I/O as a first class citizen

The IO Monad provides the context in which side effects may occur, and it also allows us to decouple pure code from I/O code. In this way, side effects are isolated and made explicit. Let's explore the ways in which I/O participates as a first-class citizen of the language:

import System.IO
import Control.Monad
import Control.Applicative

main = do
  h <- openFile "jabberwocky.txt" ReadMode
  line  <- hGetLine h
  putStrLn . show . words $ line
  hClose h

This code looks imperative in style: it seems as if we are assigning values to h and line, reading from a file, and then leaving side effects with the putStrLn function.

The openFile and hGetLine functions are I/O actions that return a file handle and string, respectively:

  openFile :: FilePath -> IOMode -> IO Handle
  hGetLine ::             Handle -> IO String

The hClose and putStrLn functions are I/O actions that return nothing in particular:

  putStrLn :: String -> IO ()
  hClose   :: Handle...