In an attempt to learn Haskell, I've built this - it accepts a number n, a source directory, and a sink directory (ending with /) copies n lines from each file in the source directory to the sink directory. It is not recursive. Thoughts?

module Main where
import System.Directory
import Control.Monad
import System.IO
import System.IO.Error
import Data.List
import Control.Applicative
import Control.Exception.Base
import System.Environment
import Data.List

--requires / or \ at end.
listFilesInDir :: FilePath -> IO [FilePath]
listFilesInDir dir= (getDirectoryContents dir) >>= filterM (\x ->doesFileExist $ dir ++ x)

readAndWrite:: Handle -> Handle -> IO ()
readAndWrite inHandle outHandle = do
    myLn<-tryIOError(hGetLine inHandle)
    case myLn of
         Left e -> if isEOFError e
                   then return ()
                   else ioError e
         Right myLn -> do hPutStrLn outHandle myLn
                          return ()

writeFirstNLines :: Int-> FilePath->FilePath -> IO ()
writeFirstNLines n source sink =do
    inHandle <- openFile source ReadMode
    outHandle <- openFile sink WriteMode
    replicateM_ n (readAndWrite inHandle outHandle)

main :: IO ()
main = do
    args <- getArgs
    let n = read $ args!!0
    let source = args!!1
    let sink   = args!!2
    let fileList=listFilesInDir  source
    fileSources <-fmap (map (source ++)) $ fileList
    fileSinks <-fmap (map (sink ++)) $ fileList
    --putStrLn "Sources"
    --mapM_ putStrLn fileSources
    --putStrLn "Sinks"
    --mapM_ putStrLn fileSinks
    q<-zipWithM (writeFirstNLines n) fileSources fileSinks
  • \$\begingroup\$ some extra whitespace here and there around operators would do good to readability! \$\endgroup\$ Commented Oct 29, 2015 at 14:50

1 Answer 1


requires / or \ at end.

Your code requires that the user supplied source and destination directories end in a path name separator. A better approach is to simply add it. See the comments below on the (</>) operator.

partial functions

When processing the command line arguments you are using functions which could throw errors, e.g. read, head, !!. When they fail you'll get a generic error message like Prelude.read: no parse or Prelude.!!: index too large.

Hackage has libraries you can use to parse and validate command line arguments like cmdargs or any of the optparse- packages. However, it's not too difficult to roll your own like this:

 import Data.Char

 parseArgs (nstr : source : dest : _) =
   if all isDigit nstr
     then ( read nstr, source, dest )
     else error "first argument is not a number"
 parseArgs _ = error "not enough arguments"

main = do
  args <- getArgs
  let (n, source, dest) = parseArgs args

parseArgs checks that there are at least three command line arguments and also that the first one consists entirely of digits.


Currently the FilePath type is a String, so using (++) to join together path name segments is fine. However, in System.FilePath Haskell defines the operator (</>) which is meant to be a cross-platform way to do this - i.e. on Windows it will use \\ and on Posix systems it will use a /. Moreover using (</>) signals to the reader that you are working with a path name string.

The module System.FilePath also has utility functions for dealing with path names such as extracting the base / directory name from a path, extracting and replacing the suffix, etc. which are good to know about and may come in handy.

A good place to use (</>) is in your listFilesInDir function, e.g.:

listFilesInDir dir =
  getDirectoryContents >>= filterM (\x -> doesFileExist (dir </> x))

Now dir does not have to end in a / or \\ because using (</>) will add the appropriate separator.


Note that fileList is an IO-action, so in these lines:

fileSources <-fmap (map (source ++)) $ fileList
fileSinks <-fmap (map (sink ++)) $ fileList

you are repeating the IO-action twice. That is, you are performing getDirectoryContents twice on the same directory. Instead you want to perform fileList only once and use its result to compute fileSources and fileSinks:

leafNames <- listFilesInDir source
let fileSources = map (source </>) leafNames
    fileSinks = map (sink </>) leafNames
zipWithM_ (writeFirstNLines n) fileSources fileSinks

Note I'm using zipWithM_ here - it works just like zipWithM but discards the results. It's more efficient because zipWithM_ discards the result of the zip operation immediately, whereas zipWithM saves the results in a list thus consuming memory for data you'll never look at.


In Haskell you need to explicitly close your file handles when you are done with them. Every process has limit on the number of handles which can be open at any one time, so if you never close any handles then eventually openFile will fail when you hit this limit.

A basic way to make sure your file handles always get closed is to use the withFile function:

withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r

withFile creates a file handle by opening a path with the specified IOMode and passes the handle to an IO-action. It will ensure that the handle is closed when the IO-action completes - whether it returns normally or if an exception is thrown.

Using withFile your writeFirstNLines would look like:

writeFirstNLines n source sink = do
  withFile source ReadMode (\hin -> do
    withFile sink WriteMode (\hout -> do
      ... copy from hin to hout ...

Now both hin and hout will automatically get closed when the copy action completes.


Note that replicateM_ n action will always execute action exactly n times - no more and no less. So this is not a good way to write a loop which could be executed a variable number of times.

As an example of how inefficient using replicateM_ here could be, suppose that n is 1000 and you are operating on a 3 line file. Then:

replicateM_ n readAndWrite

will execute readAndWrite 1000 times even though the last 997 calls won't copy any lines.

Here is a recipe for a loop which is executed at most n times with the possibility of breaking out early:

loop n | n <= 0 = return ()
loop n = do quit <- condition       -- check condition
            if quit
              then return ()        -- break out early
              else ... do stuff...
                   loop (n-1)       -- perform next iteration

You can use this to copy the first n lines by substituting:

condition        =  hIsEOF hin
...do stuff...   =  hGetLine hin >>= hPutStrLn hout
  • \$\begingroup\$ You are the man. Let me work through this stuff. Thanks. \$\endgroup\$
    – Carbon
    Commented Oct 29, 2015 at 5:34

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