filewriter.hs

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

listFilesInDir :: FilePath -> IO [FilePath]
listFilesInDir dir =
  getDirectoryContents dir >>= filterM doesFileExist


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 ()

firstNLines :: Int -> FilePath -> FilePath -> IO ()
firstNLines n source sink = do
  inHandle <- openFile source ReadMode
  outHandle <- openFile sink WriteMode
  mapM_ (\_ -> readAndWrite inHandle outHandle) [1..n]

-- join . fmap sequence_ $ mapM (\_ -> readAndWrite inHandle outHandle) [1..n]

-- one problem:
-- Prelude> :t readAndWrite <$> blah <*> woot
-- readAndWrite <$> blah <*> woot :: IO (IO ())

-- This is because you didn't bind over them, you put them in the where clause which isn't going to
-- bind the operations over the IO.