the output, and keeps the count along the way. Notice that this conduit doesn't need any IO, it works with any monad.

Now it's easy to filter a stream of numbered lines with a pattern:

parseLines :: (Monad m) =&gt; String -&gt; Conduit String m (Int, String)
parseLines pattern = numLines =$= C.filter f
  where
    f (_, x) = isInfixOf pattern x


This function fuses two conduits together. The first one numbers lines, the second filters them according to the pattern.

printMatch :: (MonadIO m) =&gt; Sink (Int, String) m ()
printMatch = C.mapM_ (\(n, x) -&gt; liftIO $ putStrLn $ show n ++ ": " ++ x)


In printMatch we separate the logic that prints out the output. For each pair it receives it prints the line number and its content.

Combining and running these conduits is then easy:


runResourceT $ sourceFileLines file $= parseLines pattern $$ printMatch



(runResourceT is needed because of bracketP.) So the rest of the program would look like

processFile :: String -&gt; String -&gt; IO ()
processFile _ [] = return ()
processFile pattern file = do 
    exists &lt;- doesFileExist file
    if not exists
        then putStrLn $ file ++ ": file does not exists"
        else do
            putStrLn file
            runResourceT $ sourceFileLines file $= parseLines pattern $$ printMatch

processFiles :: String -&gt; [String] -&gt; IO ()
processFiles pattern = mapM_ (processFile pattern)

main = do 
    args &lt;- getArgs
    processFiles (head args) (tail args)