How to do it...

1. Create a new project io-monad with the simple Stack template:

        stack new io-monad simple

2. Open src/Main.hs; we will be editing this file.

3. After initial module definition, add the following imports. Only those functions that are used in the program are imported from the corresponding module:

        import System.IO (hGetLine, hIsEOF, withFile, Handle, 
        IOMode(..))
        import System.Environment (getArgs)
        import Control.Monad
        import Data.List (intercalate)

4. Write the function getLineSeq, which returns a list of lines when given a file handle:

         -- From the file handle, check if we have reached end of file,
         -- otherwise read the file line by line
         getLinesSeq :: Handle -> IO [String]
         getLinesSeq h = do
         eof <- hIsEOF h
         -- Use (:) to get the line and append remaining ...
        if eof then return [] else (:) <$> hGetLine h <*> getLinesSeq h

5. Write the function printLine to print the line number and line in the format line number : line. It uses the intercalate function, which separates the items in the list by a given element:

         -- Print line number and string separated by :
         printLine :: (Int, String) -> IO ()
         printLine (lineno, line) = putStrLn $ intercalate " : " [show 
         lineno, line]

6. Write the withLineNumbers function, which takes any monad that emits a list of strings and returns a list of tuple. Each tuple contains the line number and line itself: 

         -- Given a monad that gives us list of strings, return the 
         list of
         -- (int,string) where int is the line number, and string 
         represents
         -- the corresponding line.
         withLineNumbers :: Monad m => m [String] -> m [(Int,String)]
         withLineNumbers m = zip <$> pure [1..] <*> m

7. Now, write the main function to open a file and print the contents along with the line number:

        main :: IO ()
        main = do
        -- Throw an error if number of arguments is not 1
        args <- getArgs
        when (length args /= 1) $ do
        putStrLn $ "Incorrect arguments " ++ (show args)
        error "Provide file name"
        -- Open the file and print the lines with line number
        withFile (head args) ReadMode (\h -> do
           -- Each line is zipped with line number
           lines <- withLineNumbers (getLinesSeq h)
           -- Print line
           forM_ lines printLine
           )

8. Build and run the project. Note that we give the argument Setup.hs to the command prompt:

      stack build
      stack exec -- io-monad Setup.hs

9. Upon running the project, you should see the following output--contents of Setup.hs along with the line number: