Working with IO monad

In the recipes that we saw earlier, we all worked with IO, and used functions such as putStrLn :: String -> IO () or print :: Show a => a -> IO (). We already know that these functions print the string or a value to standard output.

In this recipe, we will open a file, read it line by line, and output it on the stdout along with the line number. We will also understand how IO works as a monad and how IO allows a Haskell program to interact with the outside world.

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...